Gemeenschapsstucturerende processen in Mediterrane tijdelijke poelen.

Temporary wetlands are among the most characteristic and remarkable habitats in the Mediterranean region; their future, however, remains uncertain in many areas. Despite the recognized importance of this priority habitat (Habitat Directive, Natura code 3170, 92/43/CEE) for biodiversity conservation and other ecosystem services, our current understanding of diversity patterns, ecosystem functioning and community structuring processes in this habitat is still incomplete. In this dissertation, we studied key environmental factors, biotic interactions and dispersal dynamics shaping invertebrate communities in temporary wetlands of the Camargue(in the nature reserve of Tour du Valat, Southern France). Since Mediterranean temporary wetlands are particularly susceptible to changes in the quantity and quality of their water supply, we discuss our results in the light of predicted scenarios of climate change and intensive water management.Firstly, we investigated the diversity patterns and the relative contributions of local and spatial factors structuring the invertebrate communities using a fieldsurvey of 30 temporary wetlands. The strong association of invertebrates with local habitat characteristics indicated that the local environment may have acted as a selective force, resulting in efficient species sorting. The two most important factors explaining invertebrate species distributions and community structure in the studied wetlands were salinity and hydroregime. Both impose a considerable stress on the biota of temporary wetlands: osmotic stress and time stress, respectively. The importance of these key-factors suggests that human-inducedalterations of these factors, such as secondary salinisation of hydrological modification, may have a strong impact on the biota of temporary wetlands. This is especially true for the permanent residents (mainly crustaceans: zooplankton and large branchiopods) who rely on hatching from their resting egg banks and do not have the ability to actively escape when conditions become unfavorable.We therefore investigated the long term effects of salinity and hydroregime on the active and dormant crustacean communities of Camargue temporary wetlands using a large scale outdoor mesocosm experiment on the experimental field of Tour du Valat research station. The interacting effects of both stressors significantly influenced diversity, density, reproduction, succession and overall community structure of the permanent invertebrate communities of temporary wetlands. Although the presence of crustacean resting egg banks can temporarily buffer against unfavorable conditions, our results suggested that persisting bad conditions may lead totheir exhaustion within four to ten years. Based on our results we concluded that predicted aridification (leading to shortening of hydroregimes of temporary wetlands) may lead the loss of species that come late in the succession (cladocerans and ostracods), while salinisation may lead to the loss of already fragile freshwater species such as large branchiopods. We also argue that predictions of the effects of changing environmental factors are not always straightforward, since they are often complicated by unknown sublethal effects, changed biotic interactions or indirect abiotic effects. In our case, especially the biotic pressure of large branchiopods may have influenced the salinity-response of the zooplankton community in the mesocosms.Using a microcosm experiment we therefore investigated the ability of large branchiopod to shape zooplankton communities. We found that the notostracan Triops cancriformis can be considered a top predator in temporary wetlands, preying both on active zooplankton populations and dormant propagules. We also hypothesized that Triops can be considered as an ecosystem engineer in these habitats due to their bioturbation behavior. The anostracan Chirocephalus diaphanus also effectively limited most zooplankton groups. The ability of large branchiopods to shape zooplankton communities confirmed their keystone role in temporary aquatic systems.The results of our community analyses concerning the field survey suggested that dispersal is probably not limiting in the studied invertebratemetacommunity. We therefore hypothesized that frequent dispersal is mediated by the joint effect of a number of known dispersal vectors: aquatic connections, wind, waterfowl, amphibians and aquatic insects. Other potential vectors for the dispersal of aquatic invertebrates that were abundant on the study site, such as aquatic mammals (e.g. nutria, muskrats), terrestrial wallowing mammals (e.g. wild boar, cattle, horses) and humans (e.g. footwear and vehicles) were rarely mentioned in literature, although their role as dispersal vectors for plant seeds was studied rather intensely. In this dissertation, we provided ample evidence that mammals (case studies: wild boar (Sus scrofa) and nutria (Myocastor coypus)) and humans (case study: shoes and cars of field biologists) are efficient dispersal vectors of freshwater invertebrates. Comparing the importance of these vectors, we concluded that wild boar probably is the most important one on the local scale of Tour du Valat.Based on our results we formulated some implications for conservation that should apply to many temporary wetlands in the Mediterranean region.The high degree of nestedness in our field survey implies that protection of wetlands with long hydroperiods or low salinities may be more important than protection of species-poor wetlands with short hydroperiods or high salinities. In view of the expected climate changes, we need to be prepared to manage hydroregimes artificially, but there is still a long way to go due to the persisting lack of long-term data. We also need to become aware of the risks of reducing the quality and quantity of habitats or reducing vector abundances, as they may disrupt metacommunity dynamics, leading to an increased extinction risk. On the other hand, it is crucial that we understand that we might not only be observers but also actors (in this case as vectors) and, given the rapid spread of invasive zooplankton species, we promote caution when moving material and organisms from one wetland area to another.nrpages: 211status: publishe