Scale-Dependent Effects Of Grazing And Topographic Heterogeneity On Plant Species Richness In A Dutch Salt Marsh Ecosystem

AbstractQuestion

For over three decades, low￢ﾀﾐ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￢ﾀﾐintensity grazing and topographic heterogeneity affect plant species richness at different spatial scales?

Long￢ﾀﾐterm grazed and ungrazed salt marshes of the Dutch Wadden Sea island of Schiermonnikoog.

We selected ten plots of 2200ￂﾠm2 in grazed and ungrazed areas of our study sites, and recorded and compared plant species richness in 0.1, 1, 10, 100 and 1000ￂﾠm2 subplots. Topographic heterogeneity was quantified at the plot scale using the standard deviation of the elevation derived from a high￢ﾀﾐresolution (5ￂﾠmￂﾠￃﾗￂﾠ5ￂﾠm) digital elevation model. We calculated species￢ﾀﾓarea relationships to analyse our data.

We found that large￢ﾀﾐscale topographic heterogeneity (based on the whole plot of 2200ￂﾠm2) positively affects plant species richness at all scales (even at the smallest 0.1￢ﾀﾐm2 scale), and that grazing has a positive additive effect at the small scales (0.1 and 10ￂﾠm2). While grazing also had a positive effect on species richness at larger scales (1000ￂﾠm2), 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.

Our results suggest that the variation in elevation is an important predictor of whether low￢ﾀﾐ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￢ﾀﾐintensity grazing as tool for conservation management in salt marshes and other natural areas.