Cyclic grazing in vertebrates and the manipulation of the food resource

Two facets of the putative power of herbivores to manipulate their food supply according to the ideas of McNaughton are reviewed with special reference to a young salt marsh in the Dutch Wadden Sea. A general feature of vegetations described as 'grazing lawns' seems to be that return visits by the herbivore enhance the quality of food on offer. In the salt marsh we studied, intensive and repeated grazing by staging geese in spring protracted the period of high nutritive status of the food plants, effectively extending the exploitation window for the herbivores by 7 weeks. Detailed quantification of the visit interval in relation to regrowth of previously grazed ramets indicate that the geese time their grazing cycle in order to maximize the intake of nutritious regrowth. However, the observed visit interval tends to fall on the early side of the model outcome for intake maximization, suggesting that the competitive interaction between flocks on the marsh needs further attention. In the longer term, vertebrate herbivory on the marsh extends the period of persistence of the younger successional stages of the vegetation supporting the heaviest level of exploitation. In our island study, the year-round presence of the brown hare, Lepus europaeus, retarded establishment of the dwarf shrub Atriplex on the young marsh, and thus facilitated access by the geese to the palatable species otherwise screened by the bushy Atriplex. Exclosures placed on the marsh at sites of different age up to 40 years, combined with experimental removal or addition of Atriplex plants, allow the conclusion that hares facilitate grazing by geese and extend the goose exploitation period by 25 years. Why the herbivores fail to arrest the onward march of plant succession on the marsh is unresolved but may be caused by ongoing changes in the nutrient status of the soil. The grazing lawns are thus not permanent in our system, but the general features of the plant-herbivore interaction agree with the framework hypothesized by McNaughton.