Three ecological postulates that underlie conservation biology

Modern conservation biologists must often transcend the traditional boundaries of academic disciplines, for these scientists increasingly need to know about politics, economics, philosophy, anthropology, and sociology in order maintain or restore the health of ecosystems. However, because conservation biology is fundamentally concerned with the dynamics of wildlife populations, a solid understanding of biology and ecology is paramount for workers within the discipline. Three ecological principles are fundamental for understanding the relationship between population dynamics and conservation.

MANY ORGANISMS ARE THE PRODUCTS OF COEVOLUTION

If most species in an ecosystem were generalists, then in the absence of one generalist species, another generalist species would broaden its niche slightly, and the system would continue to function without important changes. However, if species tend to be specialized, then they are not interchangeable parts in the system; when one is lost, the local ecological community may be affected. For the conservation biologist, interdependent specialization is particularly important, and interdependent specialization often arises through coevolution.

Coevolution involves a series of reciprocal adaptive steps during which two or more interacting species respond to one another evolutionarily. A study of mammalian grazing ecology offers many classic examples of coevolution. Ruminant artiodactyls have evolved fermentation chambers that shelter legions of microscopic flora and fauna. These microbial sym-biants extract the energy and nutrients they need from the vegetation consumed by the host-ruminant. In return, the gut-flora ferment cellulose, providing energy and repackaging nitrogen for their hosts. Grazing mammals, in turn, structure the vegetative communities of their grassland habitats. Higher-order coevolution has been demonstrated among

At the Animal Welfare and Conservation Organization in Okonjima, Namibia, lions are encouraged to play to help stay fit. (Photo by Nigel J. Dennis/Photo Researchers, Inc. Repoduced by permission.)

species of grazing ungulates, particularly in Africa. Thomson's gazelles, or "tommies" (Gazella thomsonii), for example, are so small that they cannot effectively exploit the tall grass that grows rapidly after the first rains of the wet season. So, just as other grazers depart the depleted grasslands surrounding a recovering waterhole, tommies move in to exploit the flush of tender, new grass. Eventually tommies disperse to exploit grasslands grazed low by other ungulates (particularly zebra, Equus burchellii, and wildebeest, Connochaetes taurinus). The seasonal ecology, anatomy, and gut flora of G. thomsonii evolved in response to the seasonal ecology of the larger ungulates; without these other animals, populations of the little tommies would be much smaller indeed.

Some species, called keystone species, are especially important for the interdependent functioning of an ecosystem. Keystone species may comprise only a small proportion of the total biomass of a given community and yet have fundamental impacts on the community's organization and survival. The loss of such species may have dramatic and far-reaching consequences in the broader ecological community. Primates and bats are believed to play key roles in maintaining ecosystems through dispersing seeds (some primates), pollinating plants (bats and some primates) and serving as prey items. The loss of these species from ecosystems would be predicted to have deep impacts on ecosystem health. For example, throughout many areas in Trinidad, large mammalian species such as deer,

An African wild cat (Felis libyca) in the Kalahari Gemsbok Park, South Africa. Game preserves have been established to provide natural habitats for wild animals. (Photo by Animals Animals ©Ingrid Van Den Berg. Reproduced by permission.)

paca, agouti, and peccaries have been extirpated—and yet the ecosystems still remain functional. On the other hand, within these ecosystems, Trinidad's bat and primate populations may be fulfilling ecological roles for which few other occupants remain. Thus the monkeys and bats may now be keystone species, whose presence is vital to the now-fragile existence of the Trinidad ecosystems. Similarly, in pre-European South Carolina, cougars (Felis concolor) and a large, social canid (Ca-nis sp.) structured the forest herbivores. Now, in the absence of these top predators, whitetail deer (Odocoileus virginianus) obliterate populations of several species of forest herb.

IN ECOLOGICAL SYSTEMS SOME CRITICAL VARIABLES HAVE THRESHOLD LEVELS

Changes in one of these variables may make very little difference in ecosystem operation—until a threshold is crossed, and then dramatic systems-alterations will occur. The mathematical study of nonlinear "threshold relationships" is the province of bifurcation theory, which has been used to model catastrophic phenomena ranging from domestic violence to human heart failure. Many conservation biologists emphasize a particular corollary of this general threshold postulate: some ecological processes may suddenly fail when the landscape patch in which they operate is reduced below a threshold size.

Biologist-activist Paul R. Ehrlich has written several books on ecology and conservation, a recent one is A World of Wounds: Ecologists and the Human Dilemma, Ecology Institute, Oldendorf-Luhe, 1997. He illustrated potential dangers of ecological non-linearities by the following metaphor. Pretend that the world ecological system is an aircraft and that species within the system are rivets holding the aircraft together. If one or two rivets are lost, the aircraft continues to fly as if nothing had happened. More rivets are lost and the airplane still flies. But eventually the loss of "just one more rivet" may bring the flight to a sudden, disastrous end.

GENETIC AND DEMOGRAPHIC SYSTEMS HAVE THRESHOLDS

Like ecological systems, genetic and demographic systems can be nonlinear and have thresholds below which nonadap-tive, random processes begin to displace adaptive, "statistically deterministic" processes.

One example of this is the loss of alleles in small populations because of genetic "drift." Another is the extinction of a small population through random binomial processes. This point can be illustrated by an extreme demographic example. Consider a hypothetical species that does not breed during the dry season and suffers high dry-season mortality. More specifically, assume that each female entering the dry season has a 50% probability of surviving until the end of the dry season. Now consider the probable fates of two different populations:

• 10,000 females enter the dry season. The chances are about 95% that the population at the end of the dry season will include 4900 to 5100 females. In other words, the chances of population extinction are almost exactly 0%. (These statements can be demonstrated by an approximation of the binomial theorem.)

• Two females enter the dry season. The chances are about 25% that the population at the end of the dry season will include 0 females. In other words, the chances of extinction for this small population are about 1 in 4.

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