Top-Down Control of Small Herbivores on Salt-Marsh Vegetation Along a Productivity Gradient

Abstract

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.

Ecosystem assembly rules: the interplay of green and brown webs during salt marsh succession

“…During intermediate succession, when vertebrate herbivores are present in high numbers, herbivores seem to react strongly to the much higher biomass. Earlier experiments have already shown that these very same herbivores change the course of succession strongly but fail to halt it completely, and thus show a strong decline after intermediate successional stages (45 years; Van de Koppel et al 1996, Kuijper andBakker 2005). We conclude that vegetation succession can be understood only in light of food web dynamics, and vice versa.…”

“…Increasing primary production of high quality, and thus more food, likely explains the increasing abundance of vertebrate herbivores over early succession. Previous work in this system has indicated that these vertebrate herbivores not only respond to but also affect the course of vegetation succession (Olff et al 1997, Van Wijnen and Bakker 1999, Kuijper and Bakker 2005. As a consequence, vertebrate herbivores may strongly affect various groups of invertebrates in the intermediate stages of this succession (interactions within the green web).…”

“…In turn, this has led to dominance of plant species with high structural support and hence low palatability (Van der Wal et al 2000a, Bos et al 2005. In intermediate successional stages, small vertebrate herbivores still interact strongly with the course of succession by increasing forage quality (Kuijper and Bakker 2005), while in later stages, the ongoing vegetation succession causes herbivores to ''lose control'' over vegetation composition (Van der Wal et al 2000b, Kuijper andBakker 2005). As a result, macrodetritivores make up most of the animal biomass in late succession, with the terrestrial amphipod Orchestia gammarellus as a dominant species, comprising ;90% of the total faunal biomass.…”

“…Hare and goose densities were estimated every two weeks during 1-14 May and 1-15 July 2008 by averaging pellet counts of accumulated droppings (Kuijper andBakker 2005, Stahl et al 2006). Plant biomass was estimated by clipping the vegetation in 50 3 50 cm squares, to 1 cm height.…”

Variation in Elevation and Sward Height Facilitate Coexistence of Goose Species through Allometric Responses in Wetlands

“…Forage quality and quantity and sward structure depend to some extent on inundation patterns (Kuijper and Bakker 2005), and hence spatial differences in forage quantity and quality along the elevation gradient are expected to influence the abundance of different herbivorous species.…”