Richard D. Stevens

Compensatory dynamics are rare in natural ecological communities

In population ecology, there has been a fundamental controversy about the relative importance of competition-driven (density-dependent) population regulation vs. abiotic influences such as temperature and precipitation. The same issue arises at the community level; are population sizes driven primarily by changes in the abundances of cooccurring competitors (i.e., compensatory dynamics), or do most species have a common response to environmental factors? Competitive interactions have had a central place in ecological theory, dating back to Gleason, Volterra, Hutchison and MacArthur, and, more recently, Hubbells influential unified neutral theory of biodiversity and biogeography. If competitive interactions are important in driving year-to-year fluctuations in abundance, then changes in the abundance of one species should generally be accompanied by compensatory changes in the abundances of others. Thus, one necessary consequence of strong compensatory forces is that, on average, species within communities will covary negatively. Here we use measures of community covariance to assess the prevalence of negative covariance in 41 natural communities comprising different taxa at a range of spatial scales. We found that species in natural communities tended to covary positively rather than negatively, the opposite of what would be expected if compensatory dynamics were important. These findings suggest that abiotic factors such as temperature and precipitation are more important than competitive interactions in driving year-to-year fluctuations in species abundance within communities.

The latitudinal gradient in niche breadth: concepts and evidence.

We examine Robert MacArthur’s hypothesis that niche breadth is positively associated with latitude (the latitude–niche breadth hypothesis). This idea has been influential and long standing, yet no studies have evaluated its generality or the validity of its assumptions. We review the theoretical arguments suggesting a positive relationship between niche breadth and latitude. We also use available evidence to evaluate the assumptions and predictions of MacArthur’s latitude–niche breadth hypothesis. We find that neither the assumptions nor the predictions of the hypothesis are supported by data. We propose an alternative hypothesis linking latitude with niche breadth. Unlike previous ideas, our conceptual framework does not require equilibrial assumptions and is based on recently uncovered patterns of species interactions.

GEOGRAPHICAL ECOLOGY AT THE COMMUNITY LEVEL: PERSPECTIVES ON THE DIVERSITY OF NEW WORLD BATS

The ubiquity of the latitudinal gradient of species richness is well documented at coarse scales of resolution, but the extent to which the pattern is recapitulated at the level of local communities for any aspect of biodiversity (i.e., richness, evenness, or di- versity) is unclear. We examined how attributes of New World bat diversity vary with each other and with latitude at two scales of resolution: local communities and regional species pools. We calculated 14 indices of diversity (species richness (three), evenness (four), dominance (three), and diversity (four)) from species abundance distributions for 32 in- tensively sampled local sites between 42.258 N and 24.128 S latitude. The species richness of each corresponding regional pool was estimated from published range maps. In general, the gradient of local species richness was less steep than the corresponding gradient of regional species richness; beta diversity in the tropics is greater than that for temperate communities. All aspects of diversity at the local scale did not vary with latitude in the same manner. The latitudinal gradient in species diversity of local communities was pri- marily a consequence of the corresponding gradient in species richness. Local richness increased and became more variable with decreasing latitude. In contrast, species evenness did not vary in a systematic fashion with latitude. Although the absolute number of rare species in communities increased faster with latitude than did that of common species, both abundance classes proportionately increased with latitude in equivalent ways throughout the New World. In general, latitudinal variation at the community level was detected in diversity indices that were insensitive to the abundance of species. The dramatic increase in species richness at broad scales of resolution toward the tropics (gamma diversity) was as much a consequence of increased richness at the local level (alpha diversity) as it was a consequence of the latitudinal increase in species turnover among communities (beta diversity). Future theoretical research should examine the correlates of latitude that enhance differentiation among communities at low latitudes (i.e., those which enjoy high produc- tivities). Conservation strategies based on assessments of diversity at coarse levels of resolution (gamma diversity) should be implemented with caution because beta diversity inflates regional estimates of diversity.

Historical processes enhance patterns of diversity along latitudinal gradients

One of the more vexing issues in ecology is how historical processes affect contemporary patterns of biodiversity. Accordingly, few models have been presented. Two corollary models (centre of origin, time-for-speciation) can be used to make quantitative predictions characterizing the tropical niche conservatism hypothesis and describe diversification as diffusion and subsequent cladogenesis of species away from the place of origin of a higher taxon in the tropics. Predictions derived from such models are: (i) species richness declines toward the periphery of the range of a higher taxon; (ii) taxa are more derived toward the periphery than the centre; (iii) ages of taxa are lower toward the periphery than the centre; and (iv) ages and measures of derivedness are less variable toward the periphery of the range of a higher taxon. I tested these predictions to better understand the formation of one of the most ubiquitous patterns of biodiversity—the latitudinal gradient in species richness. Results indicate well-supported predictions for New World leaf-nosed bats and that diversification has had strong influences on latitudinal gradients of species richness. A better understanding of how evolutionary diversification of taxa contributes to formation of patterns of species richness along environmental gradients is necessary to fully understand spatial variation in biodiversity.

Elements of metacommunity structure of Paraguayan bats: multiple gradients require analysis of multiple ordination axes

Techniques to evaluate elements of metacommunity structure (EMS; coherence, species turnover and range boundary clumping) have been available for several years. Such approaches are capable of determining which idealized pattern of species distribution best describes distributions in a metacommunity. Nonetheless, this approach rarely is employed and such aspects of metacommunity structure remain poorly understood. We expanded an extant method to better investigate metacommunity structure for systems that respond to multiple environmental gradients. We used data obtained from 26 sites throughout Paraguay as a model system to demonstrate application of this methodology. Using presence–absence data for bats, we evaluated coherence, species turnover and boundary clumping to distinguish among six idealized patterns of species distribution. Analyses were conducted for all bats as well as for each of three feeding ensembles (aerial insectivores, frugivores and molossid insectivores). For each group of bats, analyses were conducted separately for primary and secondary axes of ordination as defined by reciprocal averaging. The Paraguayan bat metacommunity evinced Clementsian distributions for primary and secondary ordination axes. Patterns of species distribution for aerial insectivores were dependent on ordination axis, showing Gleasonian distributions when ordinated according to the primary axis and Clementsian distributions when ordinated according to the secondary axis. Distribution patterns for frugivores and molossid insectivores were best described as random. Analysis of metacommunities using multiple ordination axes can provide a more complete picture of environmental variables that mold patterns of species distribution. Moreover, analysis of EMS along defined gradients (e.g., latitude, elevation and depth) or based on alternative ordination techniques may complement insights based on reciprocal averaging because the fundamental questions addressed in analyses are contingent on the ordination technique that is employed.

Relative effects of time for speciation and tropical niche conservatism on the latitudinal diversity gradient of phyllostomid bats

Determinants of contemporary patterns of diversity, particularly those spanning extensive latitudinal gradients, are some of the most intensely debated issues in ecology. Recently, focus has shifted from a contemporary environmental perspective to a historical one in an attempt to better understand the construction of latitudinal gradients. Although the vast majority of research on historical mechanisms has focused on tropical niche conservatism (TNC), other historical scenarios could produce similar latitudinal gradients. Herein, I formalize predictions to distinguish between two such historical processes—namely time for speciation (TFS) and TNC—and test relative support based on diversity gradients of New World bats. TFS and TNC are distinctly spatial and environmental mechanisms, respectively. Nonetheless, because of the way that environmental characteristics vary spatially, these two mechanisms are hard to distinguish. Evidence provided herein suggests that TNC has had a more important effect than TFS in determining diversity gradients of New World bats. Indeed, relative effects of different historical mechanisms, as well as relative effects of historical and contemporary environmental determinants, are probably context-dependent. Future research should move away from attempting to identify the mechanism with primacy and instead attempt to understand the particular contexts in which different mechanisms have greater influence on diversity gradients.

Size assortment in new world bat communities

We evaluated ubiquity of morphological overdispersion, which is characteristic of competitive interactions, in feeding guilds from 15 bat communities from North, Central, and South America. Overdispersion was detected in 10 of 15 communities and 4 of 5 feeding guilds. Although overdispersed morphological patterns exist, they are not a consistent at-tribute of the structure of feeding guilds within bat communities. Because of factors ranging from environmental attributes such as heterogeneity to species-specific attributes such as high mobility, morphological overdispersion in bats may at best only be a transitory out-come of the effect of competitive interactions on community structure.

Survey Sequencing Reveals Elevated DNA Transposon Activity, Novel Elements, and Variation in Repetitive Landscapes among Vesper Bats

The repetitive landscapes of mammalian genomes typically display high Class I (retrotransposon) transposable element (TE) content, which usually comprises around half of the genome. In contrast, the Class II (DNA transposon) contribution is typically small (<3% in model mammals). Most mammalian genomes exhibit a precipitous decline in Class II activity beginning roughly 40 Ma. The first signs of more recently active mammalian Class II TEs were obtained from the little brown bat, Myotis lucifugus, and are reflected by higher genome content (∼5%). To aid in determining taxonomic limits and potential impacts of this elevated Class II activity, we performed 454 survey sequencing of a second Myotis species as well as four additional taxa within the family Vespertilionidae and an outgroup species from Phyllostomidae. Graph-based clustering methods were used to reconstruct the major repeat families present in each species and novel elements were identified in several taxa. Retrotransposons remained the dominant group with regard to overall genome mass. Elevated Class II TE composition (3–4%) was observed in all five vesper bats, while less than 0.5% of the phyllostomid reads were identified as Class II derived. Differences in satellite DNA and Class I TE content are also described among vespertilionid taxa. These analyses present the first cohesive description of TE evolution across closely related mammalian species, revealing genome-scale differences in TE content within a single family.

COMPARATIVE COMMUNITY ECOLOGY OF BATS FROM EASTERN PARAGUAY: TAXONOMIC, ECOLOGICAL, AND BIOGEOGRAPHIC PERSPECTIVES

Abstract Based on more than a year and a half of intensive fieldwork, we document the community structure of bats at 2 sites (Reserva Natural del Bosque Mbaracayú and Yaguareté Forests) in eastern Paraguay. Twenty-nine species from 17 genera and 3 families were documented based on more than 7,000 captures. In general, these communities share greatest affinities with either Atlantic rainforest or savanna-grassland sites of South America. Significant differences exist between these communities in terms of structure based on rank-abundance distributions and species composition. Moreover, Yaguareté exhibits significantly more species than does Mbaracayú even after removing the effects of differences due to high-flying insectivores. No significant differences existed between communities regarding other aspects of taxonomic diversity (e.g., evenness, diversity, or dominance of species) or any aspect of functional diversity. A higher degree of habitat heterogeneity at Yaguareté likely underlies these differences in community structure.

Phylogenetic structure illuminates the mechanistic role of environmental heterogeneity in community organization

1. Diversity begets diversity. Numerous published positive correlations between environmental heterogeneity and species diversity indicate ubiquity of this phenomenon. Nonetheless, most assessments of this relationship are phenomenological and provide little insight into the mechanism whereby such positive association results. 2. Two unresolved issues could better illuminate the mechanistic basis to diversity begets diversity. First, as environmental heterogeneity increases, both productivity and the species richness that contributes to that productivity often increase in a correlated fashion thus obscuring the primary driver. Second, it is unclear how species are added to communities as diversity increases and whether additions are trait based. 3. We examined these issues based on 31 rodent communities in the central Mojave Desert. At each site, we estimated rodent species richness and characterized environmental heterogeneity from the perspectives of standing primary productivity and number of seed resources. We further examined the phylogenetic structure of communities by estimating the mean phylogenetic distance (MPD) among species and by comparing empirical phylogenetic distances to those based on random assembly from a regional species pool. 4. The relationship between rodent species diversity and environmental heterogeneity was positive and significant. Moreover, diversity of resources accounted for more unique variation than did total productivity, suggesting that variety and not total amount of resource was the driver of increased rodent diversity. Relationships between environmental heterogeneity and phylogenetic distance were negative and significant; species were significantly phylogenetically over-dispersed in communities of low environmental heterogeneity and became more clumped as environmental heterogeneity increased. 5. Results suggest that species diversity increases with environmental heterogeneity because a wider variety of resources allow greater species packing within communities.