Biodiversity patterns diverge along geographic temperature gradients

Models applying space-for-time substitution, including those projecting ecological responses to climate change, generally assume an elevational and latitudinal equivalence that is rarely tested. However, a mismatch may lead to different capacities for providing climatic refuge to dispersing species. We compiled community data on zooplankton, ectothermic animals that form the consumer basis of most aquatic food webs, from over 1200 mountain lakes and ponds across western North America to assess biodiversity along geographic temperature gradients spanning nearly 3750 m elevation and 30° latitude. Species richness, phylogenetic relationships, and functional diversity all showed contrasting responses across gradients, with richness metrics plateauing at low elevations but exhibiting intermediate latitudinal maxima. The nonmonotonic/hump-shaped diversity trends with latitude emerged from geographic interactions, including weaker latitudinal relationships at higher elevations (i.e. in alpine lakes) linked to different underlying drivers. Here, divergent patterns of phylogenetic and functional trait dispersion indicate shifting roles of environmental filters and limiting similarity in the assembly of communities with increasing elevation and latitude. We further tested whether gradients showed common responses to warmer temperatures and found that mean annual (but not seasonal) temperatures predicted elevational richness patterns but failed to capture consistent trends with latitude, meaning that predictions of how climate change will influence diversity also differ between gradients. Contrasting responses to elevation- and latitude-driven warming suggest different limits on climatic refugia and likely greater barriers to northward range expansion.

Climate warming moderates the impacts of introduced sportfish on multiple dimensions of prey biodiversity

Human-assisted introductions of exotic species are a leading cause of anthropogenic change in biodiversity; however, context dependencies and interactions with co-occurring stressors impede our ability to predict their ecological impacts. The legacy of historical sportfish stocking in mountainous regions of western North America creates a unique, natural quasiexperiment to investigate factors moderating invasion impacts on native communities across broad geographic and environmental gradients. Here we synthesize fish stocking records and zooplankton relative abundance for 685 mountain lakes and ponds in the Cascade and Canadian Rocky Mountain Ranges, to reveal the effects of predatory sportfish introduction on multiple taxonomic, functional and phylogenetic dimensions of prey biodiversity. We demonstrate an innovative analytical approach, combining exploratory random forest machine learning with confirmatory multigroup analysis using multivariate partial least-squares structural equation models, to generate and test hypotheses concerning environmental moderation of stocking impacts. We discovered distinct effects of stocking across different dimensions of diversity, including negligible (nonsignificant) impacts on local taxonomic richness (i.e. alpha diversity) and trophic structure, in contrast to significant declines in compositional uniqueness (i.e. beta diversity) and body size. Furthermore, we found that stocking impacts were moderated by cross-scale interactions with climate and climate-related land-cover variables (e.g. factors linked to treeline position and glaciers). Interactions with physical morphometric and lithological factors were generally of lesser importance, though catchment slope and habitat size constraints were relevant in certain dimensions. Finally, applying space-for-time substitution, a strong antagonistic (i.e. dampening) interaction between sportfish predation and warmer temperatures suggests redundancy of their size-selective effects, meaning that warming will lessen the consequences of introductions in the future and stocked lakes may be less impacted by subsequent warming. While both stressors drive biotic homogenization, our results have important implications for fisheries managers weighing the costs/benefits of stocking-or removing established non-native populations-under a rapidly changing climate.

Spatio-temporal variation of zooplankton community structure in tropical urban waterbodies along trophic and urban gradients

The ecological impacts of urbanization and eutrophication on zooplankton communities in urban waterbodies have recently gained wide interest. Study findings vary across species and urban waterbodies. How the development of such changes affects the variation of biotic assemblages has only been explored to a limited extent in tropical urban lakes.

Spatial and temporal variations of zooplankton community structure in 11 urban waterbodies in the Klang Valley, Malaysia were investigated along trophic and urbanization gradients. Zooplankton and water quality samples were collected three times, between May and November 2017, from two different locations in each lake. All three main zooplankton groups (rotifera, copepoda, and cladocera) were recorded from the study areas throughout the sampling period. The zooplankton community structure, particularly with regard to rotifers and cladocerans, varied between lakes and seasons. Zooplankton diversity does not vary with lake size or distance from the city center but does vary with shoreline development index and urbanization impacts. The zooplankton populations were dominated by rotifers, mainly Brachionus angularis at all study sites during the study period followed by copepods and cladocerans. The total density of zooplankton was significantly highest (p < 0.05) in the hypereutrophic lakes and during the dry season. Zooplankton diversity and rotifer species richness were negatively correlated with total phosphorus (TP). Diversity increased with urbanization and shoreline development, with rotifers as a potential bioindicator of trophic state in urban tropical lakes, due to their close relationship with TP.