Mechanisms Influencing Grazing Success for Livestock and Wild Herbivores

McNaughton (1984) argued that herbivores do better on vegetation that has been heavily grazed, suggesting that this phenomenon explains why many herbivores aggregate. I pointed out contrary experimental evidence from rangelands research (Westoby 1985). McNaughton (1986) disputed the relevance of this evidence. Here I enlarge somewhat on my earlier comment (Westoby 1985), in the hope of clarifying the differences between my views and McNaughton's. Consider two pastures, one grazed more heavily and the other less, but similar in other respects. Which pasture will be more favorable for foragers? Several mechanisms may be operating. The more heavily grazed pasture may provide herbage with a greater concentration of biomass in a layer near the ground, allowing bigger bites. The herbage may be more digestible and nutritious, on the average, because it includes less dead material and more green regrowth. However, the less heavily grazed pasture contains more total herbage. This may allow foragers to select bigger or more nutritious bites from the larger array available, or it may allow them to bite more frequently. Moreover, in the less heavily grazed pasture, a larger proportion of the food available may be from preferred species. A priori, then, arguments can be made for either pasture's being more favorable. The outcome depends on which mechanisms are more important in the real world. I (Westoby 1985) cited reviews of more than 30 experiments, all of which found that sheep and cattle experience lower per capita weight gains or reproductive success on more heavily grazed pastures. Similarly, a recent review by Allison (1985) found that food intake is generally lower on more heavily grazed pastures. The only controlled experiment I know of that had contrary results, and the one used as evidence by McNaughton (1984), is the work by Stobbs and his colleagues with cattle grazing improved tropical pastures (see references in McNaughton 1984; Westoby 1985). So, in most of the situations studied thus far with sheep and cattle, the mechanisms that make heavily grazed pastures more favorable seem to carry less weight than the mechanisms that make lightly grazed pastures more favorable. (Note that the benefit to the individual forager is being considered here; the total weight gain of the herd is low when there are few animals and generally maximal at an intermediate stocking density.) McNaughton and I agree that experiments making this general comparison are the proper tests of the question, but we disagree about which organisms are relevant. McNaughton (1986), invoking coevolution, argued that wild animals are quite different from livestock. (He used evidence from Stobbs' cattle, how-