Top-down control of small herbivores on salt-marsh vegetation along a productivity gradient

Exploitation theory predicts strongest plant-herbivore interactions at sites of intermediate productivity. Recent studies illustrate the importance of top-down effects by small to intermediate-sized herbivores in structuring salt-marsh communities. How long-term effects of herbivory are modified by productivity of the system is a largely unexplored area. We studied how herbivory by geese and hares affected plant species replacement by erecting exclosures along a natural productivity gradient in a temperate salt-marsh system. After seven years, the largest shifts in species composition were observed when both hares and geese were excluded from plots. Only excluding geese did not have a large impact on species replacement, indicating that geese alone did not control the vegetation. Herbivory slowed down succession by retarding the establishment and spread of late-successional species on the low salt marsh. Effects of herbivory were less clear on the high marsh. Vegetation change after excluding herbivores was most pronounced in the most unproductive regions of the salt marsh. As a result, different successional trajectories emerged when herbivores were absent or present at the onset of salt-marsh succession. At sites of intermediate productivity, where grazing pressure was highest, no effect of herbivory on species composition was found. We hypothesize that the high dominance of grazing tolerant species at these sites retarded the invasion of late-successional species. The intensity of herbivory did not accurately predict the importance of herbivory in structuring plant communities. To understand herbivore effects along productivity gradients, the selectivity patterns of the herbivores and the bottom-up effects of plants on these interactions need to be considered.