Spatial and temporal homogenisation of freshwater macrofaunal communities in ditches

Abstract

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. 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. 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. 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.

Partitioning the impact of environmental drivers and species interactions in dynamic aquatic communities

“…The low impact of pesticides can be regarded as surprising given the high and persistent pesticide loads in parts of the area due to the flower bulb industry (Hunting et al 2016, Barmentlo et al 2018; http://www.pesticidesatlas.nl), although it may also suggest that the community is welladapted to pesticides due to long-term exposure, so that very sensitive taxa may be missing from the area. 2) as well as the decreased dissimilarity between samples (Musters et al 2019) coincides with this transition, reinforcing the idea that the community becomes more strongly regulated over time, either by life histories of taxa, nutrients, or both. When lumping pesticides and the other chemicals, the suggestion that these drivers become less important over the seasons is even stronger (Appendix S3: Fig.…”

“…A detailed description of the research area, macrofaunal sampling strategy, and taxonomic identification level for each group is given in Ieromina et al (2015, Musters et al 2019. Briefly, the research area of ca.…”

“…However, it has been shown that this measure is biased toward correlated predictors (Strobl et al 2007). Indeed, our research area is so small and the sampling sites are so well connected, that the distribution of our taxa is not spatially delimited ❖ www.esajournals.org (Viana et al 2015, Musters et al 2019). We regard each conditional importance of a taxon for predicting another taxon as an estimate of the unidirectional interaction between the two taxa.…”

“…This procedure assumes that the sampling sites differ in their values of predictor variables and abundance of the taxa, but that each taxon can be present at any of the sampling sites, and that the effects of dispersal and order of arrival can be ignored (Little and Altermatt 2018). Indeed, our research area is so small and the sampling sites are so well connected, that the distribution of our taxa is not spatially delimited (Viana et al 2015, Musters et al 2019). The conditional random forests and importance were calculated with the party package of R (Hothorn et al 2006, Strobl et al 2007, 2008.…”

“…We illustrate the applicability of this novel approach by analyzing a comprehensive dataset of aquatic ditch macrofaunal samples (Verdonschot et al 2011). This dataset describes a set of ditches that are exposed to different types of land uses that create a mosaic of different abiotic pressures affecting the adjacent aquatic system (Ieromina et al 2015, Hunting et al 2016, Musters et al 2019. Over the seasons, we expect to find in the macrofaunal community a shift in the relative importance of community processes, from those related to population growth toward an increased importance of interspecific interactions.…”