Search for: All records

Many well‐supported hypotheses seek to explain drivers of nonnative species richness across spatial scales, but evidence for common patterns among regions and taxa remains inconclusive. This study investigates why consistent patterns are elusive by estimating and assessing cross‐scale interactions, wherein large‐scale factors contextualize patterns measured at smaller scales. We investigated whether local relationships of disturbance and native species richness with nonnative species richness are moderated by regional native gamma diversity. Regions with higher gamma diversity, we hypothesized, would be unfavorable to nonnative species due to high levels of competition and reduced niche availability, thus mediating local effects of native richness and disturbance on nonnative species richness. Using a fine resolution stream fish community dataset covering 159 regional watersheds in the conterminous United States during 2000–2023, we quantified cross‐scale interactions using a two‐level Bayesian hierarchical model. In the first level, we estimated the effects of disturbance and native richness on nonnative richness in local stream segments indexed by region. In the second level, we used this regional index to estimate cross‐scale interactions of native gamma diversity (regional‐level richness) on the first‐level relationships. Local nonnative richness was generally positively associated with native richness and disturbance. However, these relationships were reduced in regions with more diverse native stream fish assemblages. Thus, native gamma diversity provided an important mechanistic context for local nonnative richness relationships across regional watersheds through a negative cross‐scale interaction. As large spatial datasets become increasingly available, accounting for cross‐scale interactions in inter‐regional observational studies will be critical for understanding ecological relationships and may provide a predictive framework for studies with conflicting support for differing conceptual models. 

The abundant‐center hypothesis (ACH) provides a conceptual model for predicting range‐wide distributions of species abundance, suggesting that abundance peaks in the center of the geographic range and declines towards range edges. Empirical studies testing the ACH and its subsequent derivations predominantly occurred in terrestrial systems and reported mixed support. Moreover, none of these models consider the possibility of multiple geographic areas of elevated abundance (which we refer to as ‘abundant cores'). Naturally dispersal limited species may exhibit multiple abundant cores, requiring refinement of the ACH. We used fish species abundances from 29 206 community monitoring surveys and weighted geospatial kernel density estimation to identify the number of abundant cores for 64 freshwater fish species. We regressed the number of abundant cores against range size and body size to test if larger geographic distributions and body sizes contain more abundant cores than smaller distributions and body sizes. The two predictors are surrogates for evolutionary age and dispersal ability, respectively, because older species are generally associated with larger ranges, and large‐bodied fishes have greater dispersal ability than small‐bodied fishes in dendritic networks. For studied species, 43 exhibited multi‐core distributions, and 21 exhibited a single‐core distribution. Species range size, but not body size, was significantly and positively associated with the number of abundant cores. The ACH was not a good descriptor of the abundance patterns of most stream fishes we studied, suggesting that an abundant center model may not be well‐suited for freshwater fishes. Recent geo‐climatic events in evolutionary time have isolated populations of the same species by a matrix of unsuitable habitat and/or hard dispersal barriers, providing the basis for multi‐core distributions. Biogeographic and ecological mechanisms likely underpin observed multi‐core patterns, and our work indicates that the ACH and related concepts still present opportunities for testing and refinement. 

ABSTRACT AimEmpirical tests of conceptual hypotheses describing species invasions often differ depending on the spatial scale (spatial resolution and extent of study area) at which they were conducted. Some of this disparity may arise from tradeoffs in data quality necessitating the use of different indices of community invadedness among scales. Local‐scale studies typically use fine‐resolution, descriptive measures of community invadedness (‘dominance’, the proportion nonnative individuals) at limited spatial extents, while macroscale studies often aggregate datasets to cover large spatial extents but use coarser spatial resolution and less descriptive indices (nonnative species richness). We investigated the consequences of using different indices to represent community invadedness at different spatial scales, and explored the implications for hypothesis testing when nonnative richness and dominance are not related. Location23,793 stream segments within 17 regional watersheds, conterminous United States. Time Period2000–2023. Major Taxa StudiedFreshwater fishes. MethodsUsing a large‐extent, fine‐resolution dataset, we evaluated the correlation between nonnative species richness and dominance in communities, and compared empirical support for prominent invasion hypotheses (biotic resistance, disturbance facilitation) in identical Bayesian hierarchical models with community invadedness represented by each metric. ResultsNonnative richness and dominance were weakly correlated, allowing us to classify communities into four archetypes based on relationships between the two indices. Empirical support for both invasion hypotheses differed between the two indices of community invadedness both overall and within regional watersheds. Main ConclusionsNonnative species richness and dominance describe different facets of the invasion process and may under‐ or over‐represent community invadedness when considered alone. Empirical disparity between models estimating the two metrics may be an important source of scale‐dependent inference in invasion ecology. When assembling datasets for macroscale studies, retaining fine spatial resolution as much as possible will allow researchers opportunities to use more descriptive and potentially complementary indices of community invadedness. 

ABSTRACT Scale‐Dependency of Native StatusClassifying populations as native or nonnative requires well‐defined range boundaries for species. While many studies define native status according to large biogeographic realms, natural dispersal barriers often limit species distributions at regional or smaller spatial extents. As such, native/nonnative definitions are inherently scale‐dependent and estimates of community invadedness thus depend on the spatial resolution at which native status is defined. For example, nonnative species can be introduced among realms, among regions within realms, and among ecological provinces within regions (hereafter, simply “provinces”). By explicitly considering the scale‐dependency of native/nonnative status definitions, we can more effectively compare results across studies, more comprehensively evaluate the degree of invasion levels, and more objectively communicate the native status of a species. Location30,034 stream segments, conterminous United States. Time Period2000–2023. Major Taxa StudiedFreshwater fishes. Quantifying Fish Community Invadedness Across<styled-content style='fixed-case'>US</styled-content>StreamsWe illustrate the importance of scale‐dependent native status definitions by quantifying nonnative species richness and relative abundance in stream fish communities across the United States, finding that provincially nonnative species are nearly four times as prevalent as extra‐realm nonnative species, and represented approximately 10% of all individuals in average community surveys. ImplicationsUnrealistically broad native status definitions underestimate community invadedness. Dismissing regionally and provincially nonnative species can have severe ecological consequences, including displacement and hybridisation with native species and the loss of unique communities through biotic homogenisation. These consequences may undermine efforts to maintain and protect distinct local biodiversity and conserve endemic species. 

ABSTRACT AimInvasion ecology is replete with a body of well‐supported yet contradictory evidence for numerous invasion hypotheses, likely as a result of context dependency. Context dependency in invasion studies can arise in two ways: (1)apparent, when results differ between studies solely due to methodical differences, or (2)mechanistic, when results truly differ due to ecological processes. One form of apparent context dependency occurs when causally linked factors associated with invasion success (hereafter,invasion drivers) either mask or enhance each other's effect on invasion success. Mechanistic context dependency can occur when regional scale processes modify the influence of local scale invasion processes. Together, apparent and mechanistic context dependency likely give rise to conflicting support between invasion hypotheses via confounding effects of causally related invasion drivers and region‐specific invasion processes. Location2339 stream segments in two ecoregions of the United States. MethodsUsing local scale stream fish community data for two distinct ecoregions, we constructed identical path models to estimate the direct and indirect effects of invasion drivers on nonnative richness. We chose one variable to index invasion drivers from each of the following categories: propagule pressure, natural abiotic, anthropogenic abiotic and biotic factors. ResultsWe found evidence of apparent context dependency through the presence of indirect effects, in which the effects of propagule pressure and biotic factors on nonnative richness were modulated by abiotic factors. The indirect effects of invasion drivers differed between both regions, providing evidence of mechanistic context dependency. Main ConclusionsApparent and mechanistic context dependency can lead to conflicting evidence between studies of invasion hypotheses. Accounting for indirect effects of invasion drivers is important in gaining a more general understanding of the invasion process. Furthermore, because indirect effects varied regionally, it is important to understand the large‐scale processes that contextualise local invasion processes. 

Characterizing the population density of species is a central interest in ecology. Eastern North America is the global hotspot for biodiversity of plethodontid salamanders, an inconspicuous component of terrestrial vertebrate communities, and among the most widespread is the eastern red-backed salamander,Plethodon cinereus. Previous work suggests population densities are high with significant geographic variation, but comparisons among locations are challenged by lack of standardization of methods and failure to accommodate imperfect detection. We present results from a large-scale research network that accounts for detection uncertainty using systematic survey protocols and robust statistical models. We analysed mark–recapture data from 18 study areas across much of the species range. Estimated salamander densities ranged from 1950 to 34 300 salamanders ha⁻¹, with a median of 9965 salamanders ha⁻¹. We compared these results to previous estimates forP. cinereusand other abundant terrestrial vertebrates. We demonstrate that overall the biomass ofP. cinereus, a secondary consumer, is of similar or greater magnitude to widespread primary consumers such as white-tailed deer (Odocoileus virginianus) andPeromyscusmice, and two to three orders of magnitude greater than common secondary consumer species. Our results add empirical evidence thatP. cinereus, and amphibians in general, are an outsized component of terrestrial vertebrate communities in temperate ecosystems.