Exotic species richness and native species endemism increase the impact of exotic species on islands

Aim Exotic species pose one of the most significant threats to biodiversity, especially on islands. The impacts of exotic species vary in severity among islands, yet little is known about what makes some islands more susceptible than others. Here we determine which characteristics of an island influence how severely exotic species affect its native biota. Location We studied 65 islands and archipelagos from around the world, ranging from latitude 65° N to 54° S. Methods We compiled a global database of 10 island characteristics for 65 islands and determined the relative importance of each characteristic in predicting the impact of exotic species using multivariate modelling and hierarchical partitioning. We defined the impact of exotic species as the number of bird, amphibian and mammal (BAM) species listed by the International Union for Conservation of Nature (IUCN) as threatened by exotics, relative to the total number of BAM species on that island. Results We found that the impact of exotic species is more severe on islands with more exotic species and a greater proportion of native species that are endemic. Unexpectedly, the level of anthropogenic disturbance did not influence an island's susceptibility to the impacts of exotic species. Main conclusions By coupling our results with studies on the introduction and establishment of exotic species, we conclude that colonization pressure, or invasion opportunities, influences all stages of the invasion process. However, species endemism, the other important factor determining the impact of exotic species, is not known to contribute to introduction and establishment success on islands. This demonstrates that different factors correlate with the initial stages of the invasion process and the subsequent impacts of those invaders, highlighting the importance of studying the impacts of exotic species directly. Our study helps identify islands that are at risk of impact by exotics and where investment should focus on preventing further invasions.     

Seed predation as a barrier to alien conifer invasions

Interactions between exotic plants and animals can play a major role in determining success or failure of plant introductions. Seed predation has been seen as important in explaining biotic resistance to plant invasion, but this hypothesis has rarely been tested. We studied seed predation on exotic forest plants on an island in Patagonia, Argentina where 43 pine species, including 60% of the world’s known invasive Pinaceae, were introduced ca. 80 years ago, but where exotics attain relatively high densities only near the original plantings. To test if seed predation limits exotic conifer establishment in this area, we compared seed predation in areas close to plantations (colonized by exotics) and far from them (not invaded). Seeds of exotics were preferred over seeds of native species, possibly because exotic seeds are bigger. Predation was more intense in areas far from plantations than in areas close to them, substantially reducing the chances of exotic seed establishment. Using automatic cameras, we found that both rodents and birds preyed on exotic seeds. This study suggests that native seed predators can be an important component of biological resistance to plant invasion.     

Introduced mammalian predators induce behavioural changes in parental care in an endemic New Zealand bird

The introduction of predatory mammals to oceanic islands has led to the extinction of many endemic birds. Although introduced predators should favour changes that reduce predation risk in surviving bird species, the ability of island birds to respond to such novel changes remains unstudied. We tested whether novel predation risk imposed by introduced mammalian predators has altered the parental behaviour of the endemic New Zealand bellbird (Anthornis melanura). We examined parental behaviour of bellbirds at three woodland sites in New Zealand that differed in predation risk: 1) a mainland site with exotic predators present (high predation risk), 2) a mainland site with exotic predators experimentally removed (low risk recently) and, 3) an off-shore island where exotic predators were never introduced (low risk always). We also compared parental behaviour of bellbirds with two closely related Tasmanian honeyeaters (Phylidonyris spp.) that evolved with native nest predators (high risk always). Increased nest predation risk has been postulated to favour reduced parental activity, and we tested whether island bellbirds responded to variation in predation risk. We found that females spent more time on the nest per incubating bout with increased risk of predation, a strategy that minimised activity at the nest during incubation. Parental activity during the nestling period, measured as number of feeding visits/hr, also decreased with increasing nest predation risk across sites, and was lowest among the honeyeaters in Tasmania that evolved with native predators. These results demonstrate that some island birds are able to respond to increased risk of predation by novel predators in ways that appear adaptive. We suggest that conservation efforts may be more effective if they take advantage of the ability of island birds to respond to novel predators, especially when the elimination of exotic predators is not possible.     

Across island and continents, mammals are more successful invaders than birds (Reply)

Aim  The establishment success of exotic species is calculated as the fraction of introduced species that have become established, and invasion success is estimated as the fraction of established species that have spread significantly from their points of introduction. Records on species introductions are highly incomplete, so strong conclusions about the tens rule and invasibility of island and continents cannot be drawn.
Location  Global.
Methods  Using Jeschke (2008) as an example, we explain the issue of the inadequacy of data to draw conclusions about the tens rule and invasibility of island and continents.
Results  Lack of adequate data.
Main Conclusion  Jeschke (2008) probably overestimates the establishment and invasion success rates, so his conclusions about violation of the tens rule and that islands are not more susceptible to invasion than continents are misleading.     

Contemporary morphological diversification of passerine birds introduced to the Hawaiian archipelago

Species that have been introduced to islands experience novel and strong selection pressures after establishment. There is evidence that exotic species diverge from their native source populations; further, a few studies have demonstrated adaptive divergence across multiple exotic populations of a single species. Exotic birds provide a good study system, as they have been introduced to many locations worldwide, and we often know details concerning the propagule origin, time of introduction, and dynamics of establishment and dispersal within the introduced range. These data make them especially conducive to the examination of contemporary evolution. Island faunas have received intense scrutiny, therefore we have expectations concerning the patterns of diversification for exotic species. We examine six passerine bird species that were introduced to the Hawaiian archipelago less than 150 years ago. We find that five of these show morphological divergence among islands from the time since they were established. We demonstrate that some of this divergence cannot be accounted for by genetic drift, and therefore we must consider adaptive evolution to explain it. We also evaluate evolutionary divergence rates and find that these species are diverging at similar rates to those found in published studies of contemporary evolution in native species.     

Patterns of bird invasion are consistent with environmental filtering

Predicting invasion potential has global significance for managing ecosystems as well as important theoretical implications for understanding community assembly. Phylogenetic relationships of introduced species to the extant community may be predictive of establishment success because of the opposing forces of competition/shared enemies (which should limit invasions by close relatives) versus environmental filtering (which should allow invasions by close relatives). We examine here the association between establishment success of introduced birds and their phylogenetic relatedness to the extant avifauna within three highly invaded regions (Florida, New Zealand, and Hawaii). Published information on both successful and failed introductions, as well as native species, was compiled for all three regions. We created a phylogeny for each avifauna including all native and introduced bird species. From the estimated branch lengths on these phylogenies, we calculated multiple measurements of relatedness between each introduced species and the extant avifauna. We used generalized linear models to test for an association between relatedness and establishment success. We found that close relatedness to the extant avifauna was significantly associated with increased establishment success for exotic birds both at the regional (Florida, Hawaii, New Zealand) and sub‐regional (islands within Hawaii) levels. Our results suggest that habitat filtering may be more important than interspecific competition in avian communities assembled under high rates of anthropogenic species introductions. This work also supports the utility of community phylogenetic methods in the study of vertebrate invasions.     

Historical legacies and ecological determinants of grass naturalizations worldwide

The global distribution of exotic species is the result of abiotic, biotic and dispersal filtering processes that shape the movement and success of species outside their native range. In this study we aim to understand how these filtering processes drive the fluxes of grass species among regions, the factors that influence which species establish outside of their native range, and where they do so. We used national and subnational checklists of native and introduced grass species to determine the extent to which each region was a source or recipient of exotic grass species. We asked how species traits may distinguish those grass species that have naturalized outside their native range from those that have not, and how environmental conditions are related to the distribution of exotic grass species. We found that exotic grass establishment is shaped by an array of factors including characteristics of regions, traits of species and their interactions. Regions with a longer history of human occupation and larger numbers of native grass species were generally the most important sources of exotic species. Global flows of species were mostly driven by a climate match between the native and exotic ranges, but were also highly asymmetric, with regions with recent human arrival being the major hosts of exotic grass species. Tall, annual and C4 grass species exhibited particularly high probabilities of establishment outside their native range. Despite the idiosyncrasy and stochasticity characteristic of exotic species establishment, this biogeographical analysis revealed important generalities across this large plant group. Our results suggest that grass species that have co-occurred with humans for a longer time may be better adapted to living in anthropogenic landscapes, explaining the global asymmetry in species introductions.     

Harmonia axyridis: What will stop the invader?

In recent years Harmonia axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) has become a very popular insect among biological control practitioners and scientists, not only for its potential to be an efficient biological control agent but also because it is considered invasive. Individuals of this species were deliberately introduced into several countries for biological control of different arthropods pests. However the predator itself became an invasive species, affecting the dynamics and composition of several guilds through direct or indirect interactions with established species, including intraguild predation. In this paper we discuss the reasons why the species has a high invasiveness and what are the limits to invasion by this species. It is not clear if the invasiveness of the beetle is linked to its biological, ecological and behavioural abilities, or to other factors such as invasibility and interactions between the invaders, the noninvaders, and the habitat, which may in part explain the reasons of its success and help us to answer the question “what will stop the invader?” We also discuss the reason for the absence of the predator in the Azores islands. Despite the intentional introduction of H. axyridis in the Azores and the high number of individuals released, there are no records of this species in the wild, despite recent extensive sampling effort. In this paper we discuss the reasons for the apparent failure or the delay in establishment of the predator. One factor which may hamper the establishment of H. axyridis in some of the Azores islands is the absence of winter environmental conditions, mainly the temperature which is seldom lower than 12°C, essential for the induction of diapause. The lack of success in the establishment could be also related to functional diversity saturation, that is species saturation and competitive exclusion of H. axyridis by other previously established species may be operating.     

Biodiversity Loss following the Introduction of Exotic Competitors: Does Intraguild Predation Explain the Decline of Native Lady Beetles?

Exotic species are widely accepted as a leading cause of biodiversity decline. Lady beetles (Coccinellidae) provide an important model to study how competitor introductions impact native communities since several native coccinellids have experienced declines that coincide with the establishment and spread of exotic coccinellids. This study tested the central hypothesis that intraguild predation by exotic species has caused these declines. Using sentinel egg experiments, we quantified the extent of predation on previously-common (Hippodamia convergens) and common (Coleomegilla maculata) native coccinellid eggs versus exotic coccinellid (Harmonia axyridis) eggs in three habitats: semi-natural grassland, alfalfa, and soybean. Following the experiments quantifying egg predation, we used video surveillance to determine the composition of the predator community attacking the eggs. The extent of predation varied across habitats, and egg species. Native coccinellids often sustained greater egg predation than H. axyridis. We found no evidence that exotic coccinellids consumed coccinellid eggs in the field. Harvestmen and slugs were responsible for the greatest proportion of attacks. This research challenges the widely-accepted hypothesis that intraguild predation by exotic competitors explains the loss of native coccinellids. Although exotic coccinellids may not be a direct competitor, reduced egg predation could indirectly confer a competitive advantage to these species. A lower proportion of H. axyridis eggs removed by predators may have aided its expansion and population increase and could indirectly affect native species via exploitative or apparent competition. These results do not support the intraguild predation hypothesis for native coccinellid decline, but do bring to light the existence of complex interactions between coccinellids and the guild of generalist predators in coccinellid foraging habitats.     

Introduction history and invasion success in exotic vines introduced to Australia

The ecological damage caused by invasive vines poses a considerable threat to many natural ecosystems. However, very little data are available for this potentially environmentally destructive functional group in Australia. In order to address this paucity of information, we assembled the first inventory of exotic vines that have become established in natural ecosystems across Australia. The influence that introduction history attributes, variables that relate to the introduction of a species to a new area, may have on the occurrence and distribution of exotic vines was also determined. We asked whether the continent of origin, reason for introduction, and residence time related to the prevalence and distribution of exotic vines across Australia. A total of 179 exotic climbing plant species from 40 different families were found to have become established across continental Australia. However, five families accounted for over 50% of these species. Most exotic vines originated from South America, and were introduced for ornamental purposes. The length of time in which an exotic vine had been present in its new range was significantly related to its distribution, with a positive relationship found between residence time and area of occupancy across the continent. No other introduction history attribute was significantly related to the area of occupancy, or distribution, of a species. This suggests that while the trends found among introduction history attributes are important in explaining the prevalence of exotic vines in Australia, only residence time is currently a useful predictor of their future success.     

Mistakes in the analysis of exotic species establishment: source pool designation and correlates of introduction success among parrots (Aves: Psittaciformes) of the world

Aim To evaluate the effect of mis‐specifying the correct comparison of species pools in the study of species characteristics associated with the biological introduction of exotic species. Methods We use a high quality data set on biological introductions of parrots (Aves: Psittaciformes). These data allow us to examine relationships between life history traits and probability of successful transition through an introduction stage when the species pool is both correctly and incorrectly specified. Results For the establishment of introduced parrot species, nearly half of the predictor variables showed different patterns of significance when an incorrect pool was specified. Multivariate analysis identified entirely different sets of variables as independent predictors of establishment success, depending on the species pool used. Correct pool specification identified that introduced parrot species have been more likely to establish if they have broader diets and are more sedentary. Main conclusions Conclusions from the analysis of biological introductions are likely to depend on the specification of the species pool for such analyses. In the analysis of parrot introductions, this was particularly apparent in establishment success following release. Further studies that analyse the introduction pathway need to examine the effects of pool mis‐specification so that the generality of our results can be assessed.     

Establishment success of introduced amphibians increases in the presence of congeneric species

Darwin's naturalization hypothesis predicts that the success of alien invaders will decrease with increasing taxonomic similarity to the native community. Alternatively, shared traits between aliens and the native assemblage may preadapt aliens to their novel surroundings, thereby facilitating establishment (the preadaptation hypothesis). Here we examine successful and failed introductions of amphibian species across the globe and find that the probability of successful establishment is higher when congeneric species are present at introduction locations and increases with increasing congener species richness. After accounting for positive effects of congeners, residence time, and propagule pressure, we also find that invader establishment success is higher on islands than on mainland areas and is higher in areas with abiotic conditions similar to the native range. These findings represent the first example in which the preadaptation hypothesis is supported in organisms other than plants and suggest that preadaptation has played a critical role in enabling introduced species to succeed in novel environments.     

Influence of invasion history on rapid morphological divergence across island populations of an exotic bird

There is increasing evidence that exotic populations may rapidly differentiate from those in their native range and that differences also arise among populations within the exotic range. Using morphological and DNA‐based analyses, we document the extent of trait divergence among native North American and exotic Hawaiian populations of northern cardinal (Cardinalis cardinalis). Furthermore, using a combination of historical records and DNA‐based analyses, we evaluate the role of founder effects in producing observed trait differences. We measured and compared key morphological traits across northern cardinal populations in the native and exotic ranges to assess whether trait divergence across the Hawaiian Islands, where this species was introduced between 1929 and 1931, reflected observed variation across native phylogeographic clades in its native North America. We used and added to prior phylogenetic analyses based on a mitochondrial locus to identify the most likely native source clade(s) for the Hawaiian cardinal populations. We then used Approximate Bayesian Computation (ABC) to evaluate the role of founder effects in producing the observed differences in body size and bill morphology across native and exotic populations. We found cardinal populations on the Hawaiian Islands had morphological traits that diverged substantially across islands and overlapped the trait space of all measured native North American clades. The phylogeographic analysis identified the eastern North American clade (C. cardinalis cardinalis) as the most likely and sole native source for all the Hawaiian cardinal populations. The ABC analyses supported written accounts of the cardinal's introduction that indicate the original 300 cardinals shipped to Hawaii were simultaneously and evenly released across Hawaii, Kauai, and Oahu. Populations on each island likely experienced bottlenecks followed by expansion, with cardinals from the island of Hawaii eventually colonizing Maui unaided. Overall, our results suggest that founder effects had limited impact on morphological trait divergence of exotic cardinal populations in the Hawaiian archipelago, which instead reflect postintroduction events.     

A population model for predicting the successful establishment of introduced bird species

One of the strongest generalities in invasion biology is the positive relationship between probability of establishment and the numbers of individuals introduced. Nevertheless, a number of significant questions remain regarding: (1) the relative importance of different processes during introduction (e.g., demographic, environmental, and genetic stochasticity, and Allee effects); (2) the relative effects of propagule pressure (e.g., number of introductions, size of introductions, and lag between introductions); and (3) different life history characteristics of the species themselves. Here, we adopt an individual-based simulation modeling approach to explore a range of such details in the relationship between establishment success and numbers of individuals introduced. Our models are developed for typical exotic bird introductions, for which the relationship between probability of establishment and the numbers of individuals introduced has been particularly well documented. For both short-lived and long-lived species, probability of establishment decreased across multiple introductions (compared with a single introduction of the same total size), and this decrease was greater when inbreeding depression was included. Sensitivity analyses revealed four predictors that together accounted for >95 % of model performance. Of these, R ₀ (the average number of daughters produced per female over her lifetime) and propagule pressure were of primary importance, while random environmental effects and inbreeding depression exerted lesser influence. Initial founder size is undoubtedly going to be important for ensuring the persistence of introduced populations. However, we found the demographic traits, which influence how introduced individuals behave, to have the greatest effect on establishment success.     

Influences on the transport and establishment of exotic bird species: an analysis of the parrots (Psittaciformes) of the world

Most studies of exotic species invasions only consider the factors that affect the establishment of populations following release, yet this is only one step on the invasion pathway. Different factors are likely to influence which species are transported and released. Here, we examine the influence of species traits on the successful transition of species through several stages in the introduction pathway (transport, release, and establishment), using parrots (Aves: Psittaciformes) as a model system. We use a species‐level supertree of parrots to test for phylogenetic auto‐correlation in the introduction process. Our analyses find that different sets of variables are related to the probability that a species enters each stage on the invasion pathway. The availability of individuals for transport and release seems to be most important for passage through these stages, but has no obvious effect on establishment following release. Rather, establishment success is higher for sedentary species, and species with broad diets.     

Predicting establishment success for introduced freshwater fishes: a role for climate matching

We modelled data comprising 1,189 successful and 489 failed introduction records for 280 species of freshwater fishes around the world. We found significant variations in establishment success between genera and families. The number of countries where introductions occurred was a significant predictor of the probability a species would establish in at least one country and all species that had been introduced to nine or more countries (46 species) had established at least one exotic population. We also conducted more detailed quantitative modelling for 135 species introduced to 10 countries to identify factors affecting establishment success. Relative to failed species, established species had better climate matches between the country where they were introduced and their geographic range elsewhere in the world. Established species were also more likely to have high establishment success rates elsewhere in the world. Neither the reason why fish were introduced nor the country where they were introduced was correlated with establishment success. Cross-validations indicated our model correctly categorised establishment success with 78% accuracy. Our findings may guide risk assessments for the import of live exotic fish to reduce the rate new species establish in the wild.     

The role of species traits in the establishment success of exotic birds

There is now abundant evidence that propagule pressure, a composite measure of the number of individuals released into the nonnative location that varies between introduction events, is the most consistent predictor of success in the establishment of exotic species. However, the reasons why we expect propagule pressure to be important – because larger propagules ameliorate the effects of demographic, environmental or genetic stochasticity, or of Allee effects – also predict an influence of species traits on success. Here, we use a quantitative meta-analytical approach to assess the effect of three categories of species-level traits in the successful establishment of nonnative bird species: traits relating to population growth rates, traits that predispose species to Allee effects, and traits that enable a species to cope with novel environments. Traits that predispose species to Allee effects tend to decrease introduction success, whereas traits that enable a species to cope with novel environments tend to increase success. The breadth of habitats a species uses has the strongest mean effect of all variables analysed here. Mean effects for traits relating to population growth rates conflict in sign: in general, success is greater for species with large body mass whereas clutch size is not consistently related to establishment success. These results suggest a likely influence of some species-level traits on exotic bird establishment success, especially traits that enable a species to cope with novel environments. We suggest that considering such traits in terms of the small-population paradigm from conservation biology may be a productive avenue for future research.     

Invasive Exotic Plants in the Tropical Pacific Islands: Patterns of Diversity

Oceanic islands are good model systems with which to explore factors affecting exotic species diversity. Islands vary in size, topography, substrate type, degree of isolation, native species diversity, history, human population characteristics, and economic development. Moreover, islands are highly vulnerable to exotic species establishment. We used AICc analyses of data on 1132 vascular plant species for 15 countries and 114 islands from the Pacific Island Ecosystems at Risk (PIER) project to examine biological, geographical, and socioeconomic correlates of exotic species richness. PIER provides data on the distribution of naturalized non-native plant species thought to pose environmental or economic risk. We hypothesized that the numbers of PIER-listed species would be positively correlated with island size, habitat diversity, and proximity to major source pools for propagules. Further, we expected numbers of PIER-listed exotic species to be similar among islands in the same country and to be greater where human populations were larger and where economic activity was high. Most species (908) were found on ≤ 10 islands. Species number was significantly correlated with island and country areas and with native plant species richness. The strongest model revealed by AICc analyses of island data included log (area) and maximum elevation as well as country membership, substrate type, and presence of an airport with paved runway (an index of economic activity). By country, AICc analyses revealed two equivalent models, both of which included log (area) and per capita gross domestic product as well as a measure of population size (either log (population size) or (population density)). Our analyses provide strong evidence of the roles of biogeographic, environmental, and socioeconomic impacts on the distribution and spread of exotic species.     

Determinants of vertebrate invasion success in Europe and North America

Species that are frequently introduced to an exotic range have a high potential of becoming invasive. Besides propagule pressure, however, no other generally strong determinant of invasion success is known. Although evidence has accumulated that human affiliates (domesticates, pets, human commensals) also have high invasion success, existing studies do not distinguish whether this success can be completely explained by or is partly independent of propagule pressure. Here, we analyze both factors independently, propagule pressure and human affiliation. We also consider a third factor directly related to humans, hunting, and 17 traits on each species' population size and extent, diet, body size, and life history. Our dataset includes all 2362 freshwater fish, mammals, and birds native to Europe or North America. In contrast to most previous studies, we look at the complete invasion process consisting of (1) introduction, (2) establishment, and (3) spread. In this way, we not only consider which of the introduced species became invasive but also which species were introduced. Of the 20 factors tested, propagule pressure and human affiliation were the two strongest determinants of invasion success across all taxa and steps. This was true for multivariate analyses that account for intercorrelations among variables as well as univariate analyses, suggesting that human affiliation influenced invasion success independently of propagule pressure. Some factors affected the different steps of the invasion process antagonistically. For example, game species were much more likely to be introduced to an exotic continent than nonhunted species but tended to be less likely to establish themselves and spread. Such antagonistic effects show the importance of considering the complete invasion process.