Grazing in heterogeneous environments: infra- and supra-parasite distributions determine herbivore grazing decisions.

We tested the hypothesis that the infra-gastrointestinal parasite population of herbivores affects their grazing behaviour in relation to the supra-parasite population of parasites in the environment. Our first objective was to create a naturally heterogeneous sward structure of gaps and tussocks using a continuous grazing scheme. We then demonstrate that a nutrition vs. parasitism grazing trade-off occurs within that sward structure and that infra-gastrointestinal parasite populations affect the grazing decisions of herbivores faced with the trade-off. A pool of 50 naturally parasitised female Soay sheep and their lambs were used to create a heterogeneous tall, faeces-contaminated tussock/short, non-contaminated gap sward structure in a 1-ha experimental plot. Tussocks offered approximately 1.5 times greater forage intake but contained 5.5 times the number of strongyle parasites compared to the gaps. Following a 10-week period in which the heterogeneous sward structure was created, two 5-day periods of observations of sward structure selection (i.e. gap vs. tussock) were carried out. Twenty female Soay lambs were divided into two groups of ten (balanced for live-weight) immediately prior to the start of the observation period. One of the groups of lambs was treated with an anthelmintic drench before the start of the second observation period creating two levels of parasitism (high and low). On each observation day 5-min focal observations were carried out on each animal at least twice a day, during which time the number of bites taken from gaps and tussocks were recorded along with the number of steps. During the first period of observations, all animals rejected the relatively tall, faeces-contaminated tussocks for grazing to a similar extent and had similar bite and step rates. During the second period of observations all animals showed reduced rejection of the tussocks relative to the first week, however, animals with a reduced parasite population showed a greater reduction in rejection as compare to the highly parasitised animals. We conclude that the infra- and supra-distributions of parasites within herbivore hosts and the environment greatly impact on herbivore grazing behaviour and foraging decisions and thus the structure and heterogeneity of grazed ecosystems.