The factors determining gradients of biodiversity are a fundamental yet unresolved topic in ecology. While diversity gradients have been analysed for numerous single taxa, progress towards general explanatory models has been hampered by limitations in the phylogenetic coverage of past studies. By parallel sampling of 25 major plant and animal taxa along a 3.7 km elevational gradient on Mt. Kilimanjaro, we quantify cross-taxon consensus in diversity gradients and evaluate predictors of diversity from single taxa to a multi-taxa community level. While single taxa show complex distribution patterns and respond to different environmental factors, scaling up diversity to the community level leads to an unambiguous support for temperature as the main predictor of species richness in both plants and animals. Our findings illuminate the influence of taxonomic coverage for models of diversity gradients and point to the importance of temperature for diversification and species coexistence in plant and animal communities.

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Aim Understanding the mechanisms controlling variation in species richness along environmental gradients is one of the most important objectives in ecology. Resource availability is often considered as the major driver of animal diversity. However, in ectotherms, temperature might play a predominant role as it modulates metabolic rates and the access of animals to resources. Here, we investigate the relative importance of resource availability and temperature in determining the diversity pattern of bees along a 3.6‐km elevational gradient. Location Mount Kilimanjaro, Tanzania. Methods We assessed bee species richness and abundance with pan traps and floral resources with transect records on 60 study sites which were equally distributed over six near‐natural and six disturbed habitat types along an elevational gradient from 870 to 4550 m a.s.l. We used path analysis to disentangle the effects of temperature, precipitation, floral resource abundance, bee abundance and land use on bee species richness. In addition, we monitored flower visitation rates during transect walks at different elevations to evaluate the temperature dependence of bee–flower interactions. Results Bee species richness continuously declined with elevation in natural and disturbed habitats. While the abundance of floral resources had a significant but only weak effect on species richness, the effect of temperature was strong. Temperature had a strong positive effect on species richness that was not mediated by bee abundance and an indirect effect via bee abundances. We observed higher levels of bee–flower interactions at higher temperatures, supporting the hypothesis that temperature limits diversity by constraining resource exploitation in ectotherms. Main conclusions Temperature and the availability of resources shape species richness patterns along environmental gradients. In ectothermic organisms like bees temperature seems to have the more important role, as it both limits the access to resources (abundance‐mediated effect) and accelerates other (abundance‐independent) ecological and evolutionary processes that drive the maintenance and origination of diversity.

Summary Physiological and energetic mechanisms have been proposed to constrain body sizes of organisms along climatic gradients; however, these provide contrasting predictions. While Bergmann's rule predicts increases in body sizes in cooler climates resulting from physiological constraints, energy‐based community assembly rules suggest declines in the mean body size of species caused by increased extinction probabilities for large‐bodied species in low‐energy habitats. We tested these contrasting hypotheses by quantifying trait distributions in bee communities along a 3·6‐km elevational gradient at Mt. Kilimanjaro. Traditionally, intra‐ and interspecific trait shifts along environmental gradients have been investigated in isolation. However, a surge of theoretical approaches and studies on plants demonstrated that the explicit integration of trait variation among and within species can be essential for identifying the mechanisms that shape traits and related ecosystem functions along environmental gradients. We therefore studied variation in body size and related morphological traits at both the intra‐ and interspecific level. We found support for both physiological constraints and energy‐based community assembly rules as drivers of trait distribution in bee communities along elevational gradients, which, however, affected different levels of biotic organization, that is the population and community level. While the number of bee species with large body sizes declined with elevation, individuals within species became on average larger, resulting in contrasting trends in morphometric parameters at the community versus population level. Furthermore, body size within bee communities became less variable at higher elevations, largely as a result of a non‐random, directive loss of species, but paralleled by a decline in intraspecific variance, suggesting intensified filtering effects with increasing elevation. Similar patterns were found for other functional traits related to the foraging ecology of bees (tongue length, relative forewing length). We conclude that along climatic gradients both physiological and energetic constraints shape trait distributions of pollinators, but at different levels of biological organization. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12786/suppinfo is available for this article.

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Background and aims-Plants are often overlooked in conservation planning, yet they are the foundation of all terrestrial ecosystems. The East Africa region is used to investigate the effectiveness of protected areas for conserving plants. With a wide range of ecosystems and 771 protected areas covering nearly one quarter of the land area, East Africa is an ideal location to assess the effectiveness of protected areas through distribution modelling of the genus Acacia. Methods-Herbarium specimen data (2,047 records) were collated from East Africa for 65 taxa (species, subspecies, varieties) from the genus Acacia. Generalised Additive Models were used to determine climatic drivers, and thence to extrapolate climatic suitability across the region. For two Acacia taxa, we investigated the potential for climate-induced range-shifts using a downscaled regional climate model under two IPCC scenarios. Key results-Approximately two thirds of Acacia diversity hotspots had < 10% coverage by protected areas. Furthermore, the protected area network covered less of the predicted ranges of the Acacia taxa and contained fewer taxa per unit area than would be expected under randomised placement. Areas with suitable climate for high-elevation, moisture-dependent taxa such as A. abyssinica subsp. calophylla are predicted to contract their potential range by up to 80% towards mountain peaks, where protected areas are dominated by low-level protection forest reserves. Conversely, the area of suitable environment for a xerophytic low-elevation species (A. turnbulliana) is predicted to increase by up to 77%. Conclusions-East Africa's national parks may not be preserving an important component of ecosystem diversity, a situation exacerbated by climate change. Even within the genus Acacia, different species are predicted to respond differently to climate change. Priority areas for research and conservation are identified based on overlap between predicted high Acacia diversity and gaps in the collection record, with northern and eastern Kenya highlighted as particularly important. High elevation protected areas are also predicted to become increasingly important as climatic refugia in a warmer future.