Scale-dependent effects of grazing and topographic heterogeneity on plant species richness in a Dutch salt marsh ecosystem

QuestionFor over three decades, low-intensity grazing has been used to maintain or increase plant species richness in European natural areas, but the effects are highly variable. Thus far, good predictors of whether grazing will have positive effects on plant species richness are limited. How does the interplay between low-intensity grazing and topographic heterogeneity affect plant species richness at different spatial scales?LocationLong-term grazed and ungrazed salt marshes of the Dutch Wadden Sea island of Schiermonnikoog.MethodsWe selected ten plots of 2200m(2) in grazed and ungrazed areas of our study sites, and recorded and compared plant species richness in 0.1, 1, 10, 100 and 1000m(2) subplots. Topographic heterogeneity was quantified at the plot scale using the standard deviation of the elevation derived from a high-resolution (5mx5m) digital elevation model. We calculated species-area relationships to analyse our data.ResultsWe found that large-scale topographic heterogeneity (based on the whole plot of 2200m(2)) positively affects plant species richness at all scales (even at the smallest 0.1-m(2) scale), and that grazing has a positive additive effect at the small scales (0.1 and 10m(2)). While grazing also had a positive effect on species richness at larger scales (1000m(2)), the strength of the effect was dependent on the topographic heterogeneity at that scale. The effectiveness of grazing for increased plant species richness was highest at low topographic heterogeneity, and lowest at intermediate topographic heterogeneity. Effects of intermediate heterogeneity were probably counterbalanced by the effects of grazing.ConclusionsOur results suggest that the variation in elevation is an important predictor of whether low-intensity grazing has positive effects on plant species richness or not. Grazing appears most beneficial at low topographic heterogeneity, but whether these findings hold for other grazed ecosystems will depend on several factors, most importantly, the relationship between topographic and abiotic heterogeneity. Results of our study are highly relevant for the application of low-intensity grazing as tool for conservation management in salt marshes and other natural areas.