Abstract
AbstractAimWhile elevational gradients in species richness constitute some of the best depicted patterns in ecology, there is a large uncertainty concerning the role of food resource availability for the establishment of diversity gradients in insects. Here, we analysed the importance of climate, area, land use and food resources for determining diversity gradients of dung beetles along extensive elevation and land use gradients on Mt. Kilimanjaro, Tanzania.LocationMt. Kilimanjaro, Tanzania.TaxonScarabaeidae (Coleoptera).MethodsDung beetles were recorded with baited pitfall traps at 66 study plots along a 3.6 km elevational gradient. In order to quantify food resources for the dung beetle community in form of mammal defecation rates, we assessed mammalian diversity and biomass with camera traps. Using a multi‐model inference framework and path analysis, we tested the direct and indirect links between climate, area, land use and mammal defecation rates on the species richness and abundance of dung beetles.ResultsWe found that the species richness of dung beetles declined exponentially with increasing elevation. Human land use diminished the species richness of functional groups exhibiting complex behaviour but did not have a significant influence on total species richness. Path analysis suggested that climate, in particular temperature and to a lesser degree precipitation, were the most important predictors of dung beetle species richness while mammal defecation rate was not supported as a predictor variable.Main conclusionsAlong broad climatic gradients, dung beetle diversity is mainly limited by climatic factors rather than by food resources. Our study points to a predominant role of temperature‐driven processes for the maintenance and origination of species diversity of ectothermic organisms, which will consequently be subject to ongoing climatic changes.
Highlights
In contrast with a nearly universal latitudinal decrease of species richness, patterns of diversity along elevation gradients on mountains are more variable, including patterns of monotonous decline, unimodal distributions or even increases of diversity with elevation
We examined the distribution of species richness and abundance of dung beetles along the elevational gradient with generalized additive models (GAMs)
We found that dung beetle abundance showed a hump‐ shaped pattern while species richness declined exponentially with increasing elevation with no significant differences between natural and anthropogenic habitats
Summary
In contrast with a nearly universal latitudinal decrease of species richness, patterns of diversity along elevation gradients on mountains are more variable, including patterns of monotonous decline, unimodal distributions or even increases of diversity with elevation. The ‘area hypothesis’ posits that elevations with larger areas maintain more species at larger populations and have a higher probability of allopatric speciation than elevations with lower total land area (Rosenzweig, 1995). The ‘more‐individuals hypothesis’ (Hutchinson, 1959) predicts a positive relationship between species richness and food resources as productive ecosystems with ample resources can sustain more and larger populations than ecosystems where resource availability is more limited. While the more‐individuals hypothesis has gained some support in past macroecological research (Storch, Bohdalková, & Okie, 2018), tests of the hypothesis are often limited by the use of primary productivity (and its proxies) for estimating the food resource availability of consumer communities. Most of the taxa typically studied in macroecology use specific kinds of food resources whose availability may not be linearly correlated with primary productivity (Storch et al, 2018)
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