Human impacts, energy availability and invasion across Southern Ocean Islands

Aim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues. Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia. Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale. Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate‐matching, were responsible for differences in the extent of invasion. Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.     

Native–Exotic Species Richness Relationships Across Spatial Scales in a Prairie Restoration Matrix

In highly invaded ecosystems, restoration of native plant communities is dependent upon reducing exotic species relative to native species. Even so, in monitoring, the native–exotic species richness ratio has been shown to be scale‐dependent. Measurement at small spatial scales (<1 m²) can reveal a negative native–exotic richness relationship, where niche occupation may prevent invasion. Conversely, at larger scales, a positive correlation may exist, where environmental heterogeneity and equally favorable conditions may drive native–exotic relationships. Here, we compare slopes of native–exotic relationships across spatial scales in a prairie undergoing active restoration. The observed native–exotic richness ratios varied considerably over scales ranging from 1 to 1,000 m², emphasizing the importance of choosing a measurement scale that is most pertinent to the treatment and ecological mechanism used to evaluate restoration success. Our native–exotic richness slopes were positive over all scales, but lower than would be expected in a random community assembly, suggesting the influence of niche‐based competition. Correspondingly, our native–exotic cover slope was more negative than a null model; however, areas of frequent fire treatments showed a significant deviation from null only for richness, indicating that burning may enhance native–exotic competitive dynamics for number of species but not cover. The negative native–exotic cover relationships appear to be driven in this system mainly by exotic graminoids, across burn treatments and native functional groups, supporting the concept that frequent burning can alter the dominant competitive mechanism from coverage of these exotic grasses to an improved environment for germination and dispersal of more native species.     

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.     

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.     

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.     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.     

Does energy availability influence classical patterns of spatial variation in exotic species richness?

Aim At macroecological scales, exotic species richness is frequently positively correlated with human population density. Such patterns are typically thought to arise because high human densities are associated with increased introduction effort and/or habitat modification and disturbance. Exotic and native species richness are also frequently positively correlated, although the causal mechanisms remain unclear. Energy availability frequently explains much of the variation in species richness and we test whether such species–energy relationships may influence the relationships of exotic species richness with human population density and native species richness. Location Great Britain. Methods We first investigate how spatial variation in the distributions of the 10 exotic bird species is related to energy availability. We then model exotic species richness using native avian species richness, human population density and energy availability as predictors. Species richness is modelled using two sets of models: one assumes independent errors and the other takes spatial correlation into account. Results The probability of each exotic species occurring, in a 10‐km quadrat, increases with energy availability. Exotic species richness is positively correlated with energy availability, human population density and native species richness in univariate tests. When taking energy availability into account, exotic species richness is negligibly influenced by human population density, but remains positively associated with native species richness. Main conclusions We provide one of the few demonstrations that energy availability exerts a strong positive influence on exotic species richness. Within our data, the positive relationship between exotic species richness and human population density probably arises because both variables increase with energy availability, and may be independent of the influence of human density on the probability of establishment. Positive correlations between exotic and native species richness remain when controlling for the influence of energy on species richness. The relevance of such a finding to the debate on the relationship between diversity and invasibility is discussed.     

Woody Encroachment Removal from Midwestern Oak Savannas Alters Understory Diversity across Space and Time

Recovering biodiversity is a common goal during restoration; however, for many ecosystems, it is not well understood how restoration influences species diversity across space and time. I examined understory species diversity and composition after woody encroachment removal in a large-scale savanna restoration experiment in central Iowa, United States. Over a 4-year time series, restoration had profound effects across space and time, increasing richness at local and site-level scales. Restoration sites had increased α (within sample) Simpson's diversity and α and γ (site level) species richness relative to control sites, although γ and β (among sample) Simpson's diversity, β richness, and α species evenness were not affected. Changes in richness were driven by graminoids at the α and γ scales and woody species (and some evidence for forbs) at the α scale. Interestingly, indicator species analysis revealed that at least some species from all functional groups were promoted by restoration, although no species were significant indicators of pre-treatment or control sites. Both savanna and nonsavanna species were indicators of restored sites. Restoration promoted exotic species at both scales, although species with spring phenologies were unaffected. Woody encroachment removal may be a means to promote species establishment in savannas; however, in this study, it resulted in establishment and proliferation of native and exotic and savanna and nonsavanna species. Future work might consider reintroduction of key savanna species to supplement those that have established. Work like this demonstrates the utility of restoration experiments for conducting research on large- and multiscale processes, such as species diversity.     

Scaling Disturbance Instead of Richness to Better Understand Anthropogenic Impacts on Biodiversity

A primary impediment to understanding how species diversity and anthropogenic disturbance are related is that both diversity and disturbance can depend on the scales at which they are sampled. While the scale dependence of diversity estimation has received substantial attention, the scale dependence of disturbance estimation has been essentially overlooked. Here, we break from conventional examination of the diversity-disturbance relationship by holding the area over which species richness is estimated constant and instead manipulating the area over which human disturbance is measured. In the boreal forest ecoregion of Alberta, Canada, we test the dependence of species richness on disturbance scale, the scale-dependence of the intermediate disturbance hypothesis, and the consistency of these patterns in native versus exotic species and among human disturbance types. We related field observed species richness in 1 ha surveys of 372 boreal vascular plant communities to remotely sensed measures of human disturbance extent at two survey scales: local (1 ha) and landscape (18 km²). Supporting the intermediate disturbance hypothesis, species richness-disturbance relationships were quadratic at both local and landscape scales of disturbance measurement. This suggests the shape of richness-disturbance relationships is independent of the scale at which disturbance is assessed, despite that local diversity is influenced by disturbance at different scales by different mechanisms, such as direct removal of individuals (local) or indirect alteration of propagule supply (landscape). By contrast, predictions of species richness did depend on scale of disturbance measurement: with high local disturbance richness was double that under high landscape disturbance.     

Ants and people: a test of two mechanisms potentially responsible for the large‐scale human population–biodiversity correlation for Formicidae in Europe

Aim: Recent coarse‐scale studies have shown positive relationships between the biodiversity of plants/vertebrates and the human population. Little is known about the generality of the pattern for invertebrates. Moreover, biodiversity and human population might correlate because they both covary with other factors such as energy availability and habitat heterogeneity. Here we test these two non‐mutually exclusive mechanisms with ant species‐richness data from the Fauna Europaea. Location Forty‐three European countries/regions. Methods We derived mixed models of total, native and exotic ant species richness as a function of human population size/density, controlling for country area, plant species richness (as a proxy for habitat heterogeneity), and mean annual temperature and precipitation (variables related to energy availability). Results Ant species richness increased significantly with increasing human population. This result was confirmed when controlling for variations in country area. Both for human population size/density and for ant species richness, there were positive correlations with temperature but not with precipitation. This finding is in agreement with the energy‐availability hypothesis. However, we observed a negative latitudinal gradient in ant and plant species richness, although not in human population size/density. Plant species richness was positively correlated with ant species richness but not with human population size/density. Thus, there is evidence that this type of habitat heterogeneity can play a role in the observed latitudinal gradient of ant species richness, but not in the positive correlation between ant species richness and human population. The results were confirmed for the 545 native and the 32 exotic ant species reported, and we observed a good correlation between exotic and native ant species richness. Main conclusions Ant species richness in European countries conforms to six macroecological patterns: (1) a negative latitudinal gradient; and a positive (2) species–energy relationship, (3) species–area relationship, (4) correlation with plant species richness, (5) exotic–native species richness correlation, and (6) species–people correlation. There is some evidence for the energy‐availability hypothesis, but little evidence for habitat heterogeneity as an explanation of the large‐scale human population–ant biodiversity correlation. This correlation has implications for the conservation of ant diversity in Europe.     

Multiscale Patterns of Riparian Plant Diversity and Implications for Restoration

Planning riparian restoration to resemble historic reference conditions requires an understanding of both local and regional patterns of plant species diversity. Thus, understanding species distributions at multiple spatial scales is essential to improve restoration planting success, to enhance long‐term ecosystem functioning, and to match restoration planting designs with historic biogeographic distributions. To inform restoration planning, we examined the biogeographic patterns of riparian plant diversity at local and regional scales within a major western U.S.A. drainage, California's Sacramento—San Joaquin Valley. We analyzed patterns of species richness and complementarity (β‐diversity) across two scales: the watershed scale and the floodplain scale. At the watershed scale, spatial patterns of native riparian richness were driven by herbaceous species, whereas woody species were largely cosmopolitan across the nearly 38,000 km² study area. At the floodplain scale, riparian floras reflected species richness and dissimilarity patterns related to hydrological and disturbance‐driven successional sequences. These findings reinforce the importance of concurrently evaluating both local and regional processes that promote species diversity and distribution of native riparian flora. Furthermore, as restoration activities become more prevalent across the landscape, strategies for restoration outcomes should emulate the patterns of species diversity and biogeographic distributions found at regional scales.     

Comparison of native and exotic distribution and richness models across scales reveals essential conservation lessons

Comparing native and exotic plant species distribution and richness models can help to reveal the causes of invasive exotic species proliferation and provide recommendations for preserving native‐dominated ecosystems. However, models may have limited applicability if potentially divergent patterns across scales, spatial autocorrelation and correspondence with community‐wide patterns such as species richness are not considered. I modeled the distributions of 20 dominant native and 20 dominant exotic species among and within patches in a heavily‐invaded and threatened ecosystem in western North America, examining the roles of scale and species origin on variable selection, spatial autocorrelation and model accuracy to determine conditions that favour native over exotic dominants, and derive recommendations for effective management. I also compared distribution models with native and exotic species richness models, to determine the extent to which dominant native and exotic species were representative of synoptic community patterns. Predictability was lower for exotic dominants, possibly because they are environmental generalists, and was lower within than among patches. Predictors were generally shared between distribution and richness models; however, species‐specific differences were common within both native and exotic species groups. Predictors for individual species across scales were frequently different and sometimes opposing. Distribution and richness models suggest that management assuming environmental affiliation at one scale may be ineffective at another; that site prioritization to maximize native versus exotic richness may not preserve the habitat of some common native species; and that intensive management to reduce exotics may be difficult due to low predictability and shared affiliations with natives. Comparing native and exotic distribution and richness models at two scales enabled scale‐specific conservation recommendations and elucidated trade‐offs between management for richness and representation that distribution models at an individual scale would not have allowed.     

Relationships among soil fertility,native plant diversity and exotic plant abundance inform restoration of forb‐rich eucalypt woodlands

Aim Water and nutrient availability are major limits to productivity in semi‐arid ecosystems; hence, ecological restoration often focuses on conserving or concentrating soil resources. By contrast, nutrient enrichment can promote invasion by exotic annuals, leading to restoration approaches that target reduction of soil nutrients. We aimed to explore potential biodiversity trade‐offs between these approaches by investigating relationships among soil nutrients, exotic annuals and native plant diversity and composition. In particular, we investigated the hypothesis that native plant diversity in semi‐arid to temperate woodlands reflects the productivity–diversity hypothesis, leading to hump‐backed relationships with soil nutrients such that (1) native plant diversity declines with increasing nutrient enrichment and (2) native diversity is limited at the lowest levels of soil fertility. Location Fragmented, long‐ungrazed Eucalyptus loxophleba subsp. loxophleba (York gum)–Acacia acuminata (jam) woodlands in the wheatbelt of South‐Western Australia. Methods We conducted stratified surveys of floristic composition and topsoil nutrient concentrations in 112 woodland patches. We used generalized linear models, structural equation models and ordinations to characterize relationships among soil nutrients, rainfall, exotic annuals and patch‐scale (100 m²) native plant composition and diversity. Results Patch‐scale native plant diversity declined strongly with increasing exotic abundance. This was partly related to elevated soil nutrient concentrations, particularly total nitrogen and available phosphorus. By contrast, there was little evidence for positive correlations between soil nutrients and native diversity, even at very low soil nutrient concentrations. Main conclusions Minimizing weed invasions is crucial for maximizing native plant diversity in E. loxophleba woodlands and could include nutrient‐depleting treatments without substantially compromising the functional capacity of soils to maintain native plant richness and composition. More broadly we emphasize that understanding relationships among ecosystem productivity, plant diversity and exotic invasions in the context of associated theoretical frameworks is fundamental for informing ecological restoration.     

A macroecological perspective of diversity patterns in the freshwater realm

1. The aim of this paper is to review literature on species diversity patterns of freshwater organisms and underlying mechanisms at large spatial scales. 2. Some freshwater taxa (e.g. dragonflies, fish and frogs) follow the classical latitudinal decline in regional species richness (RSR), supporting the patterns found for major terrestrial and marine organism groups. However, the mechanisms causing this cline in most freshwater taxa are inadequately understood, although research on fish suggests that energy and history are major factors underlying the patterns in total species and endemic species richness. Recent research also suggests that not all freshwater taxa comply with the decline of species richness with latitude (e.g. stoneflies, caddisflies and salamanders), but many taxa show more complex geographical patterns in across‐regions analyses. These complexities are even more profound when studies of global, continental and regional extents are compared. For example, clear latitudinal gradients may be present in regional studies but absent in global studies (e.g. macrophytes). 3. Latitudinal gradients are often especially weak in the across‐ecosystems analyses, which may be attributed to local factors overriding the effects of large‐scale factors on local communities. Nevertheless, local species richness (LSR) is typically linearly related to RSR (suggesting regional effects on local diversity), although saturating relationships have also been found in some occasions (suggesting strong local effects on diversity). Nestedness has often been found to be significant in freshwater studies, yet this pattern is highly variable and generally weak, suggesting also a strong beta diversity component in freshwater systems. 4. Both geographical location and local environmental factors contribute to variation in alpha diversity, nestedness and beta diversity in the freshwater realm, although the relative importance of these two groups of explanatory variables may be contingent on the spatial extent of the study. The mechanisms associated with spatial and environmental control of community structure have also been inferred in a number of studies, and most support has been found for species sorting (possibly because many freshwater studies have species sorting as their starting point), although also dispersal limitation and mass effects may be contributing to the patterns found. 5. The lack of latitudinal gradients in some freshwater taxa begs for further explanations. Such explanations may not be gained for most freshwater taxa in the near future, however, because we lack species‐level information, floristic and faunistic knowledge, and standardised surveys along extensive latitudinal gradients. A challenge for macroecology is thus to use the best possible species‐level information on well‐understood groups (e.g. fish) or use surrogates for species‐level patterns (e.g. families) and then develop hypotheses for further testing in the freshwater realm. An additional research challenge concerns understanding patterns and mechanisms associated with the relationships between alpha, beta and gamma components of species diversity. 6. Understanding the mechanistic basis of species diversity patterns should preferably be based on a combination of large‐scale macroecological and landscape‐scale metacommunity research. Such a research approach will help in elucidating patterns of species diversity across regional and local scales in the freshwater realm.     

Biotic and abiotic constraints to a plant invasion in vegetation communities of Tierra del Fuego

The biotic resistance theory relates invader success to species richness, and predicts that, as species richness increases, invasibility decreases. The relationship between invader success and richness, however, seems to be positive at large scales of analysis, determined by abiotic constraints, and it is to be expected that it is negative at small scales, because of biotic interactions. Moreover, the negative relationship at small scales would be stronger within species of the same functional group, because of having similar resource exploitation mechanisms. We studied the relationship between the cover of a worldwide invader of grasslands, Hieracium pilosella L., and species richness, species diversity and the cover of different growth forms at two different levels of analysis in 128 sites during the initial invasion process in the Fuegian steppe, Southern Patagonia, Argentina. At regional level, the invader was positively correlated to total (r = 0.28, P = 0.003), exotic (r = 0.273, P = 0.004), and native species richness (r = 0.210, P = 0.026), and to species diversity (r = 0.193, P = 0.041). At community level, we found only a weak negative correlation between H. pilosella and total richness (r = ?0.426, P = 0.079) and diversity (r = ?0.658, P = 0.063). The relationship between the invader and other species of the same growth form was positive both at regional (r = 0.484, P < 0.001) and community (r = 0.593, P = 0.012) levels. Consequently, in the period of establishment and initial expansion of this exotic species, our results support the idea that invader success is related to abiotic factors at large scales of analysis. Also, we observed a possible sign of biotic constraint at community level, although this was not related to the abundance of species of the same growth form.     

哈尔滨市主要城市绿地特征和耦合关系

采用样方法调查了哈尔滨市17块绿地的植物多样性,研究城市绿地生态系统中本土植物多样性对外来植物的影响规律。结果如下:哈尔滨市绿地有75种外来植物,外来栽培植物42种,外来非栽培植物33种。在哈尔滨市绿地植物群落中,外来栽培植物的平均盖度、平均重要值和平均出现频度均较高;外来非栽培种的平均盖度、平均重要值较低,而平均出现频度较高。绿地本土植物物种数与外来栽培种、外来非栽培种的物种数极显著正相关(P<0.01),与外来非栽培种的平均出现频度显著负相关(P<0.05)。绿地乔木层本土植物的香农威尔指数、物种丰富度与外来栽培种、外来非栽培种的物种数均显著负相关(P<0.05),乔木层本土植物的均匀度与外来非栽培种的平均出现频度极显著负相关(P<0.01);灌木层本土植物的物种丰富度与外来栽培种的平均重要值显著负相关(P<0.05);草本层本土植物的均匀度与外来非栽培种的平均相对盖度显著正相关(P<0.05)。随绿地总面积的增加,外来栽培种和外来非栽培种的物种数增加,平均出现频度均呈现显著降低的趋势。随建成时间增加,外来栽培种的平均出现频度呈非线形变化,外来非栽培种的平均出现频度呈现降低的趋势。应提高哈尔滨市绿地本土植物的多样性,严格控制新建成的、总面积较小的绿地中外来植物的引种栽培管理,以防止外来植物发展成为危害本地生态系统的入侵植物。     

From richer to poorer: successful invasion by freshwater fishes depends on species richness of donor and recipient basins

Evidence for the theory of biotic resistance is equivocal, with experiments often finding a negative relationship between invasion success and native species richness, and large‐scale comparative studies finding a positive relationship. Biotic resistance derives from local species interactions, yet global and regional studies often analyze data at coarse spatial grains. In addition, differences in competitive environments across regions may confound tests of biotic resistance based solely on native species richness of the invaded community. Using global and regional data sets for fishes in river and stream reaches, we ask two questions: (1) does a negative relationship exist between native and non‐native species richness and (2) do non‐native species originate from higher diversity systems. A negative relationship between native and non‐native species richness in local assemblages was found at the global scale, while regional patterns revealed the opposite trend. At both spatial scales, however, nearly all non‐native species originated from river basins with higher native species richness than the basin of the invaded community. Together, these findings imply that coevolved ecological interactions in species‐rich systems inhibit establishment of generalist non‐native species from less diverse communities. Consideration of both the ecological and evolutionary aspects of community assembly is critical to understanding invasion patterns. Distinct evolutionary histories in different regions strongly influence invasion of intact communities that are relatively unimpacted by human actions, and may explain the conflicting relationship between native and non‐native species richness found at different spatial scales.     

Tropical paradox: a multi-scale analysis of the invasion paradox within Miami Rock Ridge tropical hardwood hammocks

The invasion paradox describes the scale dependence of native-exotic richness relationships (NERRs), where NERRs are negative at neighborhood scales and positive at landscape scales. However, a lack of tropical surveys and past failures to isolate potential confounding variables contribute to significant gaps in our understanding of the processes producing these patterns. We surveyed the vascular flora of 13 tropical hardwood hammocks for community characteristics (e.g., native and exotic species richness, vegetative cover) with a hierarchical sampling design. Using model selection, we determined which variables best predicted patterns of exotic species richness at each spatial scale of consideration. We found that native and exotic species richness were positively correlated at neighborhood scales, but negatively correlated at landscape scales. The latter result stands in stark opposition to the patterns published in the literature thus far. We found that natural disturbance history (as approximated by vegetative cover) was positively correlated with exotic species richness at intermediate and landscape scales only. Overall, hammock identity was the most important factor driving exotic species richness patterns at all spatial scales. Hammocks with highly-disturbed hydrologies, brought about by water management, had fewer native species and more exotic species than hammocks with more natural hydrological conditions. Our results are among the first from examination of subtropical communities, and may support the hypothesis that tropical and subtropical communities are subject to more intense biotic interactions. However, given our unique sampling design, our results do not reject the hypothesis that environmental heterogeneity drives the relationship between native and exotic species richness patterns.     

Scale dependent effects of native prey diversity,prey biomass and natural disturbance on the invasion success of an exotic predator

There is growing support for the general notion that the drivers of invasion success often shift from biotic to abiotic factors with increasing spatial scale. Most of this research, however, has been conducted on a single trophic level; i.e. it has primarily looked at how the diversity of native competitors may influence invasion success. Less attention has been paid to understanding how native prey diversity may influence the invasion success of exotic predators and whether such biotic factors are scale-dependent. We used a hierarchical spatial survey of 17 stream communities to test whether native prey diversity, along with native prey biomass, algal resource abundance and annual stream discharge, influenced the abundance of an exotic crayfish predator, and whether the importance of these factors were scale-dependent. We used a hierarchical generalized linear model to evaluate the influence of these community and stream characteristics on exotic crayfish abundance at both the transect scale (1 m²) and the stream scale (400 m²). Our results indicated that at the stream scale, high stream discharge significantly limited invader abundance. However, at the smaller transect scale, native prey biomass was a significant driver of invasion success and positively correlated with invader abundance. We suggest that our results add to the emerging pattern that abiotic processes are stronger determinants of invasion success at large spatial scales, whereas biotic processes become more important with decreasing spatial scale. However, for predator invasions, prey biomass, not prey diversity may be a more important for driver of invasion success at small spatial scales.     

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.