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may be traced only as far as the herbivores and plants, or through other indirect links

to more distant herbivores, detritivores, protozoans, even microbes (Mills et al. 1993;

Power et al. 1996).

Despite these problems, we recognize that some species define the community com-

position, and removal of these can produce changes in state, whereas the loss of other

species has little effect on the rest of the community. The conservation consequences

from the loss of wildebeest as a keystone species in Serengeti would be dispropor-

tionately greater than those due to the loss of black rhino (Diceros bicornis) and wild

dog (Lycaon pictus), both of which have occurred with little impact on the system.

Keystone species can also have counterintuitive effects. Bell miners (Manorina

melanophrys) are dominant territorial insectivorous birds feeding on psyllids

(plant-sucking Homoptera) in Australian eucalypt forests. Where bell miners occur

trees appear unhealthy, the foliage infested with these insects. When bell miners were

removed, 11 other insectivore bird species moved in, fed on the psyllids, and within

4 months eradicated the infestation (Loyn et al. 1983). Interspecific territoriality

by the miners maintained their food supply but reduced the diversity of competing

predators.

The management and conservation implications for keystones such as wildebeest

or bell miners are very different from those for other species in their ecosystems. The

task of managers is to identify such species.

Top predators can increase the diversity of prey species through intermediate dis-

turbance effects (Connell 1978). However, predators can also have the opposite effect

and reduce diversity of prey. Such effects arise because rare species are secondary

prey, essentially by-catchfor predators that depend on more common prey (Chapter

10). It is, of course, the rare species that attract the attention of managers.

Exotic predators are of special management concern because they can threaten

rare species. We see this where exotic predators such as red fox (Vulpes vulpes) in

Australia, stoats (Mustela erminea) in New Zealand, and many other species on islands

in the Pacific and Indian oceans, supported by exotic prey or carrion, have caused

the extinction of numerous marsupial mammals, birds, and invertebrates (Serena 1994;

Atkinson 2001).

The concept of by-catch stems from harvesting, particularly fisheries harvests. Many

fish species are caught as by-catch in fisheries that focus on more abundant species.

The latter can be maintained while the by-catch species decline, either because they

are easier to catch or because they have a lower intrinsic rate of increase (Hilborn

et al. 2003). In the Atlantic, both common skates (Raja batis) and barndoor skates

(Raja laevis) almost declined to extinction because of by-catch (Casey and Myers 1998),

and shark numbers have declined by over 50% since 1990 due to by-catch in the

swordfish and tuna fisheries (Baum et al. 2003).

A prey species’ role can change depending on the presence of other species. In

British Columbia, moose (Alces alces) are primary prey for wolves, which in turn

are driving mountain caribou (Rangifer tarandus) to extinction (Seip 1992; Wittmer

et al. 2005); but in the nearby Banff ecosystem moose are secondary prey that are

being exterminated by wolves dependent primarily on elk (Cervus elaphus).

In general, the dynamics of predation, whether by natural predators or by humans,

affect prey species differently depending on their role in the system, their abundance,

and their intrinsic adaptations (Courchamp et al. 1999, 2000b). Therefore, the

ECOSYSTEM MANAGEMENT AND CONSERVATION 369

21.6.2Overpredation