Abstract
AbstractAimTemperature, 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.LocationMt. Kilimanjaro, Tanzania.TaxonCavity‐nesting Hymenoptera (Hymenoptera: Apidae, Colletidae, Megachilidae, Crabronidae, Sphecidae, Pompilidae, Vespidae).MethodsWe 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.ResultsElevational 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 conclusionsOur 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.
Highlights
A fundamental goal in ecology is to understand the origin and distribution of species
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
By systematically analysing different trophic levels, we found that the availability of resources gained more weight as a driver of species richness in antagonist groups than in bee and wasp hosts
Summary
A fundamental goal in ecology is to understand the origin and distribution of species. Patterns of biodiversity have been studied for more than three centuries, giving rise to a number of environmental predictors of broad-scale diversity gradients including climate, energy resources and top-down control (Gaston, 2000; Peters et al, 2016). Energy resources are often cited as important drivers of biodiversity (Hurlbert & Stegen, 2014). While the availability of resources has repeatedly been found to be positively correlated with species richness for endothermic taxa (Buckley, Hurlbert, & Jetz, 2012; Ferger, Schleuning, Hemp, Howell, & Böhning-Gaese, 2014), its relevance for ectotherms is more variable and depends on consumer groups, tropic level and habitat type (Schuldt et al, 2019). Food resource availability is often not proportional to food resource intake, as foraging is only possible within certain temperature thresholds (Classen et al, 2015; Willmer, 1983)
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