Temperature versus resource constraints: which factors determine bee diversity on Mount Kilimanjaro, Tanzania?

Abstract

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.

Predictors of elevational biodiversity gradients change from single taxa to the multi-taxa community level

“…While recent studies emphasize the positive effect of temperature on speciation rates25051, the young age of Mt. Kilimanjaro (<2 Mio years) and the fact that the trends were found for diversity estimates taken at local rather than regional spatial scales (study sites of 50 × 50 m) suggest the additional importance of ecological mechanisms, such as positive effects of temperature on rates of negative-density dependence5253 or positive effects on resource exploitation and other biological rates2349.…”

“…Along large environmental gradients, taxonomically diverse communities have larger opportunities for diversification and the use of niche space than a single taxon. For example, while bee species are typically restricted to warm environments and their species richness peaks in hot climates, syrphid flies have their highest density at considerably cooler temperatures, such that the thermal niche space used by pollinators in total is much larger than the thermal niches of the individual taxa2223. Broadening the taxonomic scope may therefore lead to changes in the importance of some potential drivers of diversity over others.…”

Leaf traits mediate changes in invertebrate herbivory along broad environmental gradients on Mt. Kilimanjaro, Tanzania

“…Path analysis was used to examine causal relationships (Shipley, ) and untangle the direct and indirect effects of explanatory variables (Classen et al, ; Herbst et al, ). The term ‘direct effect’ in a statistical context refers to the magnitude (strength) of change in a response variable caused by a unit change in a predictor variable independently of an intervening variable(s) in causal relationships, while indirect effect refers to the magnitude of change in a response variable caused by a unit change in a predictor variable completely through an intervening variable (Olobatuyi, ).…”

Small mammal species richness is directly linked to regional productivity, but decoupled from food resources, abundance, or habitat complexity

“…The relationships of the MIH are the most commonly‐assumed ecological mechanisms for shaping environment‐diversity patterns, but only one study (Classen et al., ) assessed all four implied variables simultaneously (i.e., climate‐resources‐abundance‐richness). The bivariate link that higher overall abundances can support greater numbers of species, the most frequently tested aspect, alone has only mixed support (e.g., Currie et al., ; Gillman & Wright, ; Kocher & Williams, ; McGlynn, Weiser, & Dunn, ; Storch et al., ).…”