Invasions of alien plant species have become widely recognized as one of the major human-induced changes, affecting the whole biosphere at the global scale (Tilman 1999, Manchester and Bullock 2000). In the last decade, the attention has been focused on testing, documenting and quantifying the impacts of alien invasive species upon the resident communities (Mack et al. 2000, Davis 2003, Hejda et al. 2009, Vilà et al. 2010).

It has been documented that the character of the recipient community co-determines the magnitude of the alien´s impact on diversity and, therefore, a single invasive species can have different impacts in different types of invaded communities (Mason and French 2008). Hejda et al. (2009) suggested that the impact on diversity of native species is especially strong, if the invader represents a novel and distinctive dominant to the community that has been lacking such dominants before the invasion. Some alien species, like Impatiens glandulifera or Helianthus tuberosus, have been documented to compete with native dominants without actually changing the site conditions for other native species, which results in relatively mild impacts on diversity, especially when considering these aliens´ robust stature and tendency to grow in high densities. On the contrary, other invaders (Fallopia sp. div., Heracleum mantegazzianum, Rumex alpinus) have been documented to represent distinctive and novel dominants to the invaded communities and also to impact both diversity and composition of native species substantially (Hejda et al. 2009).

Aliens can alter conditions on large areas, such as when a tree species invades a formally treeless environment, as documented on Galápagos by Järger et al. (2007). At the same time, invasive aliens are often strong competitors and are able to exclude native species at fine scales, due to e. g. competition by roots, allellopathic compounds or by production of large amounts of above-ground biomass.

This paper aims to test the ability of native species to coexist with the dominant alien Lupinus polyphyllus and attempts to test the following hypotheses:

Do species of native and invaded ranges differ in their ability to thrive on sites with high dominance of Lupinus polyphyllus?

Do native species of different invaded ranges (Czech Republic, central Europe; New Zealand) differ in their ability to coexist with the dominant invasive alien Lupinus polyphyllus?

In all of the three ranges (USA, Czech Republic, New Zealand), I sampled plots with the cover of Lupinus polyphyllus of up to 90% (Appendix II).

In the native range (Pacific NW USA), I recorded 112 native species and 52 aliens of European origin exclusively in the vegetation with Lupinus polyphyllus. In the Czech Republic, 120 native species were recored in the vegetation invaded by Lupinus polyphyllus along with 6 aliens, with origins in Europe or SW Asia. In New Zealand, I recorded only 33 native species within the stands of Lupinus polyphyllus, but also 52 alien species, exclusively of european origin (Appendix I and II).

In the Czech Republic, native species´ richness responded to the cover of Lupinus polyphyllus negativelly (r = -0.294 – Table 1, Fig. 1), but the relationship was only marginally significant (p = 0.065 – Table 1, Fig. 1). In New Zealand, native species responded to the cover of Lupinus polyphyllus negativelly (r = -0.757, p < 0.001 Table 1, Fig. 1). Species of european origin growing as aliens in New Zealand did not respond to the cover of Lupinus polyphyllus (r = -0.160, p = 0.324 – Table 1), nor did species native to New Zealand respond to the cover of other invaders of the target communities (Hieracium pillosella agg. – r = -0.104, p = 0.523, Anthoxantum odoratum – r = 0.070, p = 0.666 – Table 1). I detected no relation between species richness and the cover of Lupinus polyphyllus in its native range, Pacific Northwest, USA (r = 0.308, p = 0.163 – Table 1).

The response of species richness to the cover of Lupinus polyphyllus differed between the native and invaded ranges (p < 0.001, Chi = 32.15, DF = 2 / 94). In New Zealand, the ratios between the native / alien (of european origin) species richness responded to the cover of Lupinus polyphyllus negativelly (p = < 0.001, Chi = 23.547, DF = 1 / 36), indicating that alien species were more successful in the heavily invaded stands compared to native species.

The cover of Lupinus polyphyllus proved to be a significant predictor of species composition in all ranges (USA: p = 0.0220; Czech Republic: p = 0.0460; New Zealand: p = 0.0200 – Table 2).

In the USA (native range – Fig. 3) and Czech Republic (Fig. 4), some native species revealed negative response to the cover of Lupinus polyphyllus, while others were more abundant in plots with high cover of it. Contrary to this, all species native to New Zealand (Fig. 5) revealed negative response to the cover of the alien Lupinus polyphyllus, with the exception of Muehlenbeckia axillaris, which actually revealed a slight preference for the invaded stands.

All these results need to be interpreted with caution, mainly given by the comparative way the data were sampled. It is possible that the factor of the alien´s dominance is confounded with other environmental factors, such as anthropogenic disturbance or increased nutrient levels, which may suppress native species and enhance the alien´s dominance. It is not really possible to say if the invasion (expressed as the alien´s dominance on a given small spatial scale in this study) is promoted by these changes, or if the alien species transforms the sites actively. Alien invasive species have been documented to change site conditions massively, mainly due to substrate stabilisation (Lupinus polyphyllus – Holdaway and Sparrow 2006), litter accumulation (Fraxinus uhdei - Rothstein et al. 2004), nutrient uptake efficiency (Acacia saligna – Odat et al. 2011), water uptake efficiency (Tamarix sp. div. – Di Tomaso 1998) or light deficiency (Cinchona pubescens – Järger et al. 2007). Therefore, it is difficult to say in which way the causality between the occurrence of aliens and altered site conditions goes – does Lupinus polyphyllus alter the site conditions by itself, due to nitrification, substrate stabilisation and limited insolation, or does it just benefit from human-induced ruderalization? Both of these mechanisms are likely to work in concert, and either of them can prevail in a particular situation. This makes it very difficult to find a general answer to the question of the direction of causality between the invasions by alien plants and changes of site conditions, leading to changes in diversity and composition. The period a particular site has been invaded brings another interpretation difficulty – milder impact of the alien can be caused by the site´s having been invaded recently and vice versa. On the other hand, this factor can be presumed to play only a marginal role in regularly disturbed communities, where the succession series is blocked by flooding, grazing or mowing.

Be that as it may, the results show that in the invaded ranges, the dominance of Lupinus polyphyllus is associated with site conditions that do not favor native species, which is especially apparent in New Zealand. Contrary to that, species from the native range were successful when growing with Lupinus polyphyllus, more than species from both invaded ranges (Czech Republic and New Zealand). This is likely to be due to these species´ being well apapted to the conditions of sites distinctively dominated by Lupinus polyphyllus, which may be caused by the long-term coevolution of communities and species´ filtering in the dominant species´ native range. Due to the long-term presence of the dominant species Lupinus polyphyllus, species in the native range have been selected to coexist with it, otherwise they would have been eliminated from communities where Lupinus polyphyllus is a dominant species.

Contrary to that, Lupinus polyphyllus is a newly imported dominant species to the communities in the invaded ranges. In Europe, the invasion of Lupinus polyphyllus is associated with some loss of native species richness (see for example Valtonen et al. 2006), but around 70% of present species are capable of growing in dense stands of this alien, as documented earlier by Hejda et al. (2009). Obviously, most of the species in the invaded communities in the Czech Republic are strong competitors that are not easilly supressed by the alien Lupinus polyphyllus and majority of them can thrive even in the stands with a high cover of Lupinus polyphyllus. This ability may have evolved as a result of long-term presence of distinctive dominant species native to the Czech Republic, such as Aegopodium podagraria, Cirsium heterophyllum, Dactylis glomerata, Senecio hercynicus or Trisetum flavescens (see primary data in Appendix II). In other words, the long-term evolution of these communities have resulted in a state, when only species that successfuly compete with native dominants are present. In Europe, many native species utilize a similar niche as invasive aliens and prefer human disturbed places, or, on the contrary, places where the regular disturbance or management regime has ceased (Hobbs and Huenneke 1992, Davis and Pelsor 2001). In these communities, the competitive effect of the alien Lupinus polyphyllus may not differ substantially from competitive effects of native dominants.

Species native to New Zealand were found to be least successful when growing in the stands of Lupinus polyphyllus in this study, with the most invaded plots (with the cover of Lupinus polyphyllus of 90%) being almost free of species native to New Zealand. This can be partly related to the fact that, in New Zealand, Lupinus polyphyllus often invades relatively unstable and regularly disturbed riparian terraces where the vegetation is not really dense, so the level of interspecific competition can be expected to be low. Therefore, heliophilous species of these communities (Coriaria plumosa, Epilobium melanocaulon, Parahebe decora, Raoullia subsericera, Raoullia hookeri, Wahlenbergia albomarginata) are weak competitors when confronted with the distinctive alien dominant and this can be caused mainly by the differences in the type of invaded habitats between these two distinct invaded ranges (Czech Republic and New Zealand). On the other hand, Lupinus polyphyllus also invades more stable and less frequently disturbed riparian meadows in New Zealand, with species like Acaena inermis, Carex geminata, Gonocarpus aggregatus or Prasophyllum colensoi on wet places and Brachyglottis bellidioides, Celmisia gracillenta, Discaria toumatou or Leucopogon fraseri on dryer sites. In these communities, the vegetation is dense and the level of interspecific competition can be expected to be rather high, but native species still fail to coexist with the dominant invasive alien Lupinus polyphyllus. It is possible that the intensity of interspecific competition is generally lower in the communities of New Zealand and native species are weaker competitors due to, for example, the effects of insularity, which means a long-term isolated development and not having been confronted with competitively strong species with cosmopolitan tendencies. At the same time, the insular flora of New Zealand can be expected to be phylogenetically rather homogenous, originating from a few clades originally colonizing the islands. Such communities have been documented to be more easily invaded (Gerhold et al. 2011). The habitat-based and geography-based explanations of the low abilities of species native to New Zealand to thrive within the stands of Lupinus polyphyllus are definitelly not exclusive and it is very likely that they work together. The only species native to New Zealand that revealed a positive relation to the cover of the alien Lupinus polyphyllus was Muehlenbeckia axillaris, as shown by the multivariate ordination analysis – Fig. 5.

In New Zealand, Lupinus polyphyllus has been documented to accumulate silt material and therefore accelerate the stabilization of riparian terraces (Holdaway and Sparrow 2006). This effect may facilitate the site for some species, a similar pattern was observed in the native range, where a related species Lupinus lepidus was observed to accelerate the succession by remediating site conditions on pumice fields, which are unstable and experience periods of thermic and hydric stress during the vegetation season (Del Moral and Rozzell 2005). A similar principle can be expected to work on the unstable riparian terraces in New Zealand, but this process is more likely to promote aliens of European origin rather than competitivelly weak species native to New Zealand.

The target communities in New Zealand were also heavilly invaded by other aliens, like Anthoxantum odoratum and Hieracium pilosella agg., so the potential impact of the invasive alien Lupinus polyphyllus was heavily confounded with possible impacts of other invasive species. Anthoxantum odoratum forms dense and homogenous stands, while Hieracium pilosella agg. forms dense 'pillows' of leaf rosettes. But richness of species native to New Zealand was not found to be related to the cover of either of these aliens of european origin, when sampled and tested in the same way as the relation to the cover of Lupinus polyphyllus. Some native species, like Brachyglottis bellidioides, Coprosma atropurpurea or Celmisia gracillenta were actually found to prefer places with large covers of Hieracium pilosella´s rosettes. For these reasons, it is highly likely that the site conditions that depauperate the diversity of communities in New Zealand are associated with the invasion by Lupinus polyphyllus, rather than with other abundant aliens, such as Anthoxantum odoratum and Hieracium pilosella agg.

The data shows that species in the native range are able to coexist with the dominant lupins better than species from the invaded ranges. Out of the two invaded ranges studied, species native to New Zealand were found to be most effectively eliminated from communities dominated by the alien Lupinus polyphyllus. An uncertainty remains whether this effect is caused by the invading Lupinus polyphyllus or by other environmental factors that promote the invasion, such as human induced disturbance, nitrification or substrate stabilisation, however, some of these changes can be promoted by the invasion by Lupinus polyphyllus too. Even though it is difficult to separate causes and effects of the invasion in this case (as it is with most invasions), the results show that different native species respond differently to the invasion by a single alien. In one invaded range – Europe, most species are able to coexist with the invasive Lupinus polyphyllus, while in New Zealand, native species are virtually eliminated from stands with a high cover of the dominant alien Lupinus polyphyllus. It remains questionable to which degree the results scale up from tiny vegetation plots to larger units in New Zealand. The results show that at the fine scale, the invasion is associated with a severe degradation of communities, so it is likely that its potential impacts are apparent even at larger scales, due to, for example, reduction of populations of native species. In the extreme cases, this can lead to local extinctions in areas with large stands of Lupinus polyphyllus, such as the valley of Waimakariri river in the Arthur Pass National Park, NZ. Moreover, this alien invades pristine areas with many rare species and its invasion therefore represents a serious threat to native plant diversity at the fine scale and a threat to landscape character at larger scales. High local abundances observed in the invaded ranges suggest that Lupinus polyphyllus has the potential to spread further, well beyond the boundaries of its current distribution.