COYOTE (Canis latrans)

The coyote is like a medium-sized dog with a long, bushy, black-tipped tail, prominent ears and a pointed muzzle. It differs from domestic dogs of equivalent size by having very small paws. It is omnivorous, eating carrion, cactus fruits, etc., and it hunts small animals. It is a social animal, found in small family groups in many desert regions, and is active mainly at night or early morning.

Coyotes are widespread in the USA, but are more often heard than seen - the characteristic night-time howling of family groups is a characteristic sound of the desert.

In many lowland parts of the USA, the coyote is a "top predator". Such animals need a large home range in order to forage for food, but increasing urbanisation in some of the desert regions is causing fragmentation of the habitat with interesting and perhaps unexpected ecological consequences.

A classic example of this was reported in a recent scientific paper [Crooks, KR & Soulé, ME, 1999, Nature 400, 563-566]. These authors studied the effects of urban development in native sage-scrub habitat of coastal Southern California - the sort of habitat shown below.

Sage-scrub habitat on a bluff overlooking San Diego

They surveyed 28 urban habitat fragments of sage-scrub over a 2-year period, to record the presence and abundance of coyotes, and the presence and abundance of lesser predators (termed mesopredators). The mesopredators included fox, opossum, skunk and raccoon, but mainly domestic (pet) cats.

The study revealed a significant increase in numbers of mesopredators in areas where coyotes were absent or in areas that coyotes visited infrequently (because the habitat was fragmented by urban development). Evidently, the top predator tends to deter lesser predators, but these lesser predators increase substantially when top predators are absent - a phenomenon that ecologists have termed mesopredator release.

Even more significant was the effect of this relationship on the numbers of native birds that typically breed in coastal sage-scrub habitat - California quail, wrentit, spotted towhee, Bewick's wren, California thrasher, greater roadrunner, cactus wren and California gnatcatcher. The bird diversity decreased markedly as the abundance of mesopredators increased.

So, here is a classic example of the chain of indirect effects of urbanisation:

• Urban development fragments the habitat, causing a decline in activity of a top predator.

• The decline of the top predator allows lesser predators (mesopredators) to proliferate.

• The increase in mesopredators causes substantial damage to the native breeding bird population - an environmental consequence much greater than if the top predator had remained in place.