Organisms often occur in characteristic sets of species called communities, living together in certain types of habitat: rocky shore communities, mudflat communities, and so on. The northwest coast of North America, bathed by a southward flow of cool water, has a characteristic rocky-shore community of plants and animals that occurs from British Columbia to Central California.

Each community has a set of ecological components, which are in general complementary to one another. Some organisms in the community typically act as primary producers, generating food by photosynthesis. Others feed on this primary productivity, as grazers (herbivores). In turn, in this food chain, carnivores may feed on these, and so on. Finally, there are typically recyclers: scavengers who eat dead animals, and fungi and bacteria, which break down organic matter and release nutrients back into the system.

Obviously, some communities vary a lot from this simple (but common) pattern. For example, there are communities of animals in deep water, too dark for photosynthesis. Their food chain begins with organic matter that falls into the habitat from the productive surface waters far above.

The species in communities are obviously adapted to live in the particular physical environment. Therefore the rocky shore communities have a lot of species that attach firmly to rocks, so that they are not swept away. Muddy shore communities tend to have a lot of burrowers, which use the mud for food and for protection.

A community may be diverse, with many species (e.g coral reef); or it may have only a few species (typically, polar communities have only a few species, though the populations of those species may be large).

What about the communities that are in much the same habitat, but different regions? Shouldn't they have much the same sort of creatures and much the same sort of ecology? The answer is yes and no.

[The "Mediterranean" climate of California is very much like the "Mediterranean" climate of the Mediterranean (hot dry summers, cool wet winters), and much like the "Mediterranean" climate of central Chile. However, the ecology of the Mediterranean communities is very different from those of California, while California and Chile are very much alike.]

The rocky shore communities of South Africa look nothing like the seastar/mussel dominated rocky shores of the Pacific Northwest. The reason is simple, but carries a message. In South Africa, the seabirds eat the intertidal snails, which don't eat as much seaweed as our snails do. So the South African rocks get covered over by seaweed, which leaves little room for mussels and barnacles, and little food for sea stars. The message is that the physical setting doesn't control the make-up of the community, though it affects it. Local evolution produces local results.

Community Integration

The species that make up a community are obviously adapted to it physically. But they also have to be adapted to one another. Thus on the Pacific Northwest rocky shore, sea-stars are major predators, making a living by pulling open mussels and clams, and eating them. Clearly the seastars are adapted to this mode of life, and the mussels are as well adapted as they can be to resist the predator.

Ecologists have suggested from time to time that the interactions of the species of the community knit them together into a biological and ecological unit that is highly integrated and therefore does not change much with time. This is probably too simple. Even in ecological time, modern research has documented many changes as climate and environmental disasters (storms, etc.) affect the community. [For example, the last El Nino changed the summer temperatures, and that changed how much time the sea stars could stand exposure to the sun as they roamed around in the mussel beds looking for prey. And that changed the survival of the mussels, and that changed the make-up of the community within a short time.]

It is easy to imagine that a community would evolve with time, but we have to remember that it is NOT the community that evolves, but the species in it.


The coastal communities of the world can be arranged into geographically separate provinces, with each province containing its own set of communities, such as the Oregonian and Californian Provinces of western North America.

Provinces are real phenomena, not artifacts of the human tendency to classify things. There are natural ecological breaks on the Earth's surface, usually at places where geographic or climatic gradients are sharp, so that one may pass from one environmental regime to another in a short distance. A classic example is at Point Conception on the California coast. Here, two different ocean circulation patterns cause a sharp gradient in water temperature (in human terms, Point Conception marks the northern limit of west coast beaches where one can surf without a wet suit). The communities on each side of Point Conception are very different, so a provincial boundary is drawn here, with the Oregonian province grading very sharply into the (southern) Californian province.

About 30 provinces have been defined along the world's coasts, mostly on the basis of molluscs, which are obvious, abundant, and easily identified members of coastal communities. Some provinces are very large because they inhabit long coastlines that lie in the same climatic belt (the Indo-Pacific, Antarctic, and Arctic Provinces); some are small, like the Zealandian Province, which includes only the communities around the coasts of New Zealand.

Each province contains its own communities and therefore carries unique sets of animals that fill various ecological niches. For example, the intertidal rocky-shore community in New Zealand has its ecological equivalent in British Columbia, even though the families and genera of animals are quite different in the two communities. Multiplying the number of provinces automatically multiplies the diversity of animals around the coasts of the world. The total diversity of the world's shallow marine fauna directly reflects the number of provinces, which in turn reflects climate and geography. Today, for example, there is a steep temperature gradient from equator to poles, so there is a strong climatic zonation of the oceans and continents. Each north-south continental coast has several provinces along it, each different enough in climatic conditions to hold a unique fauna.

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