Spatial and temporal homogenisation of freshwater macrofaunal communities in ditches

1. There is a widespread concern that we are witnessing an ongoing homogenisation of ecological communities. However, in contrast to human impacts on spatial patterns in biodiversity, human impacts on the temporal aspects of β-diversity have received little attention. Moreover, the interplay between spatial and temporal β-diversity is poorly understood. To address this knowledge gap, we assessed dissimilarity within freshwater macrofaunal communities of drainage ditches to determine spatiotemporal β-diversity as well as homogenisation in relation to different types of land use. 

2. We considered four distinct changes in community composition: spatial turnover, temporal turnover, spatial variation over time, and temporal variation in space, as well as the combined effects of space and time on β-diversity. As a metric of dissimilarity, we calculated the taxonomic Hellinger distance between samples from different locations and time points and correlated these with distance in space and in time, as well as with three spatial variables, including land-use type, and two temporal variables. We studied the effect of interactions between spatial and temporal variables on dissimilarity by applying a permutational analysis of variance.

3. Our results illustrate the importance of changes in community composition in time with respect to temporal turnover, spatial variation over time, and temporal variation in space. While we did not find spatial turnover in community composition, both month and year had a considerable effect. Within a year, β-diversity decreased over the months, yet these assembly patterns differed between years. This suggests major effects of seasonal and year-to-year dynamics on β-diversity. Land use was also observed to be a main driver: ditches in nature conservation areas had higher β-diversity and temporal heterogeneity was lowest in ditches adjacent to the most intensive agricultural land-use category, indicating that agricultural practices can homogenise biodiversity in both space and time.

4. By analysing the spatial and temporal β-diversity patterns in freshwater macrofaunal communities in concert, we have shown that β-diversity is a sensitive and highly informative metric of both spatial and temporal changes in community composition.