Role of grazers on the relative importance of stochastic and deterministic drivers of community assembly

ALBERTI, J.; SMIT, C.; BAKKER, J.; OLFF, H.

Saint-Malo

Workshop; Coastal Ecology Workshop; 2014

The assembly of ecological communities is the result of the joint action of both, stochastic and deterministic processes. While stochastic forces increase variability (higher b-diversity), deterministic forces promote homogenization (reduced b-diversity) of communities [1]. As has been shown with factors such as fertilization and competition [2, 3], we predict a context-dependent role of grazers regulating b-diversity. We hypothesize that herbivores can increase the relative importance of stochastic community assembly by relaxing deterministic competitive exclusion in highly productive systems with reduced species pools (e.g. salt marshes). However, in systems with larger species pools and less productivity (e.g. grasslands), herbivores can reduce stochastic competitive exclusion in small-scale patches, increasing the relative importance of deterministic community assemblages. We used two long-term experiments (20 years) manipulating the presence of herbivores to evaluate these predictions. One of the experiments was conducted in an old and high salt marsh of Schiermonnikoog (one of the Dutch Frisian Islands), and the other was conducted in a floodplain grassland within Junner Koeland, a nature reserve located in the Northeast of The Netherlands. As expected in our predictions, exclusion of herbivores led to a striking convergence between plots, while the opposite pattern was seen in the grassland. These results reveal that the already known context-dependent effect of herbivores on a-diversity can scale up affecting also b-diversity. In addition, they also suggest an interesting interaction between the size of the species pool, system productivity and herbivory that finally regulates fundamental mechanisms of community assembly.