Fragmentation and seed dispersal in freshwater wetlands

Abstract

Human impact on nature has been severe in the past centuries. Land use change is seen as the main driver of the loss of biodiversity worldwide. Conversion of natural ecosystems into agricultural land or urban areas has led to habitat loss and degradation for many plant species. Additionally, land use change typically results in a fragmented distribution of the remaining habitat. Habitat fragmentation may result in increased extinction risk of local populations of plant species, by a decreased local population size, increased edge:area ratio and increased isolation between local populations. To ensure connectivity between fragmented local populations, effective seed dispersal is of vital importance. Knowledge on dispersal capacity of plant species in fragmented habitats differing in spatial configuration is thus important for understanding (meta)population dynamics and for spatial optimisation of restoration measures. Although many studies have shown the importance of seed dispersal via surface water (hydrochory) for plant population and community patterns, and thus for biodiversity, the precise mechanisms determining hydrochorous dispersal capacity of plant seeds in different fragmented landscapes are underexposed in the literature. The aim of this thesis was to shed light on the contribution of hydrochory to seed dispersal in fragmented freshwater wetlands impacted by human activity, with special attention for the effect of landscape configuration and characteristics of the water body on realised dispersal distances. For this purpose an innovative combination of different research approaches including fieldwork, analyses of large datasets, experimental research and model development and simulations was used. The results showed that habitat fragmentation per se indeed has negative effects on species occurrence in freshwater wetlands. Therefore, it is important not only to improve habitat quality but also to consider spatial characteristics of the habitat of target species when deciding on plant conservation strategies in intensively used landscapes, such as fen areas in Western Europe and North America. Results of the seed dispersal experiments showed that unlike in rivers, seed transport in ditches was determined by wind speed and direction in stead of by water flow, enabling multidirectional seed dispersal. Model results showed that density or direction of the ditch network did not seem to influence simulated water dispersal distances substantially, whereas roughness of the ditch and obstructions in the ditch strongly limited seed dispersal distances. Hydrochorous dispersal contributed considerably to the total simulated dispersal of wetland plant seeds across agricultural landscapes, for both wind and water dispersal specialists. In contrast, simulated wind dispersal distances were nil for water dispersal specialists. Also for wind dispersal specialists, wind dispersal distances were limited. These results suggest that in a modern agricultural landscape with a network of ditches, water is an important dispersal vector for both typical wind and typical water dispersers. The importance of hydrochorous dispersal for connectivity within metapopulations of riparian species in a fragmented landscape may be greater than the importance of wind dispersal. Hence, improving the infrastructure of linear aquatic systems for hydrochrorous seeds could increase metapopulation viability of riparian species, wetland biodiversity, and the success of wetland restoration projects.