Disentangling the contributions of dispersal limitation, ecological drift, and ecological filtering to wild bee community assembly

Impacts of nutrient reduction on temporal β-diversity of rotifers: A 19-year limnology case study on Lake Wuli, China

There have been many studies on the effects of eutrophication on beta diversity (β-diversity) of species assemblages. However, few studies have focused on the effects of nutrient reduction on β-diversity and community structure, and long-time series analyses are particularly scarce. We conducted a 19-year case study on the impacts of management intervention on the temporal β-diversity of aquatic grazers in a lake at the Yangtze River Basin. In our study, we compared the changes in temporal β-diversity as well as its two components, nestedness and turnover, and the synchrony of the rotifer community after management intervention. Our results showed that while the abundance of some sensitive species increased, there was no trend in species richness. Moreover, both the seasonality and interannual stabilities of rotifer assemblages increased. The species synchrony decreased in both spring and summer after management intervention. We also found that management intervention significantly reduced nutrient concentrations but not water clarity and phytoplankton abundance. The total nitrogen (TN): total phosphorous (TP) ratio was reduced after management intervention, causing an increase in the abundance of cyanobacteria that may contribute to the increase of rotifer synchrony in autumn. Our results imply that stable environmental fluctuations after management intervention may increase temporal β-diversity and stability of herbivorous assemblages. However, imbalanced changes in TN and TP after management intervention may weaken the top-down control of zooplankton on phytoplankton and slow down water clarity improvement.

Microbial Assemblage Dynamics Within the American Alligator Nesting Ecosystem: a Comparative Approach Across Ecological Scales

Understanding the ecological processes that shape species assemblage patterns is central to community ecology. The effects of ecological processes on assemblage patterns are scale-dependent. We used metabarcoding and shotgun sequencing to determine bacterial taxonomic and functional assemblage patterns among varying defined focal scales (micro-, meso-, and macroscale) within the American alligator (Alligator mississippiensis) nesting microbiome. We correlate bacterial assemblage patterns among eight nesting compartments within and proximal to alligator nests (micro-), across 18 nests (meso-), and between 4 geographic sampling sites (macro-), to determine which ecological processes may drive bacterial assemblage patterns within the nesting environment. Among all focal scales, bacterial taxonomic and functional richness (α-diversity) did not statistically differ. In contrast, bacterial assemblage structure (β-diversity) was unique across all focal scales, whereas functional pathways were redundant within nests and across geographic sites. Considering these observed scale-based patterns, taxonomic bacterial composition may be governed by unique environmental filters and dispersal limitations relative to microbial functional attributes within the alligator nesting environment. These results advance pattern-process dynamics within the field of microbial community ecology and describe processes influencing the American alligator nest microbiome.

Ecological Drift and Directional Community Change in an Isolated Mediterranean Forest Reserve-Larger Moth Species Under Higher Threat

Long-term data are important to understand the changes in ecological communities over time but are quite rare for insects. We analyzed such changes using historic museum collections. For our study area, an isolated forest reserve in North-East Italy, data from the past 80 yr were available. We used records of 300 moth species to analyze whether extinction risk was linked to their body size or to their degree of ecological specialization. Specialization was scored 1) by classifying larval food affiliations, habitat preferences, and the northern distributional limit and 2) by analyzing functional dispersion (FDis) within species assemblages over time. Our results show that locally extinct species (mean wingspan: 37.0 mm) were larger than persistent (33.2 mm) or previously unrecorded ones (30.7 mm), leading to a smaller mean wingspan of the moth community over time. Some ecological filters appear to have selected against bigger species. By using coarse specialization categories, we did not observe any relationship with local extinction risk. However, FDis, calculated across 12 species traits, significantly decreased over time. We conclude that simple classification systems might fail in reflecting changes in community-wide specialization. Multivariate approaches such as FDis may provide deeper insight, as they reflect a variety of ecological niche dimensions. With the abandonment of extensive land use practices, natural succession seems to have shifted the moth community toward a preponderance of forest-affiliated species, leading to decreased FDis values. Multivariate analyses of species composition also confirmed that the moth community has significantly changed during the last 80 yr.

Functional and Taxonomic Beta Diversity of Saproxylic Beetles in Mediterranean Forests: On What Factors Do They Depend?

Understanding how biodiversity is distributed across geographical and environmental gradients is a main goal of diversity sciences. However, since ecosystem processes are linked to variation in functional traits of the biota, examining functional beta diversity is particularly important. Our objective was to analyze the taxonomic and functional beta diversity patterns of saproxylic beetle assemblages in evergreen Quercus forest of Spain. We tested whether environmental or geographical distance had a greater influence on taxonomic and functional beta diversity, and if both measures of beta diversity were affected by the same environmental variables. We used 45 flight interception traps distributed in three protected areas over a 12-mo period to sample saproxylic beetles. We measured 13 environmental variables around each trap and the geographical distance between traps. For functional composition, we used 12 functional traits from four functional groups (morphological, phenological, trophic, and a surrogate of physiological). Our results showed that environmental differences between areas influenced the taxonomic and functional beta diversity components (replacement and loss/gain) but in different ways. While replacement components (higher for taxonomic composition) increased with environmental distance, the loss or gain components (higher for functional composition) remained constant, indicating that species replacement mostly involved functionally redundant species. Besides, environmental variables influencing both taxonomic and functional composition were strongly dependent on each area. In conclusion, in well-preserved Mediterranean forests, environmental filtering determines the taxonomic and functional composition of saproxylic beetle assemblages, by favoring species replacement but filtering species traits.

Univariate Community Assembly Analysis (UniCAA): Combining hierarchical models with null models to test the influence of spatially restricted dispersal, environmental filtering, and stochasticity on community assembly

Identifying the influence of stochastic processes and of deterministic processes, such as dispersal of individuals of different species and trait-based environmental filtering, has long been a challenge in studies of community assembly. Here, we present the Univariate Community Assembly Analysis (UniCAA) and test its ability to address three hypotheses: species occurrences within communities are (a) limited by spatially restricted dispersal; (b) environmentally filtered; or (c) the outcome of stochasticity-so that as community size decreases-species that are common outside a local community have a disproportionately higher probability of occurrence than rare species. The comparison with a null model allows assessing if the influence of each of the three processes differs from what one would expect under a purely stochastic distribution of species. We tested the framework by simulating "empirical" metacommunities under 15 scenarios that differed with respect to the strengths of spatially restricted dispersal (restricted vs. not restricted); habitat isolation (low, intermediate, and high immigration rates); and environmental filtering (strong, intermediate, and no filtering). Through these tests, we found that UniCAA rarely produced false positives for the influence of the three processes, yielding a type-I error rate ≤5%. The type-II error rate, that is, production of false negatives, was also acceptable and within the typical cutoff (20%). We demonstrate that the UniCAA provides a flexible framework for retrieving the processes behind community assembly and propose avenues for future developments of the framework.