Herbivores Enforce Sharp Boundaries Between Terrestrial and Aquatic Ecosystems
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The transitions between ecosystems (ecotones) are often biodiversity hotspots, but we know little about the forces that shape them. Today, often sharp boundaries with low diversity are found between terrestrial and aquatic ecosystems. This has been attributed to environmental factors that hamper succession. However, ecosystem properties are often controlled by both bottom-up and top-down forces, but their relative importance in shaping riparian boundaries is not known. We hypothesize that (1) herbivores may enforce sharp transitions between terrestrial and aquatic ecosystems by inhibiting emergent vegetation expansion and reducing the width of the transition zone and (2) the vegetation expansion, diversity, and species turnover are related to abiotic factors in the absence of herbivores, but not in their presence. We tested these hypotheses in 50 paired grazed and ungrazed plots spread over ten wetlands, during two years. Excluding grazers increased vegetation expansion, cover, biomass, and species richness. In ungrazed plots, vegetation cover was negatively related to water depth, whereas plant species richness was negatively related to the vegetation N:P ratio. The presence of (mainly aquatic) herbivores overruled the effect of water depth on vegetation cover increase but did not interact with vegetation N:P ratio. Increased local extinction in the presence of herbivores explained the negative effect of herbivores on species richness, as local colonization rates were unaffected by grazing. We conclude that (aquatic) herbivores can strongly inhibit expansion of the riparian vegetation and reduce vegetation diversity over a range of environmental conditions. Consequently, herbivores enforce sharp boundaries between terrestrial and aquatic ecosystems.
KeywordsBiodiversity ecotone herbivory nutrient availability riparian vegetation species turnover succession spatial patterns transition zones vegetation N:P ratio water depth
The authors acknowledge Deltares for providing data on the water level fluctuations in each of the study areas, This Buurman (Hoogheemraadschap De Stichtse Rijnlanden) for providing data on muskrat catches. Staatsbosbeheer, Natuurmonumenten, Het Noordhollands Landschap and Waternet are acknowledged for allowing and facilitating entrance to the study areas. Dennis Waasdorp, Nico Helmsing, Roy Korthout, Jose van Paassen and Isolde van Riemsdijk, Koos Swart, Thijs de Boer, Lone Ravensbergen and Roel Janssen are acknowledged for their help with practical work. This study was funded by Agentschap NL. This is a NIOO 5671 publication. Data available from the Dryad Digital Repository: http://doi.org/10.5061/dryad.qp47h.
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