Synergistic effects of climate and land use on avian beta-diversity

Reorganization of bird communities along a rainforest elevation gradient during a strong El Niño event in Papua New Guinea

The El Niño 2015 event, most extreme since 1997, led to severe droughts in tropical wet Papua New Guinea (PNG), reducing May to October dry season rainfall by 75% in the lowlands and 25% in the highlands. Such droughts are likely to have significant effects on terrestrial ecosystems, but they have been poorly explored in Papua New Guinea. Here, we report changes in bird community composition prior to, during, and after the 2015 El Niño event along the elevational gradient ranging from 200 m to 2700 m a.s.l. at the Mt. Wilhelm rainforest in PNG. The abundance of birds in the lowlands dropped by 60% but increased by 40% at elevations above 1700 m during El Niño year. In the following year, the individual bird species reached mean population sizes similar to pre-El Niño years but did not fully recover. Species richness roughly followed the pattern of observed abundance and quickly and fully re-established after the event to the pre- El Niño values. Thus, at least some terrestrial birds seem to react quickly to the extreme droughts in lowlands and shift to less affected mountain habitats. We recorded upper elevational range limits to shifts by more than 500 m a.s.l. in 22 bird species (out of 237 recorded in total) during El Niño year, in contrast to their typical ranges. Our study suggests that a strong El Niño event can have strong but reversible effects on bird communities as long as they have an opportunity to move to more favorable sites through undisturbed habitats.

Declining salinity and increasing temperature reduce the diversity and resilience of benthic diatoms

Climate change will modify the marine ecosystem in several ways, but the effects of changing climate on benthic diatoms, which are one of the most important photosynthesizing organism groups in benthic habitats, are poorly studied. We conducted a mesocosm experiment to investigate the effects of increasing temperature and decreasing salinity on the taxonomic and functional diversity of benthic diatoms. We showed that decreasing salinity affects the taxonomic and functional composition of communities, and the threshold salinity for community composition is ~5. This indicates that when climate change leads to decreasing salinity in brackish systems, the most pronounced changes in communities occur in areas where salinity decreases from >5 to <5. We also showed that both increasing temperature and decreasing salinity exert stress on communities and, hence, lead to the decrease of the alpha and beta diversity of communities. This indicates that climate change reduces the size of the species pool of diatoms. Our results show that, along with the changing climate, we can expect benthic diatom communities to become less diverse and less resilient.

Concordant and opposing effects of climate and land-use change on avian assemblages in California's most transformed landscapes

Climate and land-use change could exhibit concordant effects that favor or disfavor the same species, which would amplify their impacts, or species may respond to each threat in a divergent manner, causing opposing effects that moderate their impacts in isolation. We used early 20th century surveys of birds conducted by Joseph Grinnell paired with modern resurveys and land-use change reconstructed from historic maps to examine avian change in Los Angeles and California's Central Valley (and their surrounding foothills). Occupancy and species richness declined greatly in Los Angeles from urbanization, strong warming (+1.8°C), and drying (-77.2 millimeters) but remained stable in the Central Valley, despite large-scale agricultural development, average warming (+0.9°C), and increased precipitation (+11.2 millimeters). While climate was the main driver of species distributions a century ago, the combined impacts of land-use and climate change drove temporal changes in occupancy, with similar numbers of species experiencing concordant and opposing effects.

Refocusing multiple stressor research around the targets and scales of ecological impacts

Ecological communities face a variety of environmental and anthropogenic stressors acting simultaneously. Stressor impacts can combine additively or can interact, causing synergistic or antagonistic effects. Our knowledge of when and how interactions arise is limited, as most models and experiments only consider the effect of a small number of non-interacting stressors at one or few scales of ecological organization. This is concerning because it could lead to significant underestimations or overestimations of threats to biodiversity. Furthermore, stressors have been largely classified by their source rather than by the mechanisms and ecological scales at which they act (the target). Here, we argue, first, that a more nuanced classification of stressors by target and ecological scale can generate valuable new insights and hypotheses about stressor interactions. Second, that the predictability of multiple stressor effects, and consistent patterns in their impacts, can be evaluated by examining the distribution of stressor effects across targets and ecological scales. Third, that a variety of existing mechanistic and statistical modelling tools can play an important role in our framework and advance multiple stressor research.

The Effect of Climate and Human Pressures on Functional Diversity and Species Richness Patterns of Amphibians, Reptiles and Mammals in Europe

The ongoing biodiversity crisis reinforces the urgent need to unravel diversity patterns and the underlying processes shaping them. Although taxonomic diversity has been extensively studied and is considered the common currency, simultaneously conserving other facets of diversity (e.g., functional diversity) is critical to ensure ecosystem functioning and the provision of ecosystem services. Here, we explored the effect of key climatic factors (temperature, precipitation, temperature seasonality, and precipitation seasonality) and factors reflecting human pressures (agricultural land, urban land, land-cover diversity, and human population density) on the functional diversity (functional richness and Rao’s quadratic entropy) and species richness of amphibians (68 species), reptiles (107 species), and mammals (176 species) in Europe. We explored the relationship between different predictors and diversity metrics using generalized additive mixed model analysis, to capture non-linear relationships and to account for spatial autocorrelation. We found that at this broad continental spatial scale, climatic variables exerted a significant effect on the functional diversity and species richness of all taxa. On the other hand, variables reflecting human pressures contributed significantly in the models even though their explanatory power was lower compared to climatic variables. In most cases, functional richness and Rao’s quadratic entropy responded similarly to climate and human pressures. In conclusion, climate is the most influential factor in shaping both the functional diversity and species richness patterns of amphibians, reptiles, and mammals in Europe. However, incorporating factors reflecting human pressures complementary to climate could be conducive to us understanding the drivers of functional diversity and richness patterns.

In transition: Avian biogeographic responses to a century of climate change across desert biomes

Transition zones between biomes, also known as ecotones, are areas of pronounced ecological change. They are primarily maintained by abiotic factors and disturbance regimes that could hinder or promote species range shifts in response to climate change. We evaluated how climate change has affected metacommunity dynamics in two adjacent biomes and across their ecotone by resurveying 106 sites that were originally surveyed for avian diversity in the early 20th century by Joseph Grinnell and colleagues. The Mojave, a warm desert, and the Great Basin, a cold desert, have distinct assemblages and meet along a contiguous, east-west boundary. Both deserts substantially warmed over the past century, but the Mojave dried while the Great Basin became wetter. We examined whether the distinctiveness and composition of desert avifaunas have changed, if species distributions shifted, and how the transition zone impacted turnover patterns. Avifauna change was characterized by (a) reduced occupancy, range contractions, and idiosyncratic species redistributions; (b) degradation of historic community structure, and increased taxonomic and climatic differentiation of the species inhabiting the two deserts; and (c) high levels of turnover at the transition zone but little range expansion of species from the warm, dry Mojave into the cooler, wetter Great Basin. Although both deserts now support more drier and warmer tolerant species, their bird communities still occupy distinct climatological space and differ significantly in climatic composition. Our results suggest a persistent transition zone between biomes contributes to limiting the redistribution of birds, and highlight the importance of understanding how transition zone dynamics impact responses to climate change.

Harmony on the prairie? Grassland plant and animal community responses to variation in climate across land-use gradients

Human induced climate and land-use change are severely impacting global biodiversity, but how community composition and richness of multiple taxonomic groups change in response to local drivers and whether these responses are synchronous remains unclear. We used long-term community-level data from an experimentally manipulated grassland to assess the relative influence of climate and land use as drivers of community structure of four taxonomic groups: birds, mammals, grasshoppers, and plants. We also quantified the synchrony of responses among taxonomic groups across land-use gradients and compared climatic drivers of community structure across groups. All four taxonomic groups responded strongly to land use (fire frequency and grazing), while responses to climate variability were more pronounced in grasshoppers and small mammals. Animal groups exhibited asynchronous responses across all land-use treatments, but plant and animal groups, especially birds, exhibited synchronous responses in composition. Asynchrony was attributed to taxonomic groups responding to different components of climate variability, including both current climate conditions and lagged effects from the previous year. Data-driven land management strategies are crucial for sustaining native biodiversity in grassland systems, but asynchronous responses of taxonomic groups to climate variability across land-use gradients highlight a need to incorporate response heterogeneity into management planning.

Projected impacts of climate change on functional diversity of frugivorous birds along a tropical elevational gradient

Climate change forces many species to move their ranges to higher latitudes or elevations. Resulting immigration or emigration of species might lead to functional changes, e.g., in the trait distribution and composition of ecological assemblages. Here, we combined approaches from biogeography (species distribution models; SDMs) and community ecology (functional diversity) to investigate potential effects of climate-driven range changes on frugivorous bird assemblages along a 3000 m elevational gradient in the tropical Andes. We used SDMs to model current and projected future occurrence probabilities of frugivorous bird species from the lowlands to the tree line. SDM-derived probabilities of occurrence were combined with traits relevant for seed dispersal of fleshy-fruited plants to calculate functional dispersion (FDis; a measure of functional diversity) for current and future bird assemblages. Comparisons of FDis between current and projected future assemblages showed consistent results across four dispersal scenarios, five climate models and two representative concentration pathways. Projections indicated a decrease of FDis in the lowlands, an increase of FDis at lower mid-elevations and little changes at high elevations. This suggests that functional dispersion responds differently to global warming at different elevational levels, likely modifying avian seed dispersal functions and plant regeneration in forest ecosystems along tropical mountains.

Local- versus broad-scale environmental drivers of continental β-diversity patterns in subterranean spider communities across Europe

Macroecologists seek to identify drivers of community turnover (β-diversity) through broad spatial scales. However, the influence of local habitat features in driving broad-scale β-diversity patterns remains largely untested, owing to the objective challenges of associating local-scale variables to continental-framed datasets. We examined the relative contribution of local- versus broad-scale drivers of continental β-diversity patterns, using a uniquely suited dataset of cave-dwelling spider communities across Europe (35-70° latitude). Generalized dissimilarity modelling showed that geographical distance, mean annual temperature and size of the karst area in which caves occurred drove most of β-diversity, with differential contributions of each factor according to the level of subterranean specialization. Highly specialized communities were mostly influenced by geographical distance, while less specialized communities were mostly driven by mean annual temperature. Conversely, local-scale habitat features turned out to be meaningless predictors of community change, which emphasizes the idea of caves as the human accessible fraction of the extended network of fissures that more properly represents the elective habitat of the subterranean fauna. To the extent that the effect of local features turned to be inconspicuous, caves emerge as experimental model systems in which to study broad biological patterns without the confounding effect of local habitat features.

Primary productivity and habitat protection predict elevational species richness and community biomass of large mammals on Mt. Kilimanjaro

Despite their diversity and their large functional and cultural importance, the patterns and predictors of large mammal diversity along elevational gradients on tropical mountains remain poorly understood. Today, large mammals are threatened by human disturbances such as habitat destruction and hunting and may increasingly depend on the conservation of protected areas. Here, we use field data on the diversity of large mammals along a 3.6 km elevational gradient on Mt. Kilimanjaro to evaluate the importance of climate, net primary productivity and human impact for the distribution, species richness and community biomass of wild mammals. Mammal species richness was explored with camera traps on 66 study plots along an elevational gradient from 870 to 4,550 m a.s.l.. We applied path analysis and variance partitioning analysis to unravel the direct and indirect effects of temperature, precipitation, primary productivity, land use, land area, the protection of habitats and the occurrence of domestic mammals on the diversity of wild mammals. Both species richness and community biomass of wild mammals showed a unimodal distribution with elevation, peaking in the montane zone of Mt. Kilimanjaro. However, the peak shifted significantly to lower elevations when only protected habitats were considered. Wild mammal diversity increased with net primary productivity, protection of habitats and temperature. Our study underscores the importance of energy resources for the establishment of diversity gradients in large mammals. While temperature has been revealed as a direct predictor of diversity in most ectothermic taxa, in endothermic organisms temperature has stronger indirect effects, via a modulation of net primary productivity. Moreover, our study reveals how patterns of diversity on tropical mountains are influenced by human impact, pointing to the pivotal role of protected areas for the long-term conservation of mountain biodiversity.

Climate-land-use interactions shape tropical mountain biodiversity and ecosystem functions

Agriculture and the exploitation of natural resources have transformed tropical mountain ecosystems across the world, and the consequences of these transformations for biodiversity and ecosystem functioning are largely unknown^1-3. Conclusions that are derived from studies in non-mountainous areas are not suitable for predicting the effects of land-use changes on tropical mountains because the climatic environment rapidly changes with elevation, which may mitigate or amplify the effects of land use^4,5. It is of key importance to understand how the interplay of climate and land use constrains biodiversity and ecosystem functions to determine the consequences of global change for mountain ecosystems. Here we show that the interacting effects of climate and land use reshape elevational trends in biodiversity and ecosystem functions on Africa's largest mountain, Mount Kilimanjaro (Tanzania). We find that increasing land-use intensity causes larger losses of plant and animal species richness in the arid lowlands than in humid submontane and montane zones. Increases in land-use intensity are associated with significant changes in the composition of plant, animal and microorganism communities; stronger modifications of plant and animal communities occur in arid and humid ecosystems, respectively. Temperature, precipitation and land use jointly modulate soil properties, nutrient turnover, greenhouse gas emissions, plant biomass and productivity, as well as animal interactions. Our data suggest that the response of ecosystem functions to land-use intensity depends strongly on climate; more-severe changes in ecosystem functioning occur in the arid lowlands and the cold montane zone. Interactions between climate and land use explained-on average-54% of the variation in species richness, species composition and ecosystem functions, whereas only 30% of variation was related to single drivers. Our study reveals that climate can modulate the effects of land use on biodiversity and ecosystem functioning, and points to a lowered resistance of ecosystems in climatically challenging environments to ongoing land-use changes in tropical mountainous regions.

A century of climate and land-use change cause species turnover without loss of beta diversity in California's Central Valley

Climate and land-use changes are thought to be the greatest threats to biodiversity, but few studies have directly measured their simultaneous impacts on species distributions. We used a unique historic resource-early 20th-century bird surveys conducted by Joseph Grinnell and colleagues-paired with contemporary resurveys a century later to examine changes in bird distributions in California's Central Valley, one of the most intensively modified agricultural zones in the world and a region of heterogeneous climate change. We analyzed species- and community-level occupancy using multispecies occupancy models that explicitly accounted for imperfect detection probability, and developed a novel, simulation-based method to compare the relative influences of climate and land-use covariates on site-level species richness and beta diversity (measured by Jaccard similarity). Surprisingly, we show that mean occupancy, species richness and between-site similarity have remained remarkably stable over the past century. Stability in community-level metrics masked substantial changes in species composition; occupancy declines of some species were equally matched by increases in others, predominantly species with generalist or human-associated habitat preferences. Bird occupancy, richness and diversity within each era were driven most strongly by water availability (precipitation and percent water cover), indicating that both climate and land-use are important drivers of species distributions. Water availability had much stronger effects than temperature, urbanization and agricultural cover, which are typically thought to drive biodiversity decline.