Native-exotic Richness Relationships Research Articles

Scale dependency in native-exotic richness relationships revisited.

In their seminal paper, Shea and Chesson (Trends in Ecology & Evolution, 2002, 17, 170) developed a highly cited model (S&C model) showing scale dependency in the native–exotic richness relationships. Two decades later, extensive additional data have been accumulated, leading to new findings and insights. Accordingly, two updates were made here to the original S&C model: (1) changing the “negative” richness relationship between natives and exotics to “non‐consistent” or “non‐significant”; and (2) modifying the original diagram to correctly represent native and exotic species richness and their correlations across both small and large scales.

Native-exotic richness relationships in second-growth forests differ along a gradient of land-use history

Native-exotic richness relationships in second-growth forests differ along a gradient of land-use history

Soil nutrients and variation in biomass rather than native species richness influence introduced plant richness in a semi-arid grassland

Several different hypotheses account for the success of introduced species in new environments. Experimental studies show a negative native-exotic richness relationship (NERR), while observational studies suggest that this relationship is usually positive. Increased resource availability and environmental variation can also enable introduced species to establish in new environments. We conducted an observational study in a semi-arid grassland in the Thompson-Nicola District of British Columbia to examine the biotic and abiotic factors that account for variation in introduced and native species richness.In each of 12 sites, an 8 × 8 m area was set up, containing 64, 1-m2 plots. We identified and categorized plant species in each site into introduced and native species. We tested the relationship between introduced species richness and native species richness at the 1-m2 sampling grain and at sampling grains up to 64 m2. We also analysed the relationship between native and introduced species, and within-plot biomass, and between native and introduced species and variation in biomass. For a representative subset of four sites, we tested the relationship between introduced and native species richness and nitrogen, phosphorus and potassium.We found no NERR at the 1 m2 sampling grain, nor for the other sampling grains up to 64 m2. Introduced species richness increased with phosphorus and nitrogen availability, and was also positively related to biomass heterogeneity.Our results indicate that introduced species richness in these grasslands is likely influenced by phosphorus and nitrogen, and by variation in vegetation biomass, but not by native species. More non-native plants are likely to occupy nutrient-rich plots compared to nutrient-poor plots in these grasslands. Variation in biomass can leave gaps for the establishment of introduced species. These results should inform management considerations for the control of invasive species to optimize preservation of grasslands.

Native-exotic diversity relationships for Eastern Mediterranean fishes reveal a weak pattern of interactions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 611:215-220 (2019) - DOI: https://doi.org/10.3354/meps12871 NOTE Native-exotic diversity relationships for Eastern Mediterranean fishes reveal a weak pattern of interactions Yehezkel Buba1,*, Jonathan Belmaker1,2 1School of Zoology, Tel Aviv University, Tel Aviv 6997801, Israel 2Steinhardt Museum of Natural History, Tel Aviv University, Tel Aviv 6997801, Israel *Corresponding author: hezibuba@mail.tau.ac.il ABSTRACT: A critical question in invasion ecology is how the native species community and the exotic species affect one another. It is commonly postulated that exotics negatively impact the native community’s biodiversity, and that a rich native community can inhibit the establishment of exotics. This hypothesis can be examined using the native-exotic richness relationship (NERR), where negative impacts would be reflected by negative correlations. We examined this hypothesis in the Mediterranean Sea, which is subject to an influx of Red Sea species, making it an invasion hotspot. We tested the potentially important, yet neglected, temporal dynamics of the NERR associated with increases in the richness of exotics over time. We used data from fish bottom trawl surveys conducted in the Eastern Mediterranean over 2 time periods, spanning 2 decades, characterized by a considerable increase in numbers of exotic species. While exotic richness in the first time period was uncorrelated to that of natives, a negative NERR was observed in the second time period. However, this pattern disappeared once we controlled for depth. Results did not change when we used a diversity index instead of richness alone or changed the scale of the analyses. Together, these results suggest that for fish communities over soft substrates, native species diversity does not strongly affect exotic species diversity, which hints at weak interactions between natives and exotics. KEY WORDS: Native-exotic richness relationship · Biological invasions · Mediterranean Sea · Lessepsian migration · Fish · Native Mediterranean fish Full text in pdf format Supplementary material PreviousCite this article as: Buba Y, Belmaker J (2019) Native-exotic diversity relationships for Eastern Mediterranean fishes reveal a weak pattern of interactions. Mar Ecol Prog Ser 611:215-220. https://doi.org/10.3354/meps12871 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 611. Online publication date: February 14, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Correlation of native and exotic species richness: a global meta-analysis finds no invasion paradox across scales.

Support for the “biotic resistance hypothesis,” that species‐rich communities are more successful at resisting invasion by exotic species than are species‐poor communities, has long been debated. It has been argued that native–exotic richness relationships (NERR) are negative at small spatial scales and positive at large scales, but evidence for the role of spatial scale on NERR has been contradictory. However, no formal quantitative synthesis has previously examined whether NERR is scale‐dependent across multiple studies, and previous studies on NERR have not distinguished spatial grain and extent, which may drive very different ecological processes. We used a global systematic review and hierarchical mixed‐effects meta‐analysis to provide a comprehensive quantitative assessment of the patterns of NERR over a range of spatial grain sizes and spatial extents, based on 204 individual cases of observational (non‐experimental) NERRs from 101 publications. We show that NERR was indeed highly scale dependent across studies and increased with the log of grain size. However, mean NERR was not negative at any grain size, although there was high heterogeneity at small grain sizes. We found no clear patterns of NERR across different spatial extents, suggesting that extent plays a less important role in determining NERR than does grain, although there was a complex interaction between extent and grain size. Almost all studies on NERR were conducted in North America, western Europe, and a few other regions, with little information on tropical or Arctic regions. We did find that NERR increased northward in temperate regions and also varied with longitude. We discuss possible explanations for the patterns we found, and caution that our results do not show that invasive species are benign or have no negative consequences for biodiversity preservation. This study represents the first global quantitative analysis of scale‐based NERR, and casts doubt on the existence of an “invasion paradox” of negative NERR at small scales and positive correlations at large scales in non‐experimental studies.

Regional climate and local-scale biotic acceptance explain native-exotic richness relationships in Australian annual plant communities.

Native and exotic species richness is expected to be negatively related at small spatial scales where individuals interact, and positive at larger spatial scales as a greater variety of habitats are sampled. However, a range of native-exotic richness relationships (NERRs) have been reported, including positive at small scales and negative at larger scales. We present a hierarchical metacommunity framework to explain how contrasting NERRs may emerge across scales and study systems, and then apply this framework to NERRs in an invaded winter annual plant system in southwest Western Australia. We analysed NERRs at increasing spatial scales from neighbourhoods (0.09 m2) to communities (225 m2) to metacommunities (greater than 10 ha) within a multilevel structural equation model. In contrast to many previous studies, native and exotic richness were positively related at the neighbourhood scale and were not significantly associated at larger scales. Heterogeneity in soil surface properties was weakly, but positively, associated with native and exotic richness at the community scale. Metacommunity exotic richness increased strongly with regional temperature and moisture availability, but relationships for native richness were negative and much weaker. Thus, we show that neutral NERRs can emerge at larger scales owing to differential climatic filtering of native and exotic species pools.

The native-exotic species richness relationship varies with spatial grain of measurement and environmental conditions.

Biological invasions can have dramatic impacts on communities and biodiversity, and are critical considerations in conservation and management decisions. We present a novel analysis to determine how exotic species success varies with community richness and scale of measurement. Using 5,022 plots representing natural vegetation of the Carolinas, we calculated native and exotic species richness of all vascular plants at five grain sizes. To avoid spatial pseudoreplication, we randomly selected unique subplots from each larger plot, re-selecting 100 times to develop an empirical distribution of the native-exotic richness relationship (NERR). Because observed NERRs vary with spatial scale, we developed separate scale-specific null-model distributions to compare to the empirical data. For each spatial scale, we compared the empirical distribution of 100 slopes to the null distribution containing 99 permutations of species origin per empirical slope. We also analyzed the dataset according to broad assignments corresponding to environmental conditions, using the formation type assigned to each community. The plots followed across most scales the general trend that exotic richness increases with native richness. At the smallest scale, however, the NERR was negative. The slope of the NERR is significantly higher than the null model at the largest observed scale and significantly lower than the null model at the smallest two observed scales. The NERR for most formations follows the general pattern with scale for the entire dataset. Warm temperate forests expressed essentially 0 slope at the largest spatial grain, decreasing to a negative relationship at 1m2 and smaller. Temperate freshwater marshes and wet meadows and shrublands expressed a positive relationship at all spatial grains, demonstrating that unique environmental and biogeographic conditions differentially affect exotic species. Further, these results indicate that exotic species are unevenly distributed across natural communities and that community assembly processes vary with scale.

Native-exotic richness relationships: a biogeographic approach using turnover in island plant populations.

Spatial variation in exotic species richness is often correlated with native species richness, for reasons that are poorly understood. To better understand the mechanisms underpinning native-exotic richness relationships, I quantified the colonization and extinction of 18 exotic and 16 native plant species on 39 small islands located off the coast of New Zealand for 8 consecutive yr. Results revealed a positive native-exotic richness relationship, which could be explained by similar demographic responses of native and exotic species to island area. However, native and exotic species showed subtle differences in their response to other island attributes. Turnover in native species declined with island isolation, whereas turnover in exotic species increased with the exposure of islands to ocean-borne disturbances. Overall results illustrate how long-term observations of species turnover can be used to better understand the mechanisms underpinning native-exotic richness relationships, and demonstrate that large, exposed islands can be especially susceptible to invasions by exotic species.

An invasive exotic grass reduced sedge meadow species richness by half

A debate over the impacts of invasive plants is fueled by varied terminology, ecosystem types, response metrics, and conflicting evidence across scales. We quantify the effects of invading Phalaris arundinacea (Pa) on species richness and abundance in Wisconsin sedge meadows dominated by Carex stricta (Cs). Among species area curves from four nested plots (0.25, 1, 4, and 16 m2) in seven sites, Pa stands averaged 48 % fewer species than the adjacent Cs. Species richness was reduced by about half regardless of plot size. Total richness for Pa stands was 50 species compared to 83 for Cs, and Pa stands had consistently lower additive percent cover of subordinate species at all scales. However, resident species responded differentially to Pa invasion. Only one species, Calamagrostis canadensis (a tall clonal grass), had similar importance values (using cover and frequency) in Cs and Pa stands at 0.25 and 16-m2 scales. Other species declined with Pa invasion, mostly in frequency. Despite losing half of their resident species, sites did not differ significantly in their native exotic richness relationship (NERR). NERR confounds the effects of invasiveness with species’ origins, and it under-represents impacts of dominant invaders in wetlands. Debates should be fewer if studies specify if the invader is exotic or native, type of invaded ecosystem, the metric used to assess responses, and the spatial scale.

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.

Multi-scale analyses of exotic and native plant species diversity within Canada’s endangered antelope-brush ecosystem

Native species richness is often significantly correlated with exotic species richness, but the direction and strength of this so-called native-exotic richness relationship (NERR) differs among studies. Furthermore, the potential influence of biotic and abiotic co-variates is less commonly considered. Our objectives were to determine (i) how two different components of scale, grain, and extent, influence the NERR, and (ii) if and how biotic and abiotic variables moderate the NERR. We addressed these objectives using data collected at 37 sites within the endangered Purshia tridentata Pursh (DC) (antelope-brush) shrub-steppe grasslands in southern British Columbia, Canada. We found a significantly positive NERR at our finest grain (1 m2), but no significant NERR at any of the three broader grains (10, 100, and 1,000 m2). Changing the extent of analysis did not affect the NERR. A multi-model inference approach revealed, at fine grains, (i) strong support for native species richness as a positive predictor of exotic species richness, (ii) Bromus tectorum L. (cheatgrass) as an important negative predictor of native species richness, (iii) foliar cover of P. tridentata as an important positive predictor of exotic species cover, and (iv) a limited predictive role for soil properties. We speculate that spatial patterning in the importance of facilitation relative to competition could influence the NERR within semi-arid ecosystems such as ours. Last, to facilitate comparative research and meta-analyses, we highlight the need for future research to be more explicit in its description of scale, and to more clearly define the grain and extent of each analysis.

Biotic and abiotic influences on native and exotic richness relationship across spatial scales: favourable environments for native species are highly invasible

Summary 1. Biological invasions threaten biodiversity, and understanding the factors that influence a community’s susceptibility to invasion informs both management of invasive species and conservation efforts towards promoting biodiversity. 2. In this study, we examined the native‐exotic richness relationships (NERR) at two spatial scales and asked what variables mediate the relationship between native and exotic species richness in 1-m 2 plots among old fields. We also tested whether the favourable environment and spatial heterogeneity mechanisms alter the relationship between native and exotic richness. Additionally, we identified biotic, abiotic and landscape factors that accounted for patterns of exotic species richness across spatial scales, from 50-m transects (i.e. local scales) to entire old fields (i.e. landscape scales). 3. We found that native and exotic richness were positively related at both local and landscape spatial scales, with the strength of their relationship greater at the landscape scale. In old-field communities with lower foliar cover, native and exotic plant richness were negatively related across 1-m 2 plots, whereas in old fields with higher foliar cover, native and exotic plant richness were positively related across 1-m 2 plots. Overall, predictors of exotic species richness depended on spatial scale: at local scales, native plant richness, light and landscape factors accounted for most of the variation, but biotic factors alone accounted for most of the variation at the landscape scale. 4. Our findings suggest that fields favourable for native species are also suitable for exotic species, and management efforts towards exotic richness should be tailored to spatial scale.

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

AbstractAim 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.

Scale, disturbance and productivity control the native-exotic richness relationship

The relationship between native and exotic species richness may be highly context-dependent. Spatial scale, including both plot size (grain) and study area (extent), is likely to influence this relationship, as are environmental conditions such as resource availability and disturbance intensity. We used experimental manipulations of soil fertility and disturbance in a California coastal grassland to directly examine the sensitivity of the native–exotic richness relationship (NERR) to these factors across five grain sizes and two spatial extents. The slope of the NERR was a function of grain size, extent and treatment. Over small spatial extents, native and exotic richness were usually uncorrelated. Across a larger extent, NERRs were negative in control plots, neutral in disturbance plots, and positive in plots with experimentally reduced soil fertility. These patterns were strongest for small grain sizes. We verify the importance of spatial grain in determining the NERR, and emphasize the role of spatial extent.

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.

THE INVASION PARADOX: RECONCILING PATTERN AND PROCESS IN SPECIES INVASIONS

The invasion paradox describes the co-occurrence of independent lines of support for both a negative and a positive relationship between native biodiversity and the invasions of exotic species. The paradox leaves the implications of native-exotic species richness relationships open to debate: Are rich native communities more or less susceptible to invasion by exotic species? We reviewed the considerable observational, experimental, and theoretical evidence describing the paradox and sought generalizations concerning where and why the paradox occurs, its implications for community ecology and assembly processes, and its relevance for restoration, management, and policy associated with species invasions. The crux of the paradox concerns positive associations between native and exotic species richness at broad spatial scales, and negative associations at fine scales, especially in experiments in which diversity was directly manipulated. We identified eight processes that can generate either negative or positive native-exotic richness relationships, but none can generate both. As all eight processes have been shown to be important in some systems, a simple general theory of the paradox, and thus of the relationship between diversity and invasibility, is probably unrealistic. Nonetheless, we outline several key issues that help resolve the paradox, discuss the difficult juxtaposition of experimental and observational data (which often ask subtly different questions), and identify important themes for additional study. We conclude that natively rich ecosystems are likely to be hotspots for exotic species, but that reduction of local species richness can further accelerate the invasion of these and other vulnerable habitats.