Among ecologists, there is general agreement on the importance of competition and herbivory in structuring plant communities. Many field studies provide evidence for either of the two, far less, however, do so for both factors simultaneously. Hitherto, hardly any manipulation of competition as well as herbivory at sites differing in productivity were carried out. Therefore almost no empirical knowledge is available about how the importance of these two biotic interactions might change along productivity gradients. With the study presented here, experimental evidence on this topic is provided.
In a field experiment at three successional stages of a temperate salt marsh, the importance of competition and herbivory were assessed. Four plant species were transplanted as seedling and mature plants in a full-factorial design into plots with neighbours removed or present and herbivores excluded or present. Productivity as well as grazing differed among the three sites.
Competition and herbivory intensively influenced the biomass of the target species. Their relative and absolute impact did not increase significantly with site productivity. Plant species differed in their response to the biotic constraints: The palatable early-successional rosette plant Plantago inaritima was more severely affected by both competition and herbivory than the mid-successional grass Festuca rubra and the late-successional shrub Halimione portulacoides. The mid-successional shrubby Arternisia maritima was only slightly less sensitive than Plantago mariti,na, despite its tall stature and unpalatable leaves.
From the results of this experiment it has to be concluded that competition as well as herbivory heavily influence plant performance and can be powerful mechanisms of vegetation succession.
