Abstract
Understanding of the role of body mass in structural-functional relationships is pressing, particularly because species losses often occur non-randomly with respect to body size. Our study examined the effects of dung beetle body mass on dung removal at two levels. First, we used the lab experiment to evaluate the efficiency of eight dung beetle species belonging to two functional groups (tunnelers, dwellers) on dung removal. Second, the same species employed in the lab were used in field mesocosms to examine the effects of the two functional groups on dung removal maintaining realistic differences in the total body mass between tunneler and dweller assemblages. Furthermore, the experimental assemblages contained one and four species within each functional group, so the effect of body mass heterogeneity was examined. We used a statistical approach (offset method) which took into account a priori constraints due to the study design allowing us to analyse the effect of larger species in mesocosm style experiments. Body size played a crucial role in dung removal: large beetles were more efficient than small ones and the percentage of removed dung increased with higher body mass heterogeneity. Tunnelers were more efficient than dwellers over both short and long time periods (one month and one year). Significant effects of dwellers were found only after one year. Moreover, our study showed that not including the body mass as an offset in the model resulted in sometimes different results, as the offset expresses dung removal independently of the body mass. This approach confirmed that body size is likely a pivotal factor controlling dung removal efficiency at multiple levels, from single species to overall dung beetle assemblages. Even though other specific traits should be examined, this study has begun to address the consequences of losing individuals with specific traits that are especially sensitive to perturbations.
Highlights
The last two decades of intensive research have provided compelling evidence for a link between biodiversity and ecosystem functioning (BD-EF)
Our study examines the effects of dung beetle body mass on dung removal at two levels
Post-Hoc Interaction tests showed that A. stercorosus was the most efficient species compared to the other three tunnelers and that G. stercorarius removed more dung than O. fracticornis
Summary
The last two decades of intensive research have provided compelling evidence for a link between biodiversity and ecosystem functioning (BD-EF). Several authors have emphasized that natural and anthropogenic diversity gradients show non-random patterns in the order and characteristics of species lost [6,7,8,9] Such extinction bias raises questions about how useful inferences from random-assembly designs will be for informing conservation efforts. Body mass may represent an indicator of the niche of each species and by extension of the entire ecological network [10] For this reason, in BD-EF studies on bacteria, protists and animals, we need to take into account the body mass of the individuals within the biodiversity entity (e.g. species or functional groups) because ecosystem processes may depend on ecological traits and on the metabolic rate of the individuals [3]. There may be different patterns in the consumption of resources within the same functional group dependent on whether is composed of small or large species, since the metabolic rate per unit body mass is thought to decrease with increasing body size [11,12]
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