Mechanisms and consequences of change in aquatic microfauna communities

Abstract

Anthropogenic influences on the natural environment are increasingly observed and we only start to comprehend how this will affect biodiversity and ecosystem functioning in the long run. This question is challenging because scientific approaches normally investigate only small parts of a community, focussing on particular taxa or the effect of a restricted number of environmental variables. The predictive power of these studies is questionable because reality is a lot more complex and direct as well as indirect interactions can lead to unexpected outcomes. Whole community approaches under natural environmental conditions are logistically impracticable in most ecosystems due to the sheer impossibility of sampling, for example, an entire forest. Phytotelma, such as bromeliads, provide an ideal solution for this dilemma. These small temporary water bodies contain communities of manageable sizes that can be easily sampled in naturally replicated micro-ecosystems. Most of the previous bromeliad studies have investigated the macrofauna living in bromeliads. Microfauna have been mostly neglected and therefore little is known about their community structure. Microfauna organisms - including flagellates, ciliates, amoeba, rotifers and crustaceans - are the part of the bromeliad-inhabiting communities that this dissertation focusses on. We used a community-level approach to explore community-structuring processes in bromeliad microfauna with the aim to better predict potential effects of environmental changes on biodiversity and ecosystem functioning. In a field survey along a canopy cover gradient (chapter 1) we investigated the effect of differences in sun-exposure in a restinga rainforest on microfauna community structure. We found strong differences in the environmental conditions which resulted in changes of habitat quality along the canopy cover gradient. This was shown to affect the community structure and beta diversity of bromeliad-inhabiting microfauna via differences in daily temperature fluctuations. With regard to the expected temperature increase through climate change, this result shows that it is not necessarily the direct effect of higher average temperatures that proposes a threat to natural communities but that indirect effects of climate change such as repeated short-time fluctuations in environmental conditions may decrease a habitat’s quality, and thus, lead to a loss of biodiversity and potentially ecological functions. To disentangle the effects of environmental change and trophic interactions on microfauna community structure we carried out a community-transplantation experiment along an elevational gradient (chapter 2). We used a full-factorial experimental design to particularly address potential interactions between environmental change and trophic interactions. The results showed that bromeliad-inhabiting microfauna communities are also shaped by predator presence and priority effects. Interacting effects played an important role in structuring communities, suggesting that we need to broaden our scientific approaches to fully understand the relationships in natural ecosystems and better predict consequences of human-induced changes. Though bromeliad plants grow mainly epiphytic, most bromeliad-related studies, including our field survey (chapter 1) and our field experiment (chapter 2), sample exclusively in the understory. Based on the assumption that sun-exposure increased with increasing height and thus leads to changed environmental conditions, we carried out a field survey sampling understory and canopy bromeliads using single-rope climbing techniques (chapter 3). The comparison of microfauna community structure in understory and canopy bromeliads revealed that no change in community structure occurs along the height gradient. This justifies the former bromeliad community approaches with exclusively understory samples. Finally, we conducted a field survey along three elevational gradients to determine if bromeliad-inhabiting communities change in a generalizable pattern along natural environmental gradients (chapter 4). There was no clear pattern detectable that would allow us to filter out driving environmental factors for community structure in bromeliads on regional scale. The lack of a clear environmental driver of community structure was probably at least partly due to the lack of environmental differences along two of the three gradients. We conclude from our results that microfauna communities are subject to complex interactions and that it is therefore important to use full-factorial approaches in future studies to disentangle the effects of potential drivers of community structure. So far, we could show that daily temperature fluctuations, predator presence, priority effects and oxygen saturation can play key roles in shaping microfauna communities, but we emphasize that these are strongly dependent on the surrounding environment making general predictions difficult. Human-induced environmental alterations such as climate change are likely to affect bromeliad-inhabiting microfauna communities via indirect effects which might result in alterations of important processes in regard to energy and matter fluxes on the ecosystem level. Based upon these results we recommend the integration of microfauna communities into conservation strategies.