Grazers and Indirect Effects

Some grazing activity can have profound effects on ecological processes, diversity, and variety of coexisting species and general structure of assemblages, involving many other species, not just the plants consumed for food. Such interactive consequences of grazing have led to defining some grazers to be ecosystem engineers - species that modify habitats so that they become more or less suitable for occupation by other species. Engineering species must make or modify patches of habitat so that they have environmental conditions not found outside the modified patches.

A particularly large-scale engineering species is the beaver, Castor canadensis, which grazes on plants on the riparian zones of rivers. Grazing by beavers has profound effects on the composition of species of plants in these areas because of major reductions in the abundances of preferred food plants.

Importantly, however, beavers also dam a stream creating a pond. In some areas, the dam and pond are maintained for less than 10 years, during which period the surrounding vegetation is substantially modified by the grazers. The dams trap sediments which store nutrients. The banks of the areas in which beavers forage are generally devoid ofwoody vegetation, allowing more light than would occur where there were no beavers.

When the beavers abandon a pond to seek new supplies of food elsewhere, the dams are eventually broken and large meadows of grasses or swamps dominated by alder (Alnus incana) develop. These engineered habitats can persist for at least 50 years before the habitat eventually reverts to the unmodified woodland.

Beavers, as grazers, have clear direct effects on the assemblages of plants where they are actively foraging. Because of their engineering (dam-building), they also create ponds with nutrient-rich sediments and plenty of light. These habitats can continue to exist for decades after the beavers cease their activity.

There are also examples of indirect effects of invertebrate grazers that can cause profound shifts in the species occupying local assemblages. On many rocky shores, intertidal assemblages contain numerous grazers (mostly snails and limpets, but also crabs, chitons, starfish, poly-chaetes, and some fish). Where grazing is sufficiently intense, the grazers remove much, if not all, of the microscopic algal food from the rocks. This food consists of truly microscopic species, such as diatoms. It also contains the spores, gametes, and early sporelings of many macroscopic species which would, if not consumed while still at unicellular or microscopic sizes, grow up to form erect, upright seaweeds.

Sometimes grazers are absent, as a result of storms or other disturbances, diseases or excessive predation by their own enemies, or simply because offailure to recruit for long enough for the existing adults in a population to die without being replaced. As a result, spores and early reproductive stages of the algae survive in very large numbers compared with the usual grazed condition. The consequence is that these algae then grow up to occupy the space on the shore.

The growth of algae has serious and sometimes long-term consequences for the other species in such an area. Most of the species in the diverse assemblage on intertidal rocky shores require open, relatively bare space to live on or to feed over. For example, barnacles and tubeworms recruit from developing stages in the plankton. These are washed along a coastline by waves, winds, and currents, developing to a stage capable of settlement and metamorphosis in a suitable adult habitat. For this to be possible, they need uncluttered space, either bare rock or a surface covered by a biofilm of unicellular and extracellular organic material. Where the surface is already covered by algae, it is impossible for the sessile species to become established. Even if larvae can settle and metamorphose, they are usually smothered by the algae growing over them, preventing them from growing or feeding.

The grazers themselves are often eliminated by the algae once they grow large enough. Many of the invertebrate grazers are quite unable to consume seaweeds once they are too large. There are some invertebrates, such as sea urchins, which have no problems, but they are not usually common components of intertidal shores.

Thus, grazing by microalgal feeders frees space which is then occupied by a range of other sessile space-users and a suite ofgrazing species. This maintains the diversity ofspecies from many Phyla in the assemblage. Algae grow more quickly in some areas, for example, lower on the shore, where habitats are subject to greater splash and spray during low tide and longer periods of submersion under water during high tide. As a result, in such areas, grazers are less effective and unable to keep surfaces free from foliose algae. Consequently, there are fewer animals and types of animals in such areas.

In addition to their direct consumption of plants, grazers, through their indirect effects on habitat, contribute a lot to the maintenance of biodiversity in assemblages of other species that are neither the food plants nor their consumers.

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