Habitat heterogeneity as a mechanism determining plant species diversity: A combined experimental and observational study

Abstract

Biodiversity is a major topic for humanity as it maintains important ecosystem services. However, biodiversity is declining dramatically. Plant species diversity is a crucial component of overall biodiversity because it determines also biodiversity at higher trophic levels. For the maintenance of biodiversity, it is important to understand the mechanisms driving plant community structure and dynamics, especially those mechanisms that affect plant species coexistence. Heterogeneity of the habitat is a key mechanism affecting biodiversity and it is generally assumed to increase plant species diversity. Because habitat heterogeneity is influenced by land use both on a local and a landscape scale, land use may influence diversity directly and indirectly via habitat heterogeneity. Understanding these impacts is important because it would enable us to manage biodiversity via land use. Grasslands are an ideal model system to experimentally test the mechanisms by which land use affects diversity, because on the one hand, we know quite a lot about direct land use effects, e.g. of mowing or fertilization, on biodiversity and on the other hand, indirect effects on biodiversity that are mediated by homogenization of habitat conditions are likely. Here, I investigated both experimentally and in an observational study the effects of land use on habitat heterogeneity and the mechanisms by which heterogeneity influences plant species diversity. For the experimental part, I created grassland microcosms in which two different types of heterogeneity were manipulated. In a first experiment five levels of compositional heterogeneity, i.e. increasing numbers of patches of different habitat types were created, ranging from a single to 16 habitat types per plot. In a second experiment, I manipulated the configurational heterogeneity by creating increasingly fragmented lots composed of two habitat types each. The habitat types in both experiments were created by imitating common land-use practices (mowing/ grazing, trampling, fertilization) as well as soil depth. The observational study looked at how land use intensity affects habitat heterogeneity across several scales. It took place in two regions of Germany along local gradients of the intensity of three common land-use practices mowing, fertilization, and grazing. The experiments showed no change in diversity with heterogeneity. A switch from deterministic extinction in homogeneous habitats to stochastic extinction in heterogeneous habitats where habitat patches were smaller was observed. This is in line with recent theory. In the second experiment, fragmentation effects interacted with the contrast between habitat types and suggest that spatial mass effects are most important for increasing diversity at high fragmentation and intermediate habitat contrasts. In the observational study, I showed homogenizing effects of fertilization and mowing on habitat characteristics, as well as an increase of habitat heterogeneity due to grazing, as suggested by previous studies. Overall, plant species diversity was more directly affected by the mean of the habitat parameters than by indirect effects caused by changes in small-scale habitat heterogeneity.