Switching and Facilitation by Grazers Mechanisms Sustaining Biodiversity

Grazers not only change the diversity of their food plants by direct action (reducing abundances and eliminating some species) or by indirect interactions as discussed above (i.e., by preventing competition and therefore increasing the number of species). By changing their own behavior, grazers can sometimes ensure that food plants are not eliminated.

The processes used by herbivores to choose what to eat are complex. Some situations conform to 'optimal' selection; the composition of the diet reflects how the grazer may be able to maximize the input of energy for the minimal expenditure of energy used to acquire the food. In some cases, the consumption of different species is altered by the risks imposed in feeding. For example, venturing into open spaces to gain access to some species of plant that is not found in shade under trees may make the herbivore vulnerable to attacks by predatory birds.

There are also defensive systems adopted by plants, that is, modifications to morphology to make tissues tougher to eat or to provide protection in the form of thorns and spines. Chemical defenses consist ofnoxious or bad-tasting chemicals which herbivores learn to avoid.

Choices and preferences among different potential components of diet are very common in all types of herbivores. They are often complex and usually require considerable care in the designs and analyses of experiments to investigate them. There is, however, no doubt that modification of behavior by individual grazers can make very marked differences to the persistence of diversity of the plants they are eating. A well-known example is switching - changing between different species of food as their relative abundances change because of grazing.

Switching is the process by which grazers preferentially consume the more abundant of the available food species, but change to different species when they become more abundant. Thus, certain butterflies (e.g., Battus philenor) lay eggs on two different species of plants. The caterpillars then hatch and consume the plant on which they have been laid. The adults tend to seek out the more common of the two plants in disproportionately large relative abundances. Thus, if species A to B are in the ratio 60:40, A will be chosen for oviposition at greater than 60% of all the plants on which eggs are laid. In areas or at times when B are more abundant, the butterflies change to lay eggs on an excessively larger proportion of B. Where a grazer actually consumes the more abundant species in disproportionally excessive amounts, it will reduce the relative abundance of that species. Under these circumstances, the grazers then alter the availability of different components of their diet and switch to whichever has now become more available. Such behavior seems to maintain different species of plants in the habitat, in contrast to herbivores that continue to attack a particular species, sometimes resulting in its elimination and therefore a reduction in biodiversity.

Complex interactions among grazers can also have as an outcome the continued persistence of a suite of grazing species. A particularly well-known example is the set of grazing mammals in the Serengeti Plain in East Africa. There are numerous grazers (>20 major species) of which the three most studied are zebra, wildebeest, and Thompson's gazelle. These are in decreasing order of size (220, 160, and 16 kg, respectively, on average). These mammals migrate over very large distances, in a cyclic pattern. All three grazers eat the same grasses, but consume different parts of the plants.

The zebra mostly eat stems and the sheaths of grasses, because these are the predominant components of older plants. This material has relatively little protein and large amounts ofindigestible lignin. They obtain adequate nutrient by consuming very large amounts of plant material. Removing the coarser and relatively indigestible components of the grasses, zebra stimulates growth of new leaves and makes the food supply more accessible to wildebeest, which eat grass sheaths and leaves. These parts of the plants, particularly the leaves, contain more protein than the parts eaten by zebra. The wildebeest, in turn, modify the grasses and also expose various understorey herbs, which are then eaten by gazelles. The gazelles' diet is leaves and herbs, the latter containing even more nutrition than grass leaves.

Experimental enclosures to keep out wildebeest have demonstrated a marked reduction in amounts of vegetation where these mammals feed. In some areas, the biomass of plants was reduced to about 15% of that in ungrazed areas. This does, however, cause an increase in the production of new growth of leaves and, after the wildebeest move to new areas, there is a great amount of food available to gazelles. These grazers avoid areas that have not been previously grazed by wildebeest.

The zebra move into areas where there has been no grazing since the previous year. Their feeding reduces the availability of suitable food plants, so they move on to new areas. Their activity has, however, facilitated the feeding by wildebeest because the zebra have made the plants more suitable for them. The wildebeest, in turn, consume the parts of the plants on which they feed and then, as their food decreases in abundance, move on to the areas recently fed on by zebra. The wildebeest have facilitated feeding by gazelles, by making the latter's food accessible. The gazelle then follow the wildebeest. The entire cycle starts again when zebra return to the first areas, where gazelles ceased feeding months before and where grasses have grown up and are now older and with more stems and fewer leaves.

This cyclic pattern of facilitated grazing maintains the suite of grazers which use different parts of the grasses, in turn, maintaining the biodiversity of large grazing mammals.

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