Chapter 14
Some benthic community ecology
Ecologists use the term community to refer to the suite of organisms constituting the
living part of an ecosystem. Unfortunately, the term is burdened with connotations
coming from its ordinary, daily use in respect to interacting groups of human beings.
A human community functions by a division of labor. Some farm, some butcher, some
manage insurance schemes, some are ballerinas, and so on. Some functions are
essential, like food gathering and distribution, some less so. Early usage of the term in
ecology implied the assumption that natural plant and animal assemblages are also
characterized by division of labor. To a degree that is clearly so: plants generate the
basic organic matter which allows all biological activity; herbivores convert that to
mobile form; carnivores crop the herbivores; decomposers recycle the raw materials.
(^) A question which quickly arises concerns the extent to which specific interactions
are obligatory. Are exact or nearly exact combinations of species required to form
functional, healthy communities and, thus, ecosystems? Or can most species get along
under suitable physical conditions in a variety of combinations with other forms?
Schools of thought developed in each direction in late 19th- to early 20th-century
plant ecology, opinions eventually associated with the names of Fredric Clements and
Henry Gleason. Clements’ followers favored the view that most plant species live in
quasi-obligatory sets (like the organs in a single organism), while those following
Gleason thought the arrays of plants found at most sites were quasi-random
assemblages of species that happened to be suitable to the physical habitat, in some
cases just those whose seeds first reached patches of open ground. The empirical facts
developed down the years fall somewhere between, but generally favor the
Gleasonian view. For example, in transects along habitat gradients, such as up the side
of a mountain, plant species are added and removed one at a time, not in matched sets
changing at common boundaries. Thus, no species is absolutely required by most of
the others. Within the limits of their basic life mode, plants (and animals, too) are
sufficiently flexible to deal with a variety of other species as competitors, predators,
and associates generally. The same conclusion applies to species of benthic animals
proceeding along gradients of depth and organic-matter availability from the intertidal
to the deep sea (e.g. Dayton & Hessler 1972). Of course, this is not to say that highly
obligate associations never occur. They are particularly frequent for parasite–host
combinations.