The island biogeography of exotic bird species

Aim   A recent upsurge of interest in the island biogeography of exotic species has followed from the argument that they may provide valuable information on the natural processes structuring island biotas. Here, we use data on the occurrence of exotic bird species across oceanic islands worldwide to demonstrate an alternative and previously untested hypothesis that these distributional patterns are a simple consequence of where humans have released such species, and hence of the number of species released.
Location   Islands around the world.
Methods   Statistical analysis of published information on the numbers of exotic bird species introduced to, and established on, islands around the world.
Results   Established exotic birds showed very similar species–area relationships to native species, but different species–isolation relationships. However, in both cases the relationship for established exotics simply mimicked that for the number of exotic bird species introduced. Exotic bird introductions scaled positively with human population size and island isolation, and islands that had seen more native species extinctions had had more exotic species released.
Main conclusion   The island biogeography of exotic birds is primarily a consequence of human, rather than natural, processes.     

Native and exotic plants of fragments of sagebrush steppe produced by geomorphic processes versus land use

Habitat fragmentation and invasion by exotic species are regarded as major threats to the biodiversity of many ecosystems. We surveyed the plant communities of two types of remnant sagebrush-steppe fragments from nearby areas on the Snake River Plain of southeastern Idaho, USA. One type resulted from land use (conversion to dryland agriculture; hereafter AG Islands) and the other from geomorphic processes (Holocene volcanism; hereafter kipukas). We assessed two predictions for the variation in native plant species richness of these fragments, using structural equation models (SEM). First, we predicted that the species richness of native plants would follow the MacArthur–Wilson (M–W) hypothesis of island biogeography, as often is expected for the communities of habitat fragments. Second, we predicted a negative relationship between native and exotic plants, as would be expected if exotic plants are decreasing the diversity of native plants. Finally, we assessed whether exotic species were more strongly associated with the fragments embedded in the agricultural landscape, as would be expected if agriculture had facilitated the introduction and naturalization of non-native species, and whether the communities of the two types of fragments were distinct. Species richness of native plants was not strongly correlated with M–W characteristics for either the AG Islands or the **kipukas. The AG Islands had more species and higher cover of exotics than the kipukas, and exotic plants were good predictors of native plant species richness. Our results support the hypothesis that proximity to agriculture can increase the diversity and abundance of exotic plants in native habitat. In combination with other information, the results also suggest that agriculture and exotic species have caused loss of native diversity and reorganization of the sagebrush-steppe plant community.     

Negative native–exotic diversity relationship in oak savannas explained by human influence and climate

Recent research has proposed a scale-dependence to relationships between native diversity and exotic invasions. At fine spatial scales, native–exotic richness relationships should be negative as higher native richness confers resistance to invasion. At broad scales, relationships should be positive if natives and exotics respond similarly to extrinsic factors. Yet few studies have examined both native and exotic richness patterns across gradients of human influence, where impacts could affect native and exotic species differently. We examined native–exotic richness relationships and extrinsic drivers of plant species richness and distributions across an urban development gradient in remnant oak savanna patches. In sharp contrast to most reported results, we found a negative relationship at the regional scale, and no relationship at the local scale. The negative regional-scale relationship was best explained by extrinsic factors, surrounding road density and climate, affecting natives and exotics in opposite ways, rather than a direct effect of native on exotic richness, or vice versa. Models of individual species distributions also support the result that road density and climate have largely opposite effects on native and exotic species, although simple life history traits (life form, dispersal mode) do not predict which habitat characteristics are important for particular species. Roads likely influence distributions and species richness by increasing both exotic propagule pressure and disturbance to native species. Climate may partially explain the negative relationship due to differing climatic preferences within the native and exotic species pools. As gradients of human influence are increasingly common, negative broad-scale native–exotic richness relationships may be frequent in such landscapes.     

Characterizing the microhabitats of exotic species in Illinois shale barrens

Like native species, exotics form part of the mosaic structure of plant communities. However, their role in these communities is unclear. Thus, we ask, are invading exotic species present in recognizable microhabitats within plant communities? We compared the microhabitat of exotic species and random locations to determine if exotic plants occupied a recognizable subset of habitats within three barren communities. Fourteen exotic species were identified. The factors that characterized their habitats varied between barrens and among species. Considered as a group, exotic microhabitats comprised a subset of available habitats within the barrens, specifically, edge habitats rather than the open conditions of the barrens interior. Soil properties such as low temperature or neutral pH, and high litter or woody species cover, were common factors associated with the occurrence of exotics. Individual exotic species occupied different habitats within those identified for exotics as a group, but the range of variation between the different species was not generally significant. Within each barren, exotics occupied a distinct subset of microhabitats. This distribution suggests that exotics are not widely dispersed within these areas. Site conditions may be restricting the establishment of exotics to specific microhabitats, and in addition, some exotics may be altering the microhabitats that they colonize. Therefore, management efforts toward maintaining open conditions in the barrens may also discourage further encroachment by exotics into these areas.     

Domestic exotics and the perception of invasibility

Susceptibility of an area to invasion by exotic species is often judged by the fraction of introduced species in the local biota. However, the degree of invasion, particularly in mainland areas, has often been underestimated because of the exclusion of ‘domestic exotics’ (those introduced to internal units from within the national border) in calculations. Because all introduced species on islands are considered as exotics, this contributes to the perception that islands are more susceptible to invasion than are continental regions. Here, we determine the contribution of domestic exotic species to the degree of invasion (exotic fraction) in mainland areas. We quantify the relationships of exotic fraction to the area, human population density and land use within each of the 48 conterminous US states to identify mechanisms that potentially influence the degree of invasion. For each of the 48 conterminous US states, we compiled the number of species introduced from outside the United States (‘foreign exotics’) and the number of exotics introduced from other conterminous US states (‘domestic exotics’). The status of each species as foreign or domestic was determined for each state by researching its precise origins through vouchered herbarium records, supplemented by literature ( Kartesz, 2010 ). We found that (1) the exotic fraction inevitably decreases with increasing area as the pool of potential exotic species decreases; (2) exotic richness of areas within large mainland regions is underestimated to the extent that species introduced among areas within a region are considered as natives; and (3) human activities contribute disproportionately more exotics to smaller than to larger administrative areas. How we define ‘exotic’ influences how we count non‐native species and perceive invasibility. Excluding domestic exotics in mainland regions leads to a biased perception of increased invasibility on islands, where all introduced species are considered exotic. Thus, future documentation and interpretation of invasion patterns and management of exotics should account for these biases in quantifying the exotic fraction.     

Long-term grazing impacts on vegetation diversity,composition, and exotic species presence across an aridity gradient in northern temperate grasslands

Little is known about the specific role of exotic species on measures of grassland plant diversity, including how this may vary with climatic conditions or large mammal herbivory. This study examined vegetation responses to long-term livestock grazing, including plant richness and diversity, as well as the contribution of exotic species to these metrics, across a network of 107 northern temperate grasslands in Alberta, Canada, spanning a broad aridity gradient. Exposure to grazing modestly increased plant richness, but did not alter Shannon’s diversity, Simpson’s diversity, or evenness, suggesting stability in floral diversity relative to grazing. However, grazing did increase grass cover while reducing shrub cover, the latter of which was only apparent in mesic grasslands. Unlike total plant diversity, exotic species richness and cover, together with exotic plant contributions to diversity, varied jointly with grazing and aridity. While long-term grazing increased exotic species, this response was most apparent in wetter areas, and non-grazed grasslands remained more resistant to the presence of exotics. Several exotic species were positive indicators of grazing in wetter grasslands, and coincided with lower native species cover, indicating grazing may be facilitating a shift from native to exotic vegetation under these conditions. Overall, our results indicate that while long-term grazing has altered the composition and cover of certain functional groups, including favoring exotics and minimizing woody vegetation in mesic areas, overall changes to plant diversity were limited. Additionally, these findings suggest that semi-arid northern temperate grasslands remain relatively resistant to grazing effects, including their susceptibility to exotic plant encroachment. These results improve our understanding of how ongoing grazing exposure may impact grassland diversity, including efforts to conserve native vegetation, as well as the important role of climate in altering fundamental grassland responses to grazing.     

Parasites of native and exotic freshwater fishes in south‐western Australia

In this study, 1429 fishes of 18 different species (12 native and six exotic) were sampled from 29 localities to compare the levels of parasitism between native and exotic fish species and to examine the relationship between environmental degradation and parasite diversity. Forty‐four putative species of parasites were found and most of these appear to be native parasites, which have not previously been described. Two parasite species, Lernaea cyprinacea and Ligula intestinalis, are probably introduced. Both were found on or in a range of native fish species, where they may cause severe disease. Levels of parasitism and parasite diversity were significantly greater in native fishes than in exotic species, and this may contribute to an enhanced demographic performance and competitive ability in invading exotics. Levels of parasitism and parasite diversity in native fishes were negatively related to habitat disturbance, in particular to a suite of factors that indicate increased human use of the river and surrounding environment. This was due principally to the absence in more disturbed habitats of a number of species of endoparasites with complex life cycles, involving transmission between different host species.     

Phylogenetic patterns differ for native and exotic plant communities across a richness gradient in Northern California

Aim Increasingly, ecologists are using evolutionary relationships to infer the mechanisms of community assembly. However, modern communities are being invaded by non‐indigenous species. Since natives have been associated with one another through evolutionary time, the forces promoting character and niche divergence should be high. On the other hand, exotics have evolved elsewhere, meaning that conserved traits may be more important in their new ranges. Thus, co‐occurrence over sufficient time‐scales for reciprocal evolution may alter how phylogenetic relationships influence assembly. Here, we examined the phylogenetic structure of native and exotic plant communities across a large‐scale gradient in species richness and asked whether local assemblages are composed of more or less closely related natives and exotics and whether phylogenetic turnover among plots and among sites across this gradient is driven by turnover in close or distant relatives differentially for natives and exotics. Location Central and northern California, USA. Methods We used data from 30 to 50 replicate plots at four sites and constructed a maximum likelihood molecular phylogeny using the genes: matK, rbcl, ITS1 and 5.8s. We compared community‐level measures of native and exotic phylogenetic diversity and among‐plot phylobetadiversity. Results There were few exotic clades, but they tended to be widespread. Exotic species were phylogenetically clustered within communities and showed low phylogenetic turnover among communities. In contrast, the more species‐rich native communities showed higher phylogenetic dispersion and turnover among sites. Main conclusions The assembly of native and exotic subcommunities appears to reflect the evolutionary histories of these species and suggests that shared traits drive exotic patterns while evolutionary differentiation drives native assembly. Current invasions appear to be causing phylogenetic homogenization at regional scales.     

Plant–soil feedbacks of exotic plant species across life forms: a meta-analysis

Invasive exotic plant species effects on soil biota and processes in their new range can promote or counteract invasions via changed plant–soil feedback interactions to themselves or to native plant species. Recent meta-analyses reveale that soil influenced by native and exotic plant species is affecting growth and performance of natives more strongly than exotics. However, the question is how uniform these responses are across contrasting life forms. Here, we test the hypothesis that life form matters for effects on soil and plant–soil feedback. In a meta-analysis we show that exotics enhanced C cycling, numbers of meso-invertebrates and nematodes, while having variable effects on other soil biota and processes. Plant effects on soil biota and processes were not dependent on life form, but patterns in feedback effects of natives and exotics were dependent on life form. Native grasses and forbs caused changes in soil that subsequently negatively affected their biomass, whereas native trees caused changes in soil that subsequently positively affected their biomass. Most exotics had neutral feedback effects, although exotic forbs had positive feedback effects. Effects of exotics on natives differed among plant life forms. Native trees were inhibited in soils conditioned by exotics, whereas native grasses were positively influenced in soil conditioned by exotics. We conclude that plant life form matters when comparing plant–soil feedback effects both within and between natives and exotics. We propose that impact analyses of exotic plant species on the performance of native plant species can be improved by comparing responses within plant life form.     

庐山外来植物物种

生物入侵正日益成为普遍关注的生态环境问题.在庐山野外调查的基础上,再结合庐山植物引种记录资料的分析,发现庐山引入的2285种植物中,有1301种是室外栽培的,其中有127种82属35科逸为野生,成为庐山的外来物种.通过对这127种外来物种从物种组成、生活型、引入时间和方式、原生地和入侵性进一步分析,可以看出:外来物种主要由菊科、玄参科、禾本科、石竹科和苋科组成,这5科含种数占到总种数的63%;外来物种绝大多数为草本植物,含114种,占总种数的86.76%;原生地主要来源北美洲、其次是欧洲和亚洲,大部分是有意引进,引种目的主要用于观赏,其次是药用;在庐山127种外来物种中,有70种是列于中国100主要外来入侵物种,可见其入侵性还是很强.     

Phylogenetically structured damage to Asteraceae: susceptibility of native and exotic species to foliar herbivores

Invasive plants often lose natural enemies while moving to new regions; however, once established in a new area, these invaders may be susceptible to attack by locally occurring enemies. Such damage may be more likely for exotics with close native relatives in the invaded area, since shifts of enemies should be more likely among closely related hosts. In this study, we evaluated whether exotics experience less herbivore damage than natives, and whether phylogenetically novel exotics experience less damage that those that are more closely related to locally occurring family members. Foliar damage was measured on 20 native and 15 exotic Asteraceae that co-occur locally in southern Ontario, Canada. The phylogenetic structure of this damage was quantified using an eigenvector decomposition method, and the relationship between damage and phylogenetic novelty of exotics was evaluated based on phylogenetic distances to other locally occurring Asteraceae. Our results show that 32% of the variation in damage was explained by phylogenetic relationship; similarity in damage tended to be associated with tribes. As predicted, exotics experienced lower damage than native species, even when the dataset was corrected for phylogenetic nonindependence. Contrary to our prediction, however, exotics that were more phylogenetically isolated from locally occurring relatives did not experience less damage. These results suggest that, though exotic Asteraceae may escape many of their natural enemies, this is not in general more likely for species phylogenetically distant from locally occurring native confamilials.     

Effects of human mediated disturbances on exotic forest insect diversity in a Chilean mediterranean ecosystem

At the current rate of exchange of goods and people among geographic areas, the introduction of insect species into new habitats represents an increasing threat to insect diversity. The situation is especially acute in Mediterranean ecosystems where the high human population density incurs multiple sources of disturbance and high propagule pressure. In this study, we characterize the relationship between native and exotic forest insect richness and evaluate how human-mediated disturbances can influence this relationship in the Mediterranean central Chile. Exotic and native species richness were positively correlated across the study area, suggesting similar effect of environmental variables on both assemblages over large scales. When the effect of human-mediated disturbances was evaluated using generalized linear and additive models, we found that native richness, human population density and habitat diversity were the most important variables affecting exotic richness. Moreover, we detected strong nonlinearities in the effect of some variables. For instance, the influence of human population density on the exotic richness followed a threshold function, where below 1,000?hab/km², the proportion of exotics in the community grew rapidly with increasing human density, but above this threshold density, human population did not produce further increases in exotic richness. Two important conclusions arise from these results: first, there is a positive effect of human-mediated disturbances on the exotic richness in central Chile, and second, the key role that human population density has on the invasibility of insect communities in rural and semi-rural Mediterranean areas.     

Deconstructing the native–exotic richness relationship in plants

Aim Classic theory suggests that species‐rich communities should be more resistant to the establishment of exotic species than species‐poor communities. Although this theory predicts that exotic species should be less diverse in regions that contain more native species, macroecological analyses often find that the correlation between exotic and native species richness is positive rather than negative. To reconcile results with theory, we explore to what extent climatic conditions, landscape heterogeneity and anthropogenic disturbance may explain the positive relationship between native and exotic plant richness. Location Catalonia (western Mediterranean region). Methods We integrated floristic records and GIS‐based environmental measures to make spatially explicit 10‐km grid cells. We asked whether the observed positive relationship between native and exotic plant richness (R²= 0.11) resulted from the addition of several negative correlations corresponding to different environmental conditions identified with cluster analysis. Moreover, we directly quantified the importance of common causal effects with a structural equation modelling framework. Results We found no evidence that the relationship between native and exotic plant richness was negative when the comparison was made within environmentally homogeneous groups. Although there were common factors explaining both native and exotic richness, mainly associated with landscape heterogeneity and human pressure, these factors only explained 17.2% of the total correlation. Nevertheless, when the comparison was restricted to native plants associated with human‐disturbed (i.e. ruderal) ecosystems, the relationship was stronger (R²= 0.52) and the fraction explained by common factors increased substantially (58.3%). Main conclusions While our results confirm that the positive correlation between exotic and native plant richness is in part explained by common extrinsic factors, they also highlight the great importance of anthropic factors that – by reducing biotic resistance – facilitate the establishment and spread of both exotic and native plants that tolerate disturbed environments.     

From plant neighbourhood to landscape scales: how grazing modifies native and exotic plant species richness in grassland

The interactive effect of grazing and soil resources on plant species richness and coexistence has been predicted to vary across spatial scales. When resources are not limiting, grazing should reduce competitive effects and increase colonisation and richness at fine scales. However, at broad scales richness is predicted to decline due to loss of grazing intolerant species. We examined these hypotheses in grasslands of southern Australia that varied in resources and ungulate grazing intensity since farming commenced 170 years ago. Fine-scale species richness was slightly greater in more intensively grazed upper slope sites with high nutrients but low water supply compared to those that were moderately grazed, largely due to a greater abundance of exotic species. At broader scales, exotic species richness declined with increasing grazing intensity whether nutrients or water supply were low or high. Native species richness declined at all scales in response to increasing grazing intensity and greater resource supply. Grazing also reduced fine-scale heterogeneity in native species richness and although exotics were also characterised by greater heterogeneity at fine scales, grazing effects varied across scales. In these grasslands patterns of plant species richness did not match predictions at all scales and this is likely to be due to differing responses of native and exotic species and their relative abundance in the regional species pool. Over the past 170 years intolerant native species have been eliminated from areas that are continually and heavily grazed, whereas transient, light grazing increases richness of both exotics and natives. The results support the observation that the processes and scales at which they operate differ between coevolved ungulate—grassland systems and those in transition due to recent invasion of herbivores and associated plant species.     

Are Local Filters Blind to Provenance? Ant Seed Predation Suppresses Exotic Plants More than Natives

The question of whether species’ origins influence invasion outcomes has been a point of substantial debate in invasion ecology. Theoretically, colonization outcomes can be predicted based on how species’ traits interact with community filters, a process presumably blind to species’ origins. Yet, exotic plant introductions commonly result in monospecific plant densities not commonly seen in native assemblages, suggesting that exotic species may respond to community filters differently than natives. Here, we tested whether exotic and native species differed in their responses to a local community filter by examining how ant seed predation affected recruitment of eighteen native and exotic plant species in central Argentina. Ant seed predation proved to be an important local filter that strongly suppressed plant recruitment, but ants suppressed exotic recruitment far more than natives (89% of exotic species vs. 22% of natives). Seed size predicted ant impacts on recruitment independent of origins, with ant preference for smaller seeds resulting in smaller seeded plant species being heavily suppressed. The disproportionate effects of provenance arose because exotics had generally smaller seeds than natives. Exotics also exhibited greater emergence and earlier peak emergence than natives in the absence of ants. However, when ants had access to seeds, these potential advantages of exotics were negated due to the filtering bias against exotics. The differences in traits we observed between exotics and natives suggest that higher-order introduction filters or regional processes preselected for certain exotic traits that then interacted with the local seed predation filter. Our results suggest that the interactions between local filters and species traits can predict invasion outcomes, but understanding the role of provenance will require quantifying filtering processes at multiple hierarchical scales and evaluating interactions between filters.     

BIODIVERSITY RESEARCH: Native‐exotic species richness relationships across spatial scales and biotic homogenization in wetland plant communities of Illinois,USA

Aim To examine native‐exotic species richness relationships across spatial scales and corresponding biotic homogenization in wetland plant communities. Location Illinois, USA. Methods We analysed the native‐exotic species richness relationship for vascular plants at three spatial scales (small, 0.25 m² of sample area; medium, 1 m² of sample area; large, 5 m² of sample area) in 103 wetlands across Illinois. At each scale, Spearman’s correlation coefficient between native and exotic richness was calculated. We also investigated the potential for biotic homogenization by comparing all species surveyed in a wetland community (from the large sample area) with the species composition in all other wetlands using paired comparisons of their Jaccard’s and Simpson’s similarity indices. Results At large and medium scales, native richness was positively correlated with exotic richness, with the strength of the correlation decreasing from the large to the medium scale; at the smallest scale, the native‐exotic richness correlation was negative. The average value for homogenization indices was 0.096 and 0.168, using Jaccard’s and Simpson’s indices, respectively, indicating that these wetland plant communities have been homogenized because of invasion by exotic species. Main Conclusions Our study demonstrated a clear shift from a positive to a negative native‐exotic species richness relationship from larger to smaller spatial scales. The negative native‐exotic richness relationship that we found is suggested to result from direct biotic interactions (competitive exclusion) between native and exotic species, whereas positive correlations likely reflect the more prominent influence of habitat heterogeneity on richness at larger scales. Our finding of homogenization at the community level extends conclusions from previous studies having found this pattern at much larger spatial scales. Furthermore, these results suggest that even while exhibiting a positive native‐exotic richness relationship, community level biotas can/are still being homogenized because of exotic species invasion.     

Climate change and invasion by intracontinental range-expanding exotic plants: the role of biotic interactions

Background and Aims

In this Botanical Briefing we describe how the interactions between plants and their biotic environment can change during range-expansion within a continent and how this may influence plant invasiveness.

Scope

We address how mechanisms explaining intercontinental plant invasions by exotics (such as release from enemies) may also apply to climate-warming-induced range-expanding exotics within the same continent. We focus on above-ground and below-ground interactions of plants, enemies and symbionts, on plant defences, and on nutrient cycling.

Conclusions

Range-expansion by plants may result in above-ground and below-ground enemy release. This enemy release can be due to the higher dispersal capacity of plants than of natural enemies. Moreover, lower-latitudinal plants can have higher defence levels than plants from temperate regions, making them better defended against herbivory. In a world that contains fewer enemies, exotic plants will experience less selection pressure to maintain high levels of defensive secondary metabolites. Range-expanders potentially affect ecosystem processes, such as nutrient cycling. These features are quite comparable with what is known of intercontinental invasive exotic plants. However, intracontinental range-expanding plants will have ongoing gene-flow between the newly established populations and the populations in the native range. This is a major difference from intercontinental invasive exotic plants, which become more severely disconnected from their source populations.     

Rapid biodiversity declines in both ungrazed and intensely grazed exotic grasslands

Exotic-dominated ecosystems with low diversity are becoming increasingly common. It remains unclear, though, whether differences between native and exotic species (driver model), or changes in disturbances or resources (passenger model), allow exotics to become competitive dominants. In our field experiment, plant species origin (native or exotic), cattle grazing (ungrazed or intensely grazed once), and species composition treatments were fully crossed and randomly assigned to four-species mixtures and monocultures of grassland plants. We found that biodiversity declined more rapidly in exotic than in native species mixtures, regardless of our grazing disturbance treatment. Early declines in species evenness (i.e., increases in dominance) led to subsequent declines in species richness (i.e., local extinctions) in exotic mixtures. Specifically, Simpson’s diversity was 29% lower after 1 year, and species richness was 15% lower after 3 years, in exotic than in native mixtures. These rapid biodiversity declines in exotic mixtures were partly explained by decreased complementarity (i.e., niche partitioning and facilitation), presumably because exotic species lack the coevolutionary history that can lead to complementarity and coexistence in native communities. Thus, our results suggest that exotic species can drive biodiversity declines in the presence or absence of a grazing disturbance, partly because exotic species interactions differ from native species interactions. This implies that restoring plant biodiversity in grasslands may require removal of exotic species, in addition to disturbance management.     

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.     

Reciprocal effects of litter from exotic and congeneric native plant species via soil nutrients

Invasive exotic plant species are often expected to benefit exclusively from legacy effects of their litter inputs on soil processes and nutrient availability. However, there are relatively few experimental tests determining how litter of exotic plants affects their own growth conditions compared to congeneric native plant species. Here, we test how the legacy of litter from three exotic plant species affects their own performance in comparison to their congeneric natives that co-occur in the invaded habitat. We also analyzed litter effects on soil processes. In all three comparisons, soil with litter from exotic plant species had the highest respiration rates. In two out of the three exotic-native species comparisons, soil with litter from exotic plant species had higher inorganic nitrogen concentrations than their native congener, which was likely due to higher initial litter quality of the exotics. When litter from an exotic plant species had a positive effect on itself, it also had a positive effect on its native congener. We conclude that exotic plant species develop a legacy effect in soil from the invaded range through their litter inputs. This litter legacy effect results in altered soil processes that can promote both the exotic plant species and their native congener.