Italy and San Marino

The native range of D. invadens is thought to be in Italy (Reise et al. 2011), but the species might nevertheless be an introduction in parts of that country. Our recent collections and their genetic analysis support this hypothesis, but the results will be published elsewhere. To summarise, we have found that D. invadens is widespread throughout mainland Italy and occurs also in Sicily and Sardinia. It is easiest to find in synanthropic sites, as in other countries, but we also found it in undisturbed woodland. Several other, less frequent, species occur which are externally indistinguishable from D. invadens. Only in one garden in northwest Italy was D. invadens found co-occurring with D. panormitanum s.s. (Table 2); the latter replaces D. invadens in parts of Sicily.

Table 2 details the first two records (2013) of D. invadens in the Republic of San Marino (a tiny state surrounded by Italian territory).

Malta

Reise et al. (2011) argued that Pollonera’s (1891) original description of D. caruanae from the capital Valletta referred to D. panormitanum s.s. Our collecting in 2000 on Malta and Gozo (supplemented by that of T. Backeljau in 1994: see Reise et al. 2011) was targeted at Deroceras but never encountered D. invadens; D. panormitanum s.s. and D. golcheri (Altena, 1962) were widespread.

Great Britain

Deroceras invadens was first found about 1930, in Cornwall, but by 1932 also from South Wales, central southern and northeast England, and Scotland (Ellis 1950, Quick 1960, Kerney 1999). This wide distribution implies that it had been overlooked for some time. Formerly, some supposed that D. invadens was native in Britain (Ellis 1951, Hayward 1954), but we are now more aware of how well the species can spread, and further work (Reuse 1983) established that the shell is insufficiently distinctive for us to trust Hayward’s (1954) identification of fossils.

The Conchological Society of Great Britain and Ireland publishes lists annually of vice-counties in the British Isles from which species have been newly recorded and the identity confirmed by experts. If we restrict attention to England and Wales (Scotland and Ireland were more sporadically recorded), the number of vice-counties increased only slowly to 13 by 1964, then jumped to 54 within 10 years (presumably at least partly an artefact of increased sampling effort in preparation of a distribution atlas). Over the next 25 years, Kerney (1999) considered that it had continued to spread rapidly. Today, confirmed records are lacking in only one of the 70 vice-counties (for which there are unconfirmed records) and the distribution now spans all of Great Britain, including such offshore islands as Orkney, Shetland, the Western Isles, the Isle of Man, Lundy, and the Scilly Isles.

At a more local scale, D. invadens has been recorded from 1849 10 km grid squares in Great Britain; this is 0.44 as many as the almost ubiquitous D. reticulatum (access of NBN gateway on 24.xi.12; https://data.nbn.org.uk). The distribution map still shows some regions of scarcity (Fig. 1). Some correspond to areas where absent records of D. reticulatum suggest light recording activity, but these are also often areas of low human population density, which itself might have hindered the spread of a synanthrope. Areas in the west with low human population densities nevertheless show a dense distribution of D. invadens. A higher abundance in the west was noted by Kerney (1999) and may derive from the milder climate (both warmer winters and wetter summers).

Some regional differences in abundance are certainly not artefacts of recording intensity. For instance, in Suffolk (eastern England, sparsely populated) D. invadens was not reported until 1982 and a particularly thorough survey up to 1990 found it to be relatively scarce, with scattered localities across the county but concentrated in a couple of areas and generally only in gardens and disturbed ground (Killeen 1992). Contrast this with a 1987–88 survey of gardens in Greater Manchester (northwest England, densely populated), which found it to be the slug species occurring in the most gardens (258 out of 372 gardens) and in the greatest numbers (North and Bailey 1989). Similarly, in a diverse sample of 16 gardens in southeast Scotland, D. invadens occurred in 14; D. reticulatum was the only mollusc occurring in more (Sumner 2002).

Kerney (1999) nicely summarises where D. invadens is most often found (writing of the British Isles, but it is typical elsewhere too): “A species of disturbed habitats, and associated with roadside rubbish, farmyards and gardens. It often shelters under stones, pieces of wood, cardboard and other litter in bare or sparsely vegetated waste ground”. He also reported the species occurring in wilder places such as woods and hedgerows but only in climatically mild areas such as the South West; in our experience this is also the case in the South East. In a survey of eight ancient woods on the Isle of Man, it was found in seven and was the only widespread non-native species (Alexander and Dubbeldam 2013). Likewise, in northeast England D. invadens occurred in 9 out of 17 woods (all contained D. reticulatum: Wardhaugh 1996). Dirzo (1980) reported the absence or rarity of D. invadens in habitats dominated by grass. It occurs in some arable fields (Quick 1949, Foster 1977, Dirzo 1980, Vernavá et al. 2004, Howlett 2005 Chapter 8), where it sometimes dominates, but usually other slugs are the more important pests. However, D. invadens was the dominant slug pest in 10 selected English nurseries growing “hardy nursery stock” (Anon. 2003; the dearth of information about this survey does raise a concern that D. laeve might have been confused sometimes). An ability to thrive in the damp warm environment of modern nurseries would predispose the species to spread into gardens nationwide.

Rowson et al. (2014a, b) reported the occurrence of D. panormitanum s.s. from a garden in Cardiff. The single occurrence of this species contrasts with the 37 British populations of D. invadens that we have checked or from which COI sequences appear in Genbank (Table 1).

Island of Ireland

The first records were in 1958, from several sites around Cork, and in 1959 from Newcastle, County Down, at the other end of the island and in a different country (Makings 1959). These dates are surprisingly late, considering the close links with Great Britain and that malacologists familiar with the species there were collecting in Ireland in these 29 years after its discovery in Britain.

Today in Ireland the number of 10 km squares occupied by D. invadens is a similar proportion of those occupied by D. reticulatum as in Great Britain (0.38 vs 0.44; access to NBN gateway on 24.xi.2012; https://data.nbn.org.uk). In an interesting contrast with Britain, Ross (1984) observed that it was commoner to the east and north; this might be explicable by a correlation with higher population densities (Fig. 1). At least in the north, the explanation for such a correlation is not an absence of collecting activity towards the west (Anderson 1983). Ross (1984) reported that its occurrence in less disturbed sites such as woodlands and marshes was increasing. By 1996, it was “widespread and abundant in disturbed habitats, woodland and marshes throughout Northern Ireland. A notable pest in gardens” (Anderson 1997). Around Cork, even soon after its first discovery, 7–27% of slugs collected in gardens were D. invadens (Makings 1959, 1962).

France

In 1910 Simroth described Agriolimax scharffi collected in 1903 from La Giandola in the extreme southeast of France. Some (e.g. Bishop 1980, Gavetti et al. 2008) have argued that this refers to the slug we call D. invadens, which would then be the first record of the species in France or anywhere else and give the species name scharffi priority. Unfortunately the description is so casual (“... fand ich neben der gemeinen Ackerschnecke eine kleinere helle Form, die nicht retikuliert, sondern fein dunkel punktiert war”: I found together with the common D. reticulatum a smaller pale form which was not reticulated but with fine dark dots) that it could refer to other species known from the area (Bodon et al. 1982), such as D. bisacchium Bodon, Boato and Giusti, 1982, D. rodnae Grossu and Lupu, 1965, or a form of D. reticulatum. Also the sibling species D. panormitanum s.s. occurs only 23 km away at Bordighera, Italy (Table 2). We visited La Giandola in 2013 and did find D. invadens, which in itself is not so informative because the species could well have spread there since Simroth’s visit. More significant is that D. invadens at this site is not paler than the D. reticulatum. Conversely some of the D. reticulatum were of a somewhat unfamiliar appearance to us and did fit the description of “pale... not reticulated... with fine dark dots”. And also in La Giandola itself we found another species fitting this description; anatomically it matched local D. rodnae s.s. but its mating behaviour was distinct. Altogether there seems plenty enough uncertainty to follow Reise et al. (2011) in regarding A. scharffi as a nomen dubium and not to treat this as a record of D. invadens.

The first clear reports of D. invadens from France are from Hameury (1958), listing two sites near Brest (NW France) from 1956. Quick (1960) had examined specimens from the Pyrenées Orientales (SW France). Later Reygrobellet (1963) described a new species D. meridionale, now considered a synonym of D. invadens (Reise et al. 2011); she mentioned it occurring in southeast France (departments of Var, Bouches-du-Rhône, and Alpes-de-Haute-Provence, without details of localities or dates). More specifically, Chevallier (1973) published an occurrence of D. invadens from this region (Marseille) from 1948. Reygrobellet (1963) also referred to work by Abeloos (1945) demonstrating the existence of two forms of Deroceras near Poitiers (western France), one of which she and Abeloos had subsequently agreed was D. meridionale (i.e. D. invadens). To summarise, the species occurred at two distant sites in France already in the 1940s and was widespread at least by 1963.

A map prepared in 1972 by Chevallier (1973) showed many localities throughout the western half of France and around Paris; their absence on the eastern half of the map, except in the south, was explained by this region not having been surveyed. However Chevallier also stated that the species seemed more at home in coastal regions than inland, and emphasised that it occurred along the very edges of sea shores, estuaries and lagoons as well as by freshwater habitats. In contrast to other habitat descriptions of D. invadens, he mentioned only in the last sentence that it was also sometimes found in gardens.

The first two records from Corsica were from 1977 (Holyoak 1983); these were identified by A. Wiktor but it would be desirable to scrutinise specimens from this island further given the taxonomic complexity of the genus on the adjacent Tuscan archipelago (Giusti 1976). The online database of the Inventaire National du Patrimoine Naturel (INPN) (http://inpn.mnhn.fr/espece/cd_nom/163204/tab/rep) shows D. invadens now to occur also widely in northeast France. Cucherat and Demuynck (2006) describe it as widespread and sometimes locally abundant in the northeast departments of Nord and Pas du Calais. Comparison of INPN maps of D. invadens and D. reticulatum suggests that the continuing absence of records from areas of central and eastern France at least partially reflects an absence of recent recording. However, there certainly are still areas of France where D. invadens is uncommon. Thus, Boulord et al. (2007) found it to be rare to uncommon in Maures (NW France), occurring in only six 5 km squares compared with the 44 with D. reticulatum (ratio = 0.14). And in a review of slugs in Alsace (E France), Hommay (2000) could report D. invadens only from greenhouses near Colmar.

Monaco

We believe that the first record of D. invadens from Monaco is our finding in 2012 under bushes in a park at a spot irrigated by an automatic watering system (Table 2).

Belgium

The first finding of D. invadens in Belgium was in 1968 in a Brussels garden (Van Goethem 1974). The species was not found again until 1972, when a project mapping terrestrial Mollusca began. Van Goethem et al. (1984) mapped on a 10 km grid all findings of D. invadens and D. reticulatum for each year from 1972–83. Again we divide the number of grid squares in which D. invadens had been found by the number for D. reticulatum (thus controlling for collecting coverage); this yields a ratio of 0.65, comparable with figures of 0.44 and 0.38 for Britain and Ireland.

Van Goethem et al. (1984) also displayed the total number of records of D. invadens and D. reticulatum within each grid square. Deroceras reticulatum occurred in almost every square, so for each grid square the ratio of number of records of D. invadens to those of D. reticulatum provides a local measure of commonness of D. invadens adjusted for collecting effort. (Only a small proportion of the records for D. reticulatum predate the occurrence of D. invadens.) By this measure D. invadens is rarer in the southeast (Fig. 2). As in Britain and Ireland, the pattern is a good match at a coarse level to human population density (Fig. 2). A partial exception is the paucity of records around the city of Liege, interesting because in 2012 we failed to find the species in this city despite screening public gardens and allotments that looked ideal habitat.

Additionally the Belgian data provide the number of grid squares in which each species of Deroceras was found each year. Surprisingly, the ratio between the counts for D. invadens and D. reticulatum does not increase over the period 1972–83 (Fig. 3, τ = –0.30, P = 0.19), suggesting that the species was already well established by 1972 despite only one record predating this. Even when we considered only the apparently less saturated southeast, the ratios did not suggest an increase. Our conclusion conflicts with that of Van Goethem et al. (1984), who did sense an increased incidence over this period.

Van Goethem et al. (1984) provided qualitative comments on the habitat of D. invadens. It was more synanthropic than D. reticulatum, and could predominate over that species in such habitats, whereas it was less common on canal banks. It seemed to be absent from the interior of woods, especially coniferous woods, but it was rare even in poplar plantations.

Netherlands

The first finding was in 1969 in Domburg at the southwest tip of the country (Gittenberger et al. 1970). By 1982 it was recorded from five provinces across the south of the country (de Winter 1984), and already in 1984 de Winter estimated that he had found it in a third of gardens and similar habitats examined. A distribution map prepared in 2005 (Atlasproject Nederlandse Mollusken; http://www.anemoon.org/anm/voorlopige-kaarten, accessed 5.vi.13) shows it spread over the whole country, including even the Wadden island of Terschelling (Mienis 2003), although mapping of the Netherlands is much less complete than for the British Isles or Belgium. This map shows D. invadens as recorded in 50 5 km squares between 1990 and 2005, compared with 199 squares for D. reticulatum. The ratio of these numbers is somewhat lower than in Britain, Ireland or Belgium (0.25 vs. 0.44, 0.38 and 0.65), which might partly reflect the lesser sampling intensity in the Netherlands: if each square is sampled rarely the ratio reflects the chance of encountering D. invadens at an individual collecting site, rather than somewhere in the whole grid square.

Three intensive local surveys from the south of the Netherlands confirm the broad pattern. In Zuid-Holland, Boesveld (2005a) found D. invadens in 34 5 km squares, compared with 80 for D. reticulatum (ratio = 0.42). For 1 km squares, the corresponding figures are 48 and 129 (ratio = 0.37). In Noord-Brabant, Boesveld (2005b) found D. invadens in 9 of the 118 1 km squares visited, and D. reticulatum in 36 (ratio = 0.25). Both studies pointed out that they had undersampled urban areas, where D. invadens is commoner. Along the coast of Zeeland (Boesveld 2005c), D. invadens occurred in 22 out of the 79 1 km squares visited compared with 38 for D. reticulatum (ratio = 0.58).

Luxembourg

There are four 1997–98 records of D. invadens provided by Weitmann and Groh in the database of the Musée d’histoire naturelle Luxembourg (accessed via http://data.gbif.org/species/5190777 Sept. 2012).

Germany

Falkner (1979) documents several cases in the 1970s of D. invadens turning up in Munich on purchased lettuce, endive and strawberries, mostly imported from Italy. The first findings outdoors in Germany were in 1977 and 1978 in and near Munich (Falkner 1979). Analysing the subsequent spread is complicated because, although Germany is well supplied with malacologists, recording schemes have been organised, if at all, at the level of the 16 states (Bundesländer). The following paragraphs deal with each state in turn, in order of the first appearance of D. invadens in each, except that the states that were part of East Germany until 1990 are considered in a second paragraph (Berlin is listed with West Germany). The first published occurrence in the former East Germany postdated by a year the opening of the border between East and West Germany (Bössneck 1994), but by this time D. invadens had not been recorded either in 6 of the 11 West German states. Moreover, the notebooks of the late V. Herdam, indicate East German occurrences in Berlin in 1982 and in Brandenburg in 1985 (E. Hackenberg pers. comm.; Table 2); it is hard to judge how reliable these records are.

Bavaria: first found in 1977 (see above). North Rhine Westphalia: first found in 1979, at several sites near Cologne (Schnell and Schnell 1981); a thorough survey of Cologne between 1990 and 1994 located it at 15 sites throughout the city, compared with 49 for D. reticulatum (ratio = 0.31: Tappert 1996). Baden–Württemberg: first found in 1982, from several distant sites (Schmid 1997). Schleswig–Holstein: first found by 1983 (Wiese 1983); the 1991 atlas (Wiese 1991) showed it in 2 10 km squares, compared with 21 for D. reticulatum (post-1960 records; ratio = 0.10). Hesse: W. Hohorst collected D. invadens in Frankfurt am Main in 1985 (Table 2); in a 2010–11 survey in Frankfurt am Main, D. invadens was recorded from 11 of 22 sites, compared with 5 for D. reticulatum (ratio = 2.2; Kappes et al. 2012). Rhineland Palatinate: first found in 1994, in Jockgrim (Schmid 1997). Lower Saxony: first found in 1998 on the island of Baltrum; other records in Lill (2001) indicate that by 2001 it occurred throughout the state, sometimes at high densities, but restricted to synanthropic sites. Hamburg: first found in greenhouses in 1998 and outdoors in 2000 (Glöer and Hausdorf 2001). Bremen: first found 2001 (Lill 2001). Berlin: we know of no published record, but note Herdam’s 1982 record mentioned above, and we collected it ourselves from a garden in Steglitz in 2001 (Table 2). Saarland: we know of no record.

Saxony: first found in 1990, in Limbach near Reichenbach (Bössneck 1994), and in 1991 at the other end of the state, in Görlitz (Reise and Backeljau 1994). Thuringia: first found in 1993, in Erfurt (Bössneck 1994); rather few sites (c. 5) have turned up since (U. Bössneck, pers. comm. 28.i.14). Mecklenburg–Vorpommern: first found in 1998; by 2006 it had been recorded in 10 5 km squares, always at synanthropic sites (64 squares contained D. reticulatum: ratio = 0.16; Zettler et al. 2006). Saxony–Anhalt: Unruh (2001) stated that it had been part of the outdoor fauna for years but by 2012 it was recorded from only 4 10 km squares (compared with c. 55 with post-1990 records for D. reticulatum: ratio = 0.07; Körnig et al. 2013). Brandenburg: we know of no published record beside our own from 2004, in Senzig at the edge of the Berlin conurbation (Reise et al. 2011), but note Herdam’s 1985 record mentioned above.

A 2000–12 survey covering a broad area of northwest Germany, extending also into the Benelux countries, found that the mean density of D. invadens slightly exceeded that of D. reticulatum in gardens and in early successional woodland (Kappes and Schilthuizen 2014) but not in grassland or mature woodlands. The ranking of habitats in decreasing order of mean density of D. invadens was: gardens (0.68 m^–2), early successional woodland, mesic open, floodplain forest, wet open, wet deciduous forest, deciduous forest, scree forest, conifer; in the later three types the density was very low or absent.

In conclusion, D. invadens is now widely distributed in Germany and can be a common synanthrope in the west, but it took decades to turn up in many states and is still uncommon in many areas. The other invading Deroceras, D. sturanyi, is often commoner (e.g., Zettler et al. 2006, Körnig et al. 2013) although not in the northwest (Kappes and Schilthuizen 2014). There is little indication of D. invadens colonising more natural habitats.

Switzerland

The first record was in Basle near the German border in 1982 (Falkner 1982), the next not until 1991 (Turner et al. 1998). By 2011 D. invadens had still been recorded from only eight 5 km squares (around Basle, Bern, Lucerne and Zürich, so only from the lowlands in the northern half of the country: Boschi 2011). This compares with records of D. reticulatum from 350 squares since 1951 (Boschi 2011; ratio = 0.02). However, an independent study in 2007–09 (Fabian et al. 2012) found D. invadens to be one of the three commonest slugs in wildflower strips edging arable fields south of Lake Neuchâtel; perhaps the mapping had tended to overlook such occurrences in mundane agricultural landscapes.

Austria

The first record was in or before 1977 from a market garden in Maissau (NE Austria; Reischütz 1977, 1980). A 1986 atlas (Reischütz 1986) shows D. invadens in 9 10 km squares, all in the eastern quarter of the country, centred on Vienna. However, D. invadens had been collected already in 1979 from Lienz (SSW corner of Austria: Kofler 1986), and in the north also the species has since been found further to the west (1990 in Schiebbs: Ressl 2005; 1999 in Braunau am Inn: collected F. Seidl, Table 2; 2009 in Linz, 2011 in Wels: Aescht and Bisenberger 2011). In 2013 we found the species still further west, in Innsbruck (Table 2).

A 1992 survey of greenhouses around Vienna found D. invadens in 3 out of 10 establishments (the same as for D. reticulatum; D. laeve occurred in 6: Leiss and Reischutz 1996).

Czech Republic

A specimen collected in Ostrava in 1996 was identified as D. invadens only in 2003 (Horsák and Dvořák 2003). By that time there was a further 2002 record from a greenhouse in Sušice. Horsák and Dvořák (2003) believed that it was liable to have been widely overlooked elsewhere, but the latest review (Horsák et al. 2013a) provides no further records. We have recently (2014) found the species at two sites within the town of Hrádek nad Nisou, close to the German and Polish borders (Table 2).

Slovakia

Deroceras invadens is known only from greenhouses of the botanical garden in Bratislava where it was first found in 2003 (Dvořák et al. 2003; Horsák et al. 2013a).

Poland

The only record of which we are aware is from 2001 from the botanical garden and adjacent areas in Wrocław (SW Poland: Wiktor 2001a, 2004). The population was still extant in 2013 (A. Wiktor and HR pers. obs.).

Lithuania

Deroceras invadens had been reported from the botanic garden in Kaunas (Skujienė G 2013 Invasive slugs in Lithuania: results, problems and perspectives of the investigations. Abstract booklet of “Slugs and snails as invasive species, a meeting of the IOBC/WPRS slugs and snails subgroup, Bergen, Norway 25–27 September 2013”, p. 11). However, Skujienė kindly lent us the specimen on which this record was based and it proved to have been misidentified.

Hungary

Deroceras invadens was listed as present in Hungary in the guide book by Kerney et al. (1983). However, the more authoritative lists of Wiktor and Szigethy (1983) and Pintér and Suara (2004) do not include this species.

Romania

Grossu (1969) retracted the earlier claim that D. invadens occurred in Romania (Grossu and Lupu 1965).

Bulgaria

Wiktor (2000) mentioned a population in the Piryn mountains of Bulgaria that he assigned to D. panormitanum. At that time the confounding of D. panormitanum and D. invadens meant that the genitalia of the species were considered very variable. Now we reject this identification; for example, the sarcobelum is strongly flattened in these Bulgarian slugs, which is utterly untypical of D. invadens, or of D. panormitanum s.s.

Greece

Wiktor (2001b) reported “Deroceras cf. panormitanum” at “scattered localities” in Greece; he considered the 202 specimens from many sites to be all one species but was tentative in placing them in this taxon. However, specimens fitting this description from Crete proved to be an undescribed species (Reise et al. 2011). We have now checked the anatomy of five Greek samples in the Museum of Natural History, Wrocław University (from Mount Lampeia in the Erymanthos mountains, Cape Sounion south of Athens, the island of Skyros, the Kallicratis figau in SW Crete, Heraklion in Crete) and one sample in the Naturalis Biodiversity Centre, Leiden (RMNH.MOL 329848 from the island of Lefkada), all labelled by Wiktor as D. panormitanum. In the light of the recent taxonomic revision (Reise et al. 2011), our opinion is that none of these are D. invadens or D. panormitanum s.s., so we concluded that probably all Greek records of D. invadens should be considered unreliable.

However, recently Rowson et al. (2014a) have published a finding of D. invadens from Lake Kournas in Crete reliably identified by a COI gene sequence (Genbank KF894343; R. Anderson pers. comm. 15.iv.2014). Anderson had found the species in a patch of disturbed scrubland in May 2011 and 2012. Crete has been fairly well sampled for slugs without this species being noted elsewhere, so a recent introduction seems likeliest.

Denmark

The first specimens were already collected in 1937, outdoors in a park in Odense. But their identity was not recognised until the mid 1950s when further specimens turned up at seven different cemeteries in another part of Denmark (northern end of Jutland: Lohmander 1959). We have found no more recent information.

See below for records from the Faroe Islands.

Sweden

The first findings date from 1957 to 1959, when D. invadens was found in six greenhouses well spread over the country (even up in Bysek at 65°N; Waldén 1960). However, subsequently the species remained rare in greenhouses or even declined (Proschwitz 1991). Outside greenhouses, occurrences also remained “extremely rare” before 1980, but by 2002 D. invadens was rapidly establishing itself, perhaps as a result of milder winters (Proschwitz 2002). It is now recorded outdoors from the provinces of Skåne, Halland, Småland, Västergötland, Dalsland, Uppland, Värmland and the Baltic islands of Öland and Gotland, with records denser in the south but extending as far north as Uppsala (59.8°N: Proschwitz 2009, 2010).

Norway

The first find was in about 1967 from a greenhouse in a botanic garden in Bergen (Olsen 2002). Outdoors, D. invadens was first found in 1983–84 at four synanthropic sites along the mild Atlantic coast, as far north as Møre og Romsdal (c. 62°N: notebooks of H.W. Waldén, cited in http://databank.artsdatabanken.no/FremmedArt2012/N79713 accessed July 2013). By 2002 it was considered common outdoors in cultural habitats (Olsen 2002), and it is spreading to more natural habitats (Sneli et al. 2006).

Finland

Brander and Kantee (1961) reported D. invadens from Finland, without stating the date of discovery. Valovirta (1967) made clear that it was known only from the greenhouses of southern Finland. We are not aware of more recent information.

Portugal

Seixas (1978) was the first to report D. invadens, from near Lisbon in 1977. Rodríguez et al. (1993) reported three further localities in the northern half of Portugal from 1983–85 (out of 55 sites visited across Portugal, 40 of which yielded D. reticulatum: ratio = 0.05).

See below for records from Madeira and the Azores. The indications of D. panormitanum s.s., together with D. invadens, on both these archipelagos suggest that it would be worthwhile rechecking specimens from mainland Portugal.

Spain

The first published record of D. invadens was from Bilbao (north coast) in 1980 (Gómez et al. 1981). However, Castillejo (1983) had identified material collected as early as 1974 from northeast Spain; over the period 1974–80 D. invadens had been found in 16 10 km squares in Galicia (compared with 36 for D. reticulatum: ratio = 0.44; records added over the next decade changed this to 20:50 = 0.40: Castillejo and Rodríguez 1991). Later mapping (Castillejo 1997) indicates a distribution along the wet northern coast, with one record in the southern foothills of the Pyrenees ENE of Huesca and some around Valencia (middle of east coast). The species is rarer in the east than in the northwest: in the east-coast province of Castellon, Borredà and Collado (1996) found D. invadens in 2 out of 105 locations sampled in 1990–92, compared with 61 for D. reticulatum (2 vs 38 10 km squares occupied by each species: ratio = 0.05). Similarly in the neighbouring east-coast province of Valencia, D. invadens occurred in 2 out of 69 localities sampled in 1990 and D. reticulatum in 34 (2 vs 27 10 km squares: ratio = 0.07; Borredà et al. 1990). Two of our own findings (Table 2) extend the range to the northeast (Bàscara = first record for Catalonia) and further south of the Pyrenees (Tudela). There remains a dearth of records from the drier middle and south of the country. Survey work in Castilla–La Mancha (central Spain) 2003–07, located only one site for D. invadens compared with about 101 for D. reticulatum (Bragado et al. 2010).

The first record for the Balearic Islands was 2001 from Majorca, where D. invadens has since been found in several other localities (Beckmann 2007). There are also records from Ibiza in 2001 (under pots in garden centre: Anderson 2003) and from Menorca in 2002 (hedgerows and/or woods: Anderson 2004).

Castillejo (1998) emphasised that D. invadens is restricted to synanthropic habitats in mainland Spain, especially gardens, whereas Altonaga et al. (1994) listed wet meadows and beside streams. In northwest Spain it occurs in arable fields and permanent cattle-grazed pasture, in both of which it is sometimes the commonest slug (Iglesias et al. 2001, 2003; Cordoba et al. 2011).

See below for records from the Canary Islands.

Egypt

A 2005–07 survey in Asyut Governate (along the Nile, upstream of Cairo) found D. invadens in 15 out of 38 gardens and farms (Obuid-Allah et. al. 2008; 8 sites contained D. reticulatum: ratio = 4.75). This was the first report from Egypt, probably partly because the authors sent some specimens abroad to be identified.

South Africa

Deroceras invadens was recorded from several widely separated sites in Cape Province (George, Wilderness, Cape Town) in 1963–65 (Altena 1966). Now (Herbert 2010), it is also known from the Eastern Cape and from Guateng, including Pretoria, which, at 25.7°S, has a humid subtropical climate (cf. the mediterranean climate of the Cape). Although most records are from gardens it has been found also in indigenous forest on Table Mountain and near Somerset East (at the former already in 1965: Altena 1966, Herbert 2010). Herbert’s (2010) distribution maps show 10 dots for D. invadens compared with 16 for D. reticulatum (ratio = 0.63), the major difference being that only D. reticulatum has been reported from the region west of Durban.

See below for records from Marion Island.

Kenya

A specimen was intercepted arriving in the USA on cut flowers (Astrantia) from Kenya (04.viii.12, USDA 110834). One area in Kenya where Astrantia is grown commercially for export is at Kipipiri, at an altitude of 2300–3000 m. Such altitudes may well provide a suitable habitat for D. invadens.

Sri Lanka

Two publications report the presence of D. invadens in Sri Lanka (Kumburegama and Ranawana 2001, Bambaradeniya 2002) and this information has been repeated in secondary sources. However, a later review paper including some of the same authors, but in conjunction with European experts, did not list the species (Naggs et al. 2003). The explanation is that the original reports were erroneous (N.P.S. Kumburegama, pers. comm. 21.viii.13).

Australia

Altena and Smith (1975) were the first to identify D. invadens in Australia. The earliest collection date of the specimens they identified was 1967, although in 1964 specimens had been intercepted arriving in New Zealand on plants from Australia (Barker 1979). However, a sample in the Museum Victoria from near Melbourne was collected in 1936 (collection no. F174271; HR confirmed identity). Previous to 1975, the species had been misidentified as D. laeve and D. reticulatum. Because they had never encountered specimens of D. laeve from Australia, Altena and Smith (1975) suggested that Limax queenslandicus Hedley, 1888, which had been synonymised with D. laeve soon after its description, might have been D. invadens. Reise et al. (2011) argued strongly against this; they showed that D. laeve was present in Queensland in the nineteenth century and that the identification of L. queenslandicus as D. laeve was reliable.

Deroceras invadens has been reported widely from Australia (Altena and Smith 1975, Smith 1992a, Stanisic et al. 2010, Atlas of Living Australia http://bie.ala.org.au/species/Deroceras+panormitanum accessed 21.vii.13). However, we have borrowed some of the material in the Australian museums on which these records were based and found that much of it is misidentified D. laeve. As in North America, D. laeve in Australia often grows larger than in Europe, which might have misled originally. Altena and Smith (1975) did not encounter D. laeve, but did not dissect material from outside Victoria that they assumed to be D. invadens. The belief that D. laeve was rare in Australia and, for instance, did not occur at all in Queensland (Stanisic et al. 2010) must have inhibited routine dissection. The latest Australian guidebook gives general coloration and paleness around the pneumostome as characters to distinguish D. laeve and D. invadens (Stanisic et al. 2010); these are certainly not reliable characters in Europe and may well explain misidentifications in Australia.

We can confirm the occurrence of D. invadens in Victoria, New South Wales, Tasmania, South Australia and Western Australia (Table 2). There is no reliable evidence of D. invadens in Queensland. We have examined 14 lots covering a wide geographic range in Queensland from the New South Wales border as far north as 16° (10 labelled as D. panormitanum from the Queensland Museum, one similarly labelled from the Museum Victoria, and 3 others). All were D. laeve. The northernmost confirmed records of D. invadens in New South Wales are 30.4°S (Table 2).

Already in the 1975, D. invadens was described as “one of the commonest and most wide-spread of introduced slugs”, “a pest both of pasture plants and those of suburban gardens”, occurring “in only slightly disturbed native bushland as well as wholly modified habitats” (Altena and Smith 1975). More recent surveys found it to be the commonest introduced mollusc in native grassland in southeast Australia (Holland et al. 2007), and to occur also around the edges of ponds and swamps (Stanisic et al. 2010). It is found in some arable fields (Nash et al. 2007). The frequent misidentifications of D. laeve as D. invadens might have coloured these claims, but we can confirm its presence in gardens, agricultural habitats, and rough grassland.

See below for erroneous records from Lord Howe Island and Norfolk Island.

New Zealand

Barker (1979) published the first records of D. invadens from New Zealand, including specimens collected in 1974. Barker (1979, 1999) concluded that earlier widespread records of D. laeve must have referred to D. invadens, because he found that D. laeve had a restricted distribution in New Zealand. For instance, Suter (1913, p. 1071) described D. laeve as cosmopolitan, and already in 1891 Musson mentioned that D. laeve occurred. An occurrence at either of these dates would predate the first records of D. invadens anywhere in the world. The argument mirrors that for an early occurrence in Australia, which we criticised above, but we find it more compelling in the case of New Zealand: New Zealand has been more thoroughly surveyed, the distribution of D. laeve was more restricted at the time of these reliable surveys, and, unlike in Australia, there are no extant specimens of D. laeve collected before the late 1950s.

Barker (1999) described the habitat of D. invadens in New Zealand thus; “a slug of moist habitats found in gardens, parks, and pastures, on arable land, stream banks, and roadsides, and in greatly disturbed areas of native forest”. It is “frequently found in roadside margins adjacent to forest, often kilometres from any cultivated areas”, “ubiquitous where there is improved grassland and cultivated plants”, “often the most abundant species in humid glasshouses and wet field situations”, and “an important pest of cultivated plants” (Barker 1982, 1992). In one old pasture, 39% of slugs were D. invadens (= 14 m^–2, cf. 16 m^–2 D. reticulatum; Barker 1990). Although most often recorded in the Auckland area of North Island, D. invadens also occurs widely around the coast of South Island and on Stewart Island (Barker 1999).

See below for records from Raoul Island and the Chatham Islands.

United States of America

Pilsbry (1948) described D. invadens as widespread in the Bay Area of California (the environs of San Francisco) by 1940; habitats he listed were a park, nurseries, a lawn and a ranch. Subsequently it has been recognised further south, in Monterey County (Lange 1944, Pilsbry 1948) and in Los Angeles (Roth and Sadeghian 2003). It also occurs further north, in Oregon and Washington: Burke (2013) described it as common in urban and suburban areas of the Pacific Northwest from California to southern British Columbia and Pearce et al. (2013) described it as well established in Olympia and Tacoma (Washington State). Table 2 includes further records from this region.

With this firm foothold, one would expect the species to have become widely distributed in the USA, because some large horticultural firms grow plants in the benign climate of the Pacific Northwest and then ship nationally. However, compared with Europe, North America has far fewer experts able to identify slugs, especially those requiring dissection. Also problematic is that D. laeve in North America often grows larger than in Europe so that D. invadens is readily mistaken for it unless specimens are dissected (Reise et al. 2006). It is not a healthy sign that all the records of D. invadens from the USA outside of the west coast are our own!

In 1998 we found D. invadens in Washington DC, under bushes in a park, the first outdoor record in eastern North America (Reise et al. 2006). Our surveys of synanthropic habitats in Colorado and Utah in 2004 and 2006 respectively (JMCH and HR, unpublished; see Table 2) turned up D. invadens in several garden centres and in the watered plantings in city centres (e.g. Park City; Memory Grove Park, Salt Lake City: Reise et al. 2011), but also in unwatered parts of a city park (Cheesman Park, Denver), along the banks of a drainage ditch (university campus, Fort Collins) and beside a stream in a ski resort (Snowbird).

The only other records from the eastern USA of which we are aware are from two sites in Kentucky (specimens in Florida Museum of Natural History, catalogue numbers 43778 and 44718, details available via http://data.gbif.org/species/5190777 accessed 24.iii.14). However, we have dissected one animal from each sample and they were not D. invadens.

Canada

The first Canadian records are from greenhouses in two cities in Quebec Province in 1966 (Chichester and Getz 1969). This thorough survey of 770 sites (although only 25 greenhouses, nurseries or gardens) in the northeast of the USA and southeast of Canada failed to find the species elsewhere, whereas D. reticulatum and other European slugs were frequently encountered. The first outdoor records are from the other side of the continent on the UBC campus in Vancouver in 1974 (Rollo and Wellington 1975). Deroceras invadens is nowadays present at other sites in Greater Vancouver and around Victoria on Vancouver Island (Forsyth et al. 2001, Forsyth 2004, http://linnet.geog.ubc.ca/efauna/Atlas/Atlas.aspx?sciname=Deroceras%20invadens; Table 2). We also found it in 2013 in a garden centre in Kamloops, in the drier interior of British Columbia (Table 2). On the Atlantic coast, D. invadens was found in 2012 on rough ground adjacent to gardens in St John’s, Newfoundland (Forsyth 2014; confirmed by HR); the maritime influence there ameliorates winter temperatures.

Identifications of D. invadens from a garden in Edmonton, Alberta and from orchards by Osoyoos Lake, British Columbia, (Neckheim 2013, 2014) were not based on dissection (C.M. Neckheim pers. comm. 02.x.13), so should be regarded as unconfirmed; winters in Edmonton are more extreme than in any area where the species is known with certainty (see Discussion). A 1994 record from wild habitat near Kingston, Ontario (Grimm et al. 2009) is also best considered as unconfirmed because of other misidentifications by the collector (Forsyth 2013).

Mexico

We have confirmed the identification of specimens of D. invadens collected by A.S.H. Breure in 1974 at 3000 m in the Desierto de los Leones National Park, above Mexico City (Table 2). The gardens of the nearby convent might have been the original point of introduction but these slugs were collected in the surrounding pine and oak woodland (A.S.H. Breure 1974, pers. comm. 27.ix.13). At this altitude the climate is cool and damp. Although the specimens were correctly identified by Altena and deposited in the Naturalis Biodiversity Centre, Leiden, the record was not published so that D. invadens is missing from a recent national checklist (Thompson 2008).

Costa Rica

In 2006, we found ten specimens under rocks in a small wood near Tierra Blanca, Provincia Cartago (USDA 131032; Table 2). The altitude was 2060 m. The only Deroceras species listed from Costa Rica by Barrientos (2003) was D. laeve.

Panamá

In July 2007, a specimen of D. invadens was found on a leaf imported into the USA from Panamá (USDA 131034). In July 2009, three further specimens were found on Allium imported into the USA from Panamá (USDA 131033). Note that Panamá connects Costa Rica to Colombia, countries for which the presence of D. invadens has been confirmed on the ground.

Colombia

Two specimens of D. invadens in the Field Museum Chicago (JK-198690, identity confirmed by HR) were collected by the University of Oxford expedition to Colombia in September 1975. There are no further locality data but in this month the expedition was both near Nazaret (Guajira state) and in the capital Bogotá (Knappett et al. 1976).

Deroceras invadens was next found in 2000 at two rural sites near Bogotá in a garden and a flower plantation (Hausdorf 2002). It was considered a serious horticultural pest. Although the localities are less than 5°N, the altitude (> 2600 m) makes the climate oceanic (subtropical highland). Until Hausdorf’s visit, the species had apparently not been recognised by local researchers, so that it may be more widespread. However, D. invadens was not found at 6 of the 8 sites where Hausdorf’s survey found D. reticulatum (ratio = 0.25).

Cut flowers imported into the USA from Colombia in March 2008 contained D. invadens (USDA 131036).

Ecuador

In April 2012, L. Manangón collected a specimen “on alder” near Bolivar, Provincia Carchi (USDA 110614: Table 2). Although this is only 56 km north of the equator, the altitude of 2600 m meliorates the climate (cf. the localities in Colombia, 600 km away along the chain of the Andes). Earlier, in August 2004, an individual of D. invadens had been intercepted on cut flowers imported into the USA from Ecuador (USDA 131035).

Peru

The only Deroceras species listed from Peru by Ramírez et al. (2003) were D. laeve and D. reticulatum. However, in March 2012, an individual of D. invadens was intercepted on lettuce imported into the USA from Peru (USDA 110831). Peru connects Ecuador and Chile, countries for which the presence of D. invadens has been confirmed on the ground.

Chile

Letelier et al. (2003) reported D. invadens as present in southern Chile. We can add three later records of our own from central Chile in, and within 70 km of, the capital Santiago (Table 2). At these latitudes, the species occurs at low altitudes.

See below for a record from the Juan Fernández Islands. The presence of D. invadens there in 1962 suggests that it was probably present in mainland Chile by this time.

Argentina

Gutiérrez Gregoric et al. (2013) reported the occurrence of D. invadens in Argentina without providing the date of first discovery. However, the online catalogue of the Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’ (http://datos.sndb.mincyt.gob.ar/portal/occurrences/search.htm?c[0].s=0&c[0].p=0&c[0].o=Deroceras+panormitanum) indicates that much of the material was collected in 2010 and 2011 in well vegetated valleys at the eastern edge of the Andes around the shores of lake Nahuel Huapi (770 m a.s.l.), and 94 km south in El Bolsón (310 m a.s.l.). A further site is a little to the east on a cultivated estate in the arid Patagonian steppe. The map in Gutiérrez Gregoric et al. (2013) indicates a further locality near the city of Neuquen 300 km to the northeast of Nahuel Huapi. So the range in Argentina is sizeable and Gutiérrez Gregoric et al. (2013) stated that localities include both urban areas and national parks.

Brazil

Barker (1999) listed two samples of D. invadens that he examined, from a park in the city of Porto Alegre and from a state park 70 km outside the city, both collected in 1991. These determinations should be considered reliable because Barker was familiar with D. invadens in New Zealand. However, recent species lists from the same state (Agudo-Padrón 2009) and from all Brazil (Simone 2006, Agudo-Padrón and Lenhard 2010) do not mention this species. They do list D. laeve, which seems likely to have masked the presence of D. invadens from local malacologists.

Recently (27.ii.14) a specimen of D. invadens was intercepted arriving in the USA on cabbage from a ship’s stores that had been loaded in Brazil (USDA 140148).

Faroe Islands (Denmark)

This sizeable archipelago (1400 km², population 50,000) lies between Scotland and Iceland at 62°N, having a maritime subarctic climate. In 1970, McMillan (1972) found D. invadens to be frequent in waste ground to the west of the capital Tórshavn.

Madeira (Portugal)

The first record of D. invadens is from 1980 (Rähle 1992). Based on fieldwork from 1980 to 1986, Rähle (1992) concluded that it was widespread but rare and restricted to synanthropic sites on the main island. Seddon (2008) provided more records, but the species had still not been recorded from Porto Santo or other minor islands. Whereas Rähle knew it from 7 sites compared with 10 for D. reticulatum (ratio = 0.7), Seddon’s distribution maps show 21 dots for D. invadens and 11 for D. reticulatum. However, Seddon’s data may have confounded occurrences of D. invadens with those of Deroceras lombricoides (Morelet, 1845): Seddon has no records of the latter species, whereas Rähle found it to be the commonest Deroceras.

Rähle’s (1992) illustrations indicate that he found D. invadens rather than D. panormitanum s.s. We have also confirmed the identity of samples in the Stuttgart State Museum of Natural History from 6 localities (collected 1980–85) and of samples in the Naturalis Biodiversity Centre, Leiden, from a further 11 localities (collected 1987–88). However, we collected D. panormitanum s.s. from the south of Madeira in 2006 and again eight years later just 0.7 km away (USDA 131030, USDA 140106: Table 2); the localities lie <2.5 km from one where D. invadens was found in 1988.

Azores (Portugal)

Waldén (1960) mentions the first finding of D. invadens in the Azores by a 1957 expedition (see Backhuys 1975, p. 22). However, Backhuys’ (1975) team of collectors did not find this species during their extensive fieldwork in 1969, in contrast to numerous records for other slugs, including D. laeve (reported as always aphallic, so not a misidentification). Backhuys (1975) reported a single later finding in 1974 from a synanthropic habitat on São Miguel. In dramatic contrast, single-island surveys 13–20 years later showed the species to have become widespread (e.g., present at 5 out of 29 sampling stations on São Miguel in 1987, 8/21 stations on Flores in 1989, 5/17 stations on Santa Maria in 1990, 2/9 stations on São Jorge in 1992, 3/18 stations on Faial in 1993: de Winter 1988; Martins et al. 1990, 1991, 1993; Cunha et al. 1994). By 2010 the species was known from all of the nine main islands except Terceira (Cunha, Rodrigues and Martin 2010).

A slug intercepted arriving in the USA from the Azores on a taro root in March 2008 was D. panormitanum s.s. (USDA 110434). So we checked specimens collected by J. Wieringa from four sites on São Miguel in 1987 (Naturalis Biodiversity Centre, Leiden: collection numbers 329842–329845). They were D. invadens, as was another U.S. interception from the Azores (USDA 131029, Dec. 2008). Probably, as on Madeira, both species occur.

Canary Islands (Spain)

The first records of D. invadens are from the island of La Palma in 1947 (Altena 1950). In the 1980s Alonso et al. (1986) found it to be commoner there than D. reticulatum (12 vs 2 localities, 8 vs 1 5 km squares) and to range in altitude from 60 to 1800 m. On the island of Tenerife, Altena (1950) failed to find D. invadens in 1947 and extensive sampling in 1982–85 located it at only one site (Alonso et al. 1986), but by 2007 it was widespread in native laurel forests (Kappes et al. 2009). The species was also present on Gran Canaria by 1984 (M. Ibáñez, pers. comm. 16.ix.13, Table 2).

Tristan da Cunha (UK)

This lies in the middle of the South Atlantic (37.1°S), 2816 km from South Africa, with a population of under 300. The climate is temperate. Preece’s (2001) survey in 1982–83 found D. invadens not only on the main island of Tristan da Cunha but also on Inaccessible Island 40 km away. The latter had been inhabited rarely, and not since 1938 (Anon. 2011). Seabirds abound, so possibly they could have dispersed slugs from Tristan da Cunha.

Raoul Island (New Zealand)

This lies 29.3°S in the South Pacific, 1100 km NNE of New Zealand’s North Island. The climate is subtropical. Deroceras invadens was found in 1973 in forest litter (Barker 1999). At this time there was a small farm on the island as well as a meteorological station, although resupply was infrequent.

Chatham Islands (New Zealand)

These lie 44°S in the South Pacific, 680 km from New Zealand. The climate is temperate and they have a sizeable agricultural community and frequent transport links. Deroceras invadens was found in 1976 in pasture on the main island and nearby Pitt Island (Barker 1979).

Marion Island (South Africa)

This is a subantarctic island (46.9°S, 290 km²) with a cool oceanic climate, unpopulated except for research stations; South Africa lies 1730 km to the northwest. Deroceras invadens was first reported in 1972, under timber and in damp mossy habitat beside the base hut; a thorough survey in 1965–66 had not reported it (Smith 1992b). By 1976–77 it occurred in further habitats but had not spread far, and by the early 1990s it was much more abundant but still known only within several hundred metres of the base (Smith 1992b, Chown et al. 2002). Further dispersal around the rest of the island over inhospitable terrain is thought to have been facilitated by its habit of sitting under wooden boxes lying on the ground, which helicopters then carried to other huts (Chown et al. 2002). The species has now spread right around the island but the cold restricts it to land under about 200 m (Lee et al. 2009). It is most abundant near the coast.

Juan Fernández Islands (Chile)

In the Field Museum Chicago and the Museum of Natural History, Wrocław University are specimens of D. invadens collected in 1962 from Robinsón Crusoe Island, formerly Más a Tierra (identities confirmed by HR; Field Museum catalogue number = 198633). This Pacific island is 48 km² in area, and lies 600 km west of mainland Chile, with a mediterranean climate and a population in 1999 of over 500. The slugs were collected in a ravine in the Valle de Lord Anson, which rises from the main village.

Errors and Absences

Lord Howe Island and Norfolk Island are small (15 and 35 km²) but well populated Pacific islands, belonging to Australia although 570 and 1400 km east from the Australian mainland. Online records from the Australian Museum (http://ozcan.ala.org.au accessed 08.iii.2013) indicated that D. invadens was collected on Lord Howe Island in 2000 (single record) and on Norfolk Island in 1999 and 2002 (6 records, most from the largest patch of woodland but two from the opposite side of the island). We have borrowed the specimen from Lord Howe Island and two from Norfolk Island (one from each year); one from each island were D. laeve and the third specimen was not identifiable. Given the high rate of similar misidentifications of other Australian material (including from this museum; see above), we consider that there is no reliable evidence of D. invadens occurring on either island.

Specimens in the Natural History Museum London labelled as D. caruanae from São Tomé in the Gulf of Guinea (collected 1993 by A. Gascoigne from Lagoa Amelia and Tras-os-Montes; BMNH 19991797, 19991798) turned out to be D. laeve (dissection by HR); these records appear not to have been published or put online.

It may also be helpful to list some oceanic islands where D. invadens has not been found even though the climate might be suitable and recent surveys have been extensive and informed enough to have probably revealed the species were it well established: Iceland (Sumner 2007), Bermuda (Bieler and Slapcinsky 2000), Cape Verde Islands (Groh 2012), Mascarene Islands (Griffiths and Florens 2006), Hawaii (Cowie 1997, Hayes et al. 2012), Samoan Islands (http://pbs.bishopmuseum.org/samoasnail/query.asp, updated 2003), Pitcairn (Preece 1995), Rarotonga (Brook 2010), Fiji Islands (Brodie and Barker 2011), Easter Island (Boyko and Cordeiro 2001). Of these ten island groups, all but the Samoan Islands have been colonised by D. laeve, and five have D. reticulatum also.

In Europe much of the area occupied by D. invadens counts as temperate according to the Köppen climate scale (Fig. 4B). This holds also for much of its range outside of Europe. Further areas of the world having this climate category, such as the southern USA and much of China and Japan, would be worth checking further for the presence of D. invadens, and should consider themselves at risk of invasion. Moreover, the presence of D. invadens in such regions as Austria and Newfoundland demonstrates that it can exist in areas with somewhat colder winters than allowed under Köppen’s temperate categories (C), whereas its presence in eastern Spain shows that it can extend into drier climates classed as steppe (BS). Conversely, even around the Mediterranean, D. invadens does not occupy all the region classed as temperate. Inevitably, the criteria of the Köppen scale do not exactly match the critical factors defining the species’ niche.

Cold seems to be one critical factor. In the laboratory, D. invadens collected from Marion Island was unable to survive brief temperatures lower than –6.4 °C, or, on a longer time scale, temperatures lower than about –3 °C on average. This neatly explained its altitudinal range on that island (Lee et al. 2009). Moreover, the gradual decline in density with altitude is explicable by a gradual reduction in performance with decreasing temperatures (Lee et al. 2009). Analogously, the slowness to penetrate Eastern Europe perhaps has as much to do with the cold winters as the restrictions on east–west trade until 1990. Temperatures experienced by slugs in winter depend on snow cover and how deep the slugs burrow underground, but weather stations normally measure only air temperature. In Europe, the coldest locations for which D. invadens has been recorded outdoors are Ultuna near Uppsala in Sweden (winter minimum air temperature averaged over last 20 years = –22 °C; coldest over same period = –29 °C), Södra Fjöle in Sweden (–22 °C and –28 °C for these statistics), and Ostrava in the Czech Republic (–19 °C and –27 °C: Klein Tank et al. 2002, http://www.ecad.eu accessed 25.ix.13). For Wrocław, we know the species has survived the last 12 winters, in three of which air temperatures dropped to –22 °C. Figure 4C suggests that winter temperatures could well block further expansion eastwards from Wrocław or northwards from Uppsala, but there does seem an opportunity for further expansion elsewhere, for instance along the Baltic coast of Poland, into Hungary, or onto Iceland. Proschwitz (2010) has suggested that in Sweden the ameliorating climate is associated with the recent range extension, and the same process could occur in eastern Europe.

In North America, three sites in Colorado and Utah where D. invadens has been found away from garden centres and plantings of annual bedding plants (Table 2) reach temperatures similarly cold as the extremes in Europe: for Fort Collins, winter minimum temperature over last 20 years averaged –22 °C with a minimum of –28 °C; for both Denver and Snowbird these statistics are –22 °C and –27 °C (http://ccc.atmos.colostate.edu/sum_form.html; http://climate.usurf.usu.edu).

At the other extreme, too much heat is probably not a restriction per se, since the species occurs in Egypt, for instance. Meyer and Cowie (2010) proposed that the occasional extremes of a temperate climate preadapt some invasive molluscs to be able to cope with the high temperatures of the tropics. However, heat may be associated with drought, at least seasonally. In such regions the automatic watering systems in use in intensive agriculture and in horticulture (particularly prevalent in prosperous countries) facilitate the survival and spread of D. invadens. Maybe these artificial habitats will provide a route for D. invadens to colonise naturally damp areas within steppe habitats.

In Central and South America, D. invadens occurs within the tropics but the accurately localised records are all from higher altitudes. This may be because higher altitudes tend to be cooler and have different precipitation patterns, or because such climatic differences have encouraged urbanisation or types of agriculture that favour the species. Higher altitudes would seem the likeliest place to search for D. invadens in other areas of the tropics, including Africa and India.

Deroceras invadens is typically associated with disturbed habitats, especially gardens, and is often easiest to find under discarded rubbish. It is one of the few molluscs to occur in the most urban sites, by surviving in the soil of flowerpots (Horsák et al. 2013b). Greenhouses and consistently watered nurseries suit it, and it occurs in some arable fields (e.g. England, N Spain, Switzerland, Australia), but is a significant pest more rarely than D. reticulatum. Although reported to avoid grassland in Britain (Dirzo 1980), D. invadens occupies that habitat in New Zealand and Australia (Barker 1982, Holland et al. 2007). Outside of the probable natural range of the species (Italy), the species has spread to natural habitats not only in western Europe (e.g. Britain, Ireland and France, although not in Germany) but also in Australia, South Africa and the Canary Islands. It seems likely that dispersal into gardens is often rapid, but that the spread into and across agricultural and natural habitats is a slower process and not inevitable. This could explain the positive association with population density even in long colonised countries such as Britain, Ireland and Belgium.

Deroceras invadens can become one of the most frequently encountered slugs, typically about half as frequent as D. reticulatum, but at some sites even commoner (e.g. Manchester gardens, British commercial greenhouses, Frankfurt am Main, Egypt). In the laboratory we have observed that D. invadens can mature a month faster than D. reticulatum, which may give it an advantage when growing seasons are short owing to either climate or agricultural activities.

Deroceras invadens is widespread over most of the western half of Europe. However, there are still areas within this region where it is scarce. In some cases this is probably because of climate (cold in Scandinavia, summer drought in central Spain), in other areas it might merely be because of a lack of time to spread there (e.g. Suffolk, Alsace). The species has yet to spread far in eastern Europe and is still much more thinly spread in Germany and Austria than in Belgium or Great Britain, for instance. Currently the most eastern outdoor records in Europe are the Baltic coast of Sweden, Wrocław in Poland and Ostrava in the Czech Republic; further east, at least in Lithuania, Latvia and the Ukraine, there is sufficient current interest in slugs that D. invadens would probably have been recorded had it become well established (Skujienė 2002, Gural-Sverlova et al. 2009, Rudzīte et al. 2010). The species also appears largely absent from the eastern Mediterranean (except for one record from Crete and some from along the Nile in Egypt in agricultural habitats). The Balkans and Turkey are a centre of diversity of Deroceras, so maybe competition with local species is a factor limiting the spread here. But this diversity also makes it harder for malacologists to spot a new arrival. The absence of D. invadens from Asia is striking, but have malacologists there been looking critically at their alien Deroceras species?

The species has long been known from the Pacific Northwest of America, and also occurs sporadically elsewhere in the USA and Canada. The pronounced scarcity of records in the east compared with the west is untypical of other introduced European slugs. The reason could be that much of the east has an unsuitable climate for D. invadens; perhaps in areas southern enough to for the winters not to be too cold, the summers are too hot and dry (Fig. 4B). But surely there is enough artificial irrigation often to overcome this restriction. Or, supposing that D. invadens has spread to America much later than most other introduced slugs, perhaps its distribution remains more biased towards the original site of introduction. However, the pattern could also be an artefact: our hunch is that D. invadens is at least somewhat more widely spread in the east than currently recognised, but has been frequently misidentified as D. laeve. This confusion had no doubt hidden its occurrence in Central and South America; the records assembled here are sparse but imply that D. invadens is widespread.

Elsewhere in the southern hemisphere, the species has been present, maybe for a long time, in the former British colonies of South Africa, Australia and New Zealand. In this context, the indication from interceptions that it may be present in Kenya is not surprising. It has also colonised a number of remote oceanic islands; the maritime influence on their climates is probably favourable, and perhaps also their depauperate faunas have left a niche vacant.

The range of D. invadens is impressive (Fig. 4), and current records probably still significantly underestimate it. However, it should be recognised that there are a number of other European slugs that are ahead of it in the extent of their non-native range. Besides D. laeve and D. reticulatum, one can list at least Lehmannia valentiana (Müller, 1774), Limacus flavus (Linnaeus, 1758), and Milax gagates (Draparnaud, 1801) (Herbert 2010). Although these species have spread at different rates, the process is continuing, so it remains to be seen whether the ranges that they eventually occupy will differ significantly.

The similar species D. panormitanum s.s. from Sicily and Malta has also been introduced elsewhere, but much more rarely than D. invadens: the only such records are from one site in northern Italy, one in Wales, two adjacent sites on Madeira and an interception from the Azores (Tables 1, 2). Although both species are probably native on Sicily, we never collected them together there (JMCH & HR unpublished), so the species might tend to exclude each other. We did collect them together from the same culvert in northern Italy (Table 2), but two subsequent visits to the site yielded only D. panormitanum s.s.

Deroceras invadens has been directly observed arriving from abroad on salads, vegetables, flowers, roots, and tiles (Falkner 1979 and USDA records). Such vectors need not be representative of the processes responsible for spread within a country, which surely must often be via garden plants, considering how frequently the species has been found in nurseries and garden centres. At both scales it seems unnecessary to invoke dispersal on bird’s feet or plumage, for which there is conspicuously little direct evidence for any slug (Pearce et al. 2012); birds are a parsimonious explanation only perhaps for colonisation of uninhabited islands (e.g. to Inaccessible Island from Tristan da Cunha).

We hoped that our review of the literature would illuminate the rate and pattern of these dispersal processes, but mostly it is hard to be sure that the apparent rate of spread is actually not the spread of awareness that this novel species is worth distinguishing from others. That is particularly a problem with a slug species that requires dissection for reliable identification. In several cases (e.g. Britain, France, New Zealand) the species was probably widespread before anyone was aware of its presence; presumably at a more local level the distribution continued to grow denser, but usually there are no follow-up surveys once someone has claimed the first record. What is really required is an initial survey reporting absences of the species, then comparable repeat surveys of the same places in subsequent years; this has rarely, if ever, been done.

There is nevertheless good evidence of a spread within one or two decades through the Azores and Tenerife. The German data are also probably reliable and representative in suggesting a time scale of one to two decades to extend over a larger country, but it is far from the case that every suitable garden or even district has been colonised within that time. Puzzling gaps in the present distribution elsewhere (e.g. Suffolk in England, Alsace in France) suggest that “filling in” can take decades longer. It is difficult to make quantitative comparisons between species, especially because the delay in spotting a new arrival depends on the ease of recognising the species, but D. invadens probably has spread a little slower than three other terrestrial molluscs that have also invaded much of Europe within the last century, the slugs Boettgerilla pallens Simroth, 1912 and Arion vulgaris (Moquin-Tandon, 1855) and the snail Hygromia cinctella (Draparnaud, 1801) (Reise et al. 2000, Beckmann and Kobialka 2008, Kozłowski and Kozłowski 2011). Quite probably when D. invadens first arrived in England and France, the more local pattern of trade at that time led to a slower spread than has occurred in more recently colonised European countries, but the quality of the data is insufficient to test this. One also expects the spread to be slower in countries with a hostile climate, which might explain the dearth of additional records in Poland, Slovakia and the Czech Republic. Similarly, the rate of spread in Sweden may reflect recent climate amelioration as much as the dispersal process (Proschwitz 2002). Marion Island is a special case, but is revealing in demonstrating the reliance on man for long-distance dispersal within the island, in contrast to the slow initial penetration of the natural habitat by natural means. Here and elsewhere, it is an interesting open question whether the spread accelerated following a period of genetic adaptation to the local environment. The climatic diversity of its non-native range would make D. invadens an appealing subject on which to test whether such adaptations have evolved.

One would expect uniparental reproduction to facilitate colonisation if adventitious human-mediated transport sometimes introduces a single slug at a time. Foltz et al. (1984) observed that self-fertilising slug species had been significantly more successful in colonising eastern North America, with D. invadens one of the species fitting this pattern (outcrossing and absent). Subsequently D. invadens has been found in eastern North America, but paternity studies in our laboratory (Reise et al. unpublished) have shown that the species does sometimes self-fertilise, producing viable offspring, especially in the absence of a partner. Nevertheless, the intra-site polymorphisms that Foltz et al. (1984) observed in populations from the British Isles and France do not suggest that the species’ spread through these regions was dependent on a succession of founder events involving self-fertilisation. In contrast, on the remote Marion Island, all 25 slugs sequenced had the same COI haplotype (Lee et al. 2009). In ongoing work, we are comparing patterns of genetic diversity elsewhere in its range.