Do biodiversity patterns in Dutch wetland complexes relate to variation in urbanisation, intensity of agricultural land use or fragmentation?

Red list species densities of birds (maximally 22 km(-2)), and angiosperms (maximally 39 km(-2)) were used as biodiversity indicators in 21 larger complexes of wetlands across the Netherlands. Their covariability with a range of indicators of human land use was assessed, including population, road and visitor density, area covered by agriculture, open water, forest and residential housing. Data were collected on the wetland complexes as well as for a perimeter with 10 km radius. In a principal components analysis (PCA) with all land use variables, it was found that the population-density-related complex of urbanisation, fragmentation (by roads), and intensity of fertilizer use together explained most of the variability present (i.e. the first PCA axis explained 50%), whilst land use within these complexes was second with an additional 19% and waterside recreation third with 12%. Red list bird species density did not correlate with that of angiosperms, nor with any of the indicators used. For the 13 complexes on organic peatland, we observed an increase in maximum red list angiosperm species density with the proportion of open marshland (P < 0.01, r(2) > 0.55), which, in turn, was negatively and closely correlated with the first PCA axis reflecting an urbanisation gradient across the Netherlands.