What do snails do in ecosystems?

Current environmental changes demand the ability to predict possible consequences for ecosystems performing important functions regulating the Earth system, and providing essential services for human well-being. Indirect impacts can occur through changes in biotic communities. Functional traits determine organisms' performance, and thus their fitness in a given environment. Therefore, traits can be used to assess communities' response to environmental variation (via response traits) and their effects on ecosystem processes (via effect traits).In this thesis, I used terrestrial snails as study organisms to examine different aspects of their response to environmental and spatial factors, and their potential effects on leaf litter decomposition, as mediated by their traits. I investigated which factors determine the assembly of communities and found that trait underdispersion was more common than trait overdispersion, indicating the dominance of environmental filtering over biotic interactions. Testing the relative importance of environmental and spatial factors, I found that both environment and space significantly contributed to the variation in community trait composition, whereby the environment had the strongest effect. This indicates that intrinsic population processes (e.g. dispersal) that are independent from the environment play only a subordinate role. Moreover, I conducted microcosm experiments to measure snail traits that affect leaf litter decomposition (consumption rate, faeces production and assimilation efficiency) testing two litter types differing in quality. With the observed relationship between species specific shell size and consumption rate as proxy I assessed snail communities' potential influence on leaf litter decomposition along an acidification gradient.Several traits responded to environmental factors in all three observational studies (shell size, number of offspring, reproduction mode, and microhabitat occurrence), or in two of the three studies (survival of dry period, and humidity preference) suggesting some generality of certain trait-environment relationships. Three traits (shell size, calcium affinity, and self-fertilization) and one measure of functional diversity (functional richness) responded both to environmental variation, and influenced the snails' potential contribution to leaf litter decomposition.