limitations set by light, production of plants is often reduced or prevented by lack of sufficient water or, in some circumstances, essential nutrients. Generally, plants are not limited by their grazers.
To understand why, in general, terrestrial grazers do not keep their food plants at small abundances therefore requires understanding of what reduces or controls the activities of the grazers. Much discussion of this, as a large-scale, worldwide phenomenon, considers three potential controls. One of these is competition among the grazers. This seems quite unlikely, given that the original phenomenon being explained is the prepondence of plant foods, not their small amounts which would lead to shortages of food. Competition will only occur among herbivorous species when more than one species of grazer requires the same plant foods and the food resources are in short supply. Competition then sets in among the grazers as they try to gain sufficient resources to breed (adults), grow (juveniles), or simply to maintain their tissues (all sizes).
Alternatively, abundances or effectiveness of herbivores may be controlled by inclement weather. There is evidence for this for many populations of insects, which do not reach their potential carrying capacity (the size of a population, given a certain availability of resources of food and shelter) because mortality due to bad weather keeps reducing the numbers. Such cases are, however, not considered to be the usual situation. Many types of grazers can deal with bad weather by alterations of activity (e.g., hibernation or estivation) or of reproduction (e.g., by producing offspring during the periods of favorable weather).
The third general mechanism is for populations of grazers to be controlled by their predators (plus pathogens and parasites). Under this proposition, large numbers of herbivores cannot be maintained, even where there is appropriate weather and an abundance of food, because their numbers are culled by their natural enemies.
This led long ago to the conclusion that primary producers - the plants - are largely regulated by availability of and resulting competition for limited resources. Presumably bad weather is also important because many species of plants will not grow outside a specific range of temperature, light intensity, and humidity. Similarly, predators are often limited by availability of their herbivorous prey - suffering from competition when populations of prey are at small abundances. Both are examples of 'bottom-up' control. Populations of plants and predators are generally supposed to be regulated by their resources.
In contrast, terrestrial grazing species are thought to be primarily limited by their consumers, which is an example of 'top-down' control.
The extent to which such a generality is correct can only be ascertained by examining a large number of experimental studies of the process controlling populations of herbivores, which is beyond the scope of this overview. Nevertheless, it is clear that, in general, plants on land are abundant despite numerous varieties of grazers.
It is evident that when grazers are released from their predators, they can completely destroy their food plants. Two sorts of examples demonstrate this. Accidental introductions of alien herbivorous species into a new biogeographical region are often followed by outbreaks of the herbivores in extremely large densities, voraciously consuming the plants they use as food. Quite often, the cause of an outbreak is that the natural enemies (predators, pathogens, parasites) of the introduced pest are not introduced with it. The herbivorous species is therefore not held in check and its numbers become excessive.
The other scenario is where human intervention (or management) is used to reduce numbers of insects (or other predators). Spraying forests or farms to remove nuisance herbivores (e.g., larval insects) sometimes also kills other insects which are predatory. As a result, populations of different herbivores that are normally killed by these predators expand and become a new nuisance because they consume trees or agricultural produce. In both cases, the grazers, by being released from their enemies, seriously reduce their food plants.
There are many exceptions to a general principle that grazers do not control their food plants. A well-known example is the destruction of plants caused by plague locusts, which break out in massive populations and greatly reduce the amounts of plants in areas where they feed.
The most notable exception is, however, the case of grazing by planktonic animals (see below). In the plankton, grazing generally reduces populations of plants to very small abundances. Given that about 70% of the Earth's surface is ocean, the pattern of populations of plants being controlled by grazing is, in fact, more widespread than that described above for terrestrial habitats.
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