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Species’ functional traits set the blueprint for pair-wise interactions in ecological networks. Yet, it is unknown to what extent the functional diversity of plant and animal communities controls network assembly along environmental gradients in real-world ecosystems. Here we address this question with a unique dataset of mutualistic bird–fruit, bird–flower and insect–flower interaction networks and associated functional traits of 200 plant and 282 animal species sampled along broad climate and land-use gradients on Mt. Kilimanjaro. We show that plant functional diversity is mainly limited by precipitation, while animal functional diversity is primarily limited by temperature. Furthermore, shifts in plant and animal functional diversity along the elevational gradient control the niche breadth and partitioning of the respective other trophic level. These findings reveal that climatic constraints on the functional diversity of either plants or animals determine the relative importance of bottom-up and top-down control in plant–animal interaction networks.

Aim We used the extensive climatic and land-use gradient on Mt Kilimanjaro, Tanzania, to study large-scale variation of nutrient use in ground-foraging ant communities. In particular, we tested the hypothesis that recruitment of ants to six different nutrients would vary with elevation and between land-use regimes. We also tested whether the nutrient space (number and evenness of nutrients) used by ants decreases as species richness declines with elevation, a pattern expected because of complementarity in nutritional intake of species.Location Mt Kilimanjaro, Tanzania.Methods Standardized bait experiments with six nutrient treatments [carbohydrates (CHO), amino acid, CHO-amino acid mix, NaCl, H 2 O and lipids] were performed at a total of 48 study sites in natural and managed ecosystems along an elevational gradient from 860 to 4390 m a.s.l. We used generalized linear models, information theory-based model inference and null model analyses to test hypotheses.Results The species richness of ant communities declined with elevation in natural ecosystems but peaked at mid-elevations in managed ecosystems. We found that the use of four nutrients (NaCl, CHO, H 2 O and lipids) varied with elevation and, in the case of NaCl and H 2 O, with land use. Use of H 2 O and CHO decreased with elevation, while lipid use increased. NaCl use increased with elevation in natural ecosystems but decreased in managed ecosystems. The nutrient space exploited by ant communities increased with ant species richness in natural ecosystems but decreased slightly in managed ecosystems. The difference could be because there are more trophic generalists in managed ecosystems and more specialists with complementary foraging niches in natural ecosystems.Main conclusions Ant communities in different environments appear to be limited by different types and numbers of nutrients. This spatial heterogeneity in nutritional ecology is probably determined by both the environmental availability of nutrients and the functional composition of ant communities.

Aim Temperature, food resources and top‐down regulation by antagonists are considered as major drivers of insect diversity, but their relative importance is poorly understood. Here, we used cavity‐nesting communities of bees, wasps and their antagonists to reveal the role of temperature, food resources, parasitism rate and land use as drivers of species richness at different trophic levels along a broad elevational gradient. Location Mt. Kilimanjaro, Tanzania. Taxon Cavity‐nesting Hymenoptera (Hymenoptera: Apidae, Colletidae, Megachilidae, Crabronidae, Sphecidae, Pompilidae, Vespidae). Methods We established trap nests on 25 study sites that were distributed over similar large distances in terms of elevation along an elevational gradient from 866 to 1788 m a.s.l., including both natural and disturbed habitats. We quantified species richness and abundance of bees, wasps and antagonists, parasitism rates and flower or arthropod food resources. Data were analysed with generalized linear models within a multi‐model inference framework. Results Elevational species richness patterns changed with trophic level from monotonically declining richness of bees to increasingly humped‐shaped patterns for caterpillar‐hunting wasps, spider‐hunting wasps and antagonists. Parasitism rates generally declined with elevation but were higher for wasps than for bees. Temperature was the most important predictor of both bee and wasp host richness patterns. Antagonist richness patterns were also well predicted by temperature, but in contrast to host richness patterns, additionally by resource abundance and diversity. The conversion of natural habitats through anthropogenic land use, which included biomass removal, agricultural inputs, vegetation structure and percentage of surrounding agricultural habitats, had no significant effects on bee and wasp communities. Main conclusions Our study underpins the importance of temperature as a main driver of diversity gradients in ectothermic organisms and reveals the increasingly important role of food resources at higher trophic levels. Higher parasitism rates at higher trophic levels and at higher temperatures indicated that the relative importance of bottom‐up and top‐down drivers of species richness change across trophic levels and may respond differently to future climate change.

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