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We examined temporal introduction patterns of 132 invasive alien plant species (IAPS) to Australia since European colonisation in 1770. Introductions of IAPS were high during 1810–1820 (10 species), 1840–1880 (51 species, 38 of these between 1840 and 1860) and 1930–1940 (9 species). Conspicuously few introductions occurred during 10-year periods directly preceding each introduction peak. Peaks during early European settlement (1810–1820) and human range expansion across the continent (1840-1860) both coincided with considerable growth in Australia’s human population. We suggest that population growth during these times increased the likelihood of introduced plant species becoming invasive as a result of increased colonization and propagule pressure. Deliberate introductions of IAPS (104 species) far outnumbered accidental introductions (28 species) and were particularly prominent during early settlement. Cosmopolitan IAPS (25 species) and those native solely to South America (53 species), Africa (27 species) and Asia (19 species) have been introduced deliberately and accidentally to Australia across a broad period of time. A small number of IAPS, native solely to Europe (5 species) and North America (2 species), were all introduced to Australia prior to 1880. These contrasting findings for native range suggest some role for habitat matching, with similar environmental conditions in Australia potentially driving the proliferation of IAPS native to southern-hemisphere regions. Shrub, tree and vine species dominated IAPS introduced prior to 1840, with no grasses or forbs introduced during early colonisation. Since 1840, all five growth forms have been introduced deliberately and accidentally in relatively large numbers across a broad period of time. In particular, a large number of grass and forb IAPS were deliberately introduced between 1840 and 1860, most likely a direct result of the introduction of legislation promoting intensive agriculture across large areas of the continent. Since the 1980s, only three IAPS have been introduced (all deliberately introduced forbs). The decline in IAPS introductions is most likely a reflection of both increased surveillance and biosecurity efforts and the likelihood that many potential IAPS are still within a pre-expansion lag period.
Biological invasions, exotic plants, introduction history, invasion ecology, residence time
Comparative analyses of alien plant introductions can pinpoint periods in history when different regions of the world have received particularly high numbers of species (e.g.
Detailed knowledge of alien floras in Europe has led to deep insights into temporal patterns of plant introductions across much of the northern hemisphere (
Permanent settlement of Australia by European colonists began in earnest in 1770. Since then, a total of 26, 242 alien plant species have been introduced to Australia, with 2, 739 of these species having become naturalized and over 130 now considered IAPS (
In this study, we compared temporal introduction patterns among IAPS introduced to Australia since European settlement. We related among-species variation in introduction times to the method of introduction of species, their native range and plant growth form. Our aim was to provide a detailed picture of when IAPS with different traits were introduced to Australia and to interpret our findings in the context of drivers of biological invasions and historical events during the development of Australia as a nation.
MethodsWe constructed a dataset of 132 IAPS in Australia using the latest compendium of the introduced flora of Australia (
We accessed the ‘Australian Census of Cultivated Plants 2009’ (Department of Agriculture, Fisheries and Forestry) to obtain the earliest year each of the study species has been recorded in Australia. The Census contains information gathered from public domain sources including over 600 plant nursery catalogues spanning 200 years, botanical and major garden plant species lists, Australian Quarantine and Inspection Service (AQIS) permitted import list, State department vegetation surveys and Commonwealth lists of imported species (R. Ingram, personal communication). We refer to the earliest year on record for each species as year of introduction. Given the high profile of the invasive species in our dataset, the extensive research, information and first-hand knowledge available in the literature for these species, we are confident that earliest records are tightly linked to actual introduction years.
We performed a literature search involving published papers, reports and government websites seeking information on the IAPS covering method of introduction to Australia, native geographic range and plant growth form. Method of introduction was recorded as either deliberate (e.g. ornamentals, forage or grafting plants, lawn species) or accidental (no known purposeful reason for introduction). The native range of each species was categorized as a single geographic region (Africa, Asia, Europe, North America, South America) or as cosmopolitan (i.e. the native range covered more than a single region). Each of the IAPS was placed into one of the following plant growth form categories: (1) Grass – monocotyledonous graminoids with narrow leaves growing from the base including Poaceae (Gramineae), Cyperaceae and Juncaceae; (2) Forb – self-supporting plants that have no woody stem above ground, with leaves and stems that may die down at the end of the growing season to the soil level; (3) Shrub – self-supporting plants that can produce a woody stem aboveground less than 6 m tall; (4) Tree – self-supporting plants with a woody stem more than 6 m tall; and (5) Vine – herbaceous or woody plants that climb or spread vertically or horizontally. Vines are usually considered as plants that must rely on external support to attain height. However, we classified other similar plants such as scramblers and rambling shrubs (which do not always require additional structural support) as one functional group (vines), consistent with Australia’s first exotic vine inventory (
Numbers of IAPS introduced to Australia have varied substantially over the years since European settlement (Fig. 1). Introduction numbers were comparatively high from 1810 to 1820 (10 species), 1840 to 1880 (51 species) and 1930 to 1940 (9 species). The 1840 to 1880 period was characterised by an especially high number of introductions between 1840 and 1860 (38 species). In contrast, low introduction numbers occurred during 10-year periods directly preceding each of the introduction peaks, with three IAPS introduced between 1800 and 1810, none introduced between 1830 and 1840 and one introduced between 1920 and 1930. Notably, only three IAPS have been introduced to Australia since 1980.
Frequency distribution of introduction times of invasive alien plant species in Australia in relation to method of introduction (accidental or deliberate).
Deliberate introductions of IAPS (104 species) began in 1770 and have continued to the present day (Fig. 1), far outnumbering accidental introductions (28 species). Indeed, deliberate introductions were particularly prominent during early settlement with no IAPS introduced accidentally either prior to 1840, or in fact since 1970. Generally, species native to all geographic regions have been introduced deliberately (Fig. 2a) and accidentally (Fig. 2b) to Australia across a broad period of time, with no particular region dominating any of the introduction peaks. The native ranges of the majority of IAPS extend over a single geographic region, with most species native to South America (53 species), Africa (27 species) and Asia (19 species). Interestingly, only five species were found to be native solely to Europe and only two species native solely to North America, and all seven were introduced prior to 1880. A total of 25 IAPS were found to be cosmopolitan (Table 1), all introduced across a broad period of time (Fig. 2).
Frequency distributions of a deliberate and b accidental introduction times of invasive alien plant species in Australia in relation to native geographic range. Rubus fruticosus is not included in (a) as it is an aggregate species consisting of 14 closely-related species (some of which may be hybrids) from a range of different regions.
Cosmopolitan species, their method of introduction and native geographic ranges.
Species | Introduction | Native range |
Acaciella angustissima | Deliberate | North America, South America |
Andropogon virginicus | Accidental | North America, South America |
Annona glabra | Deliberate | Africa, North America, South America |
Arundo donax | Deliberate | Asia, Europe |
Baccharis halimifolia | Deliberate | North America, South America |
Cardiospermum grandiflorum | Deliberate | Africa, North America, South America |
Cenchrus ciliaris | Accidental | Africa, Asia |
Gloriosa superba | Deliberate | Africa, Asia |
Hydrocotyle bonariensis | Accidental | Africa, North America, South America |
Ilex aquifolium | Deliberate | Africa, Asia, Europe |
Ipomoea cairica | Deliberate | Africa, Asia |
Juncus articulatus | Accidental | Africa, Asia, Europe, North America |
Macroptilium atropurpureum | Accidental | North America, South America |
Neonotonia wightii | Deliberate | Africa, Asia |
Olea europaea | Deliberate | Africa, Asia, Europe |
Opuntia imbricata | Deliberate | North America, South America |
Parietaria judaica | Accidental | Africa, Asia, Europe |
Parkinsonia aculeata | Deliberate | North America, South America |
Parthenium hysterophorus | Accidental | North America, South America |
Phyla canescens | Deliberate | North America, South America |
Prunus cerasus | Accidental | Asia, Europe |
Ricinus communis | Deliberate | Africa, Asia, Europe |
Salvia coccinea | Deliberate | North America, South America |
Tamarix aphylla | Deliberate | Africa, Asia |
Verbesina encelioides | Accidental | North America, South America |
Shrub, tree and vine species dominated IAPS introduced (all deliberately) prior to 1840, with no grasses or forbs introduced during early colonisation (Fig. 3). Since 1840, all five growth forms have been introduced deliberately (Fig. 3a) and accidentally (Fig. 3b) in relatively large numbers across a broad period of time. One particular growth-form pattern that emerged showed that a comparatively large number of grasses and forbs were deliberately introduced between 1840 and 1880 and since then (up until 1970) accidental introductions of grasses and forbs have been especially prominent. The small number of IAPS introduced since 1980 have all been forbs.
Frequency distributions of a deliberate and b accidental introduction times of invasive alien plant species in Australia in relation to plant growth form.
We identified three distinct periods in Australia’s recent history when introductions of IAPS were particularly high. We describe a simple null model that relates increased introductions of IAPS to increases in both ‘colonization pressure’ and ‘propagule pressure’ (sensu
For Australia, as is common in many retrospective studies of invasion (
Support for a link between human population size and the introduction of IAPS in Australia might be obtained if substantial increases in Australia’s population coincided with increased introductions of IAPS. We accessed data from the Australian Bureau of Statistics, specifically the database ‘Australian Historical Population Statistics 2008’ showing changes in Australia’s population over time (http://www.abs.gov.au, accessed February 2012). We compared increases in both IAPS and Australia’s population during both peaks, and importantly, during periods of low introductions of IAPS directly prior to the peaks. We found that peaks during early European colonization (1810–1820) and human range expansion across the continent (1840-1860) both coincided with considerable growth in Australia’s human population (Fig. 4). The introduction lows in the preceding periods were associated with comparatively smaller increases in Australia’s population. These correlated events provide support for the idea that human population increase could be a substantial driver of increased introductions of IAPS via increased colonization and propagule pressure.
Increases in IAPS (numbers of species above arrows) and Australia’s human population (number of people below arrows) in relation to time (years shown between arrows) during a early European colonization b human range expansion and c pre-World War II.
Interestingly, unlike the first two peaks in IAPS, the third peak (1930-1940) did not coincide with a marked increase in Australia’s population (Fig. 4). During the preceding period (1920-1930), there was a much larger increase in Australia’s population (c. 1 million people) but only one IAPS was introduced. Nevertheless, Australia’s population still increased by half-a-million people between 1930 and 1940, which could still explain to some extent the peak in IAPS. The significant low in IAPS introductions between 1920 and 1930 might be a result of economic difficulties during the Great Depression during the 1920s. At such a time, costs associated with importing species might have been avoided. It is also likely that during such financially challenging times that hobbies such as gardening would not have been a high priority, potentially reducing the influx of ornamental plants. Perhaps the subsequent peak in introductions may be accounted for by more targeted introductions of species during pre-World War II immigration to Australia during the 1930s. A goal of future work will be to unravel the pathways of introduction of IAPS, particularly during the 1930-1940 peak, to determine why IAPS introductions peaked during a time when Australia’s population showed a comparatively smaller increase.
What explains the predominance of IAPS in Australia that are native to the southern hemisphere? This finding suggests that invaders from other continents might possess a degree of climatic pre-adaptation or habitat matching that facilitates invasiveness (e.g. see
Our work has shown that most IAPS were deliberately introduced to Australia. This is not surprising, with previous studies documenting the importance of deliberate introductions in the increase in alien plant species in Australia (
No grass or forb IAPS were introduced during early colonisation but a comparatively large number of these were deliberately introduced between 1840 and 1860. The later introduction of forb and grass IAPS is associated with a period of time in Australia’s history when land ‘selection’ became prominent. Selection allowed settlers to have free choice of government land in some Australian colonies under land legislation acts introduced in the 1860s (
Since the 1980s, only three IAPS have been introduced (all deliberately introduced forbs). The decline in IAPS introductions is most likely due to two factors. First, increased surveillance and biosecurity efforts have been successful in controlling the influx of alien species likely to become IAPS. For example, accidental introductions are much less likely due to seed cleaning techniques and quarantine services ensuring minimal contamination (
We thank the members of the Biodiversity Research Group at UTS, Carl Arbegast, Petr Pyšek and an anonymous reviewer for providing helpful comments on a draft of the paper. We are grateful to Robert Ingram of the Department of Agriculture, Fisheries and Forestry for kindly providing access to the Census of Cultivated Plants 2009. This paper is dedicated to the memory of Ivy Murray.
Invasive (‘5A’) taxa in
Taxa | Reason for exclusion |
Acacia boliviana | Non-current synonym for Acaciella angustissima which is not listed at all in |
Crocosmia X crocosmiiflora | An engineered horticultural hybrid, thus quite distinct and not comparable to all other invasive species in our dataset |
Myrsiphyllum asparagoides | Non-current synonym for Asparagus asparagoides which is listed as 5A in |
Panicum maxiumum | Not listed as naturalized (N) in |
Pinus elliotii | A gymnosperm, our dataset contains angiosperms only |
Pinus radiata | A gymnosperm, our dataset contains angiosperms only |
Protasparagus aethiopicus | Non-current synonym for Asparagus aethiopicus which is not listed as 5A in |
Protasparagus plumosus | Non-current synonym for Asparagus plumosus which is listed as 5A in |
Salvinia molesta | A fern, our dataset contains angiosperms only |
Senecio tamoides | A taxonomic misapplication of Senecio angulatus which is not listed as 5A in |
Sporobolus pyramidalis | Not listed as naturalized (N) in |
Tradescantia albiflora | Non-current synonym for Tradescantia fluminensis which is listed as 5A in |
Turnera subulata | Not listed as naturalized (N) in |