2.1 ‘UNEXPECTED’ EFFECTSCONNECTIONS BETWEEN THEM IN THE WEB, AND IN...

20.2.1 ‘Unexpected’ effects

connections between them in the web, and interaction strengths)

and on the consequences of these species interactions for eco-

The removal of a species (experimentally, managerially or

system processes such as productivity and nutrient flux.

First, we consider the incidental effects – repercussions further

naturally) can be a powerful tool in unraveling the workings

of a food web. If a predator species is removed, we expect an

away in the food web – when one species affects the abundance

of another (Section 20.2). We examine indirect, ‘unexpected’

increase in the density of its prey. If a competitor species is

effects in general (Section 20.2.1) and then specifically the effects

removed, we expect an increase in the success of species with which

of ‘trophic cascades’ (Sections 20.2.3 and 20.2.4). This leads

it competes. Not surprisingly, there are plenty of examples of such

expected results.

naturally to the question of when and where the control of food

Sometimes, however, removing a species may lead to a

– since the deliberate aim is to solve a problem, not create further,

decrease in competitor abundance, or the removal of a predator

unexpected problems.

For example, there are many islands

mesopredators

may lead to a decrease in prey abundance. Such unexpected

effects arise when direct effects are less important than the

on which feral cats have been allowed

to escape domestication and now threaten native prey, especially

effects that occur through indirect pathways. Thus, the removal

birds, with extinction. The ‘obvious’ response is to eliminate

of a species might increase the density of one competitor, which

the cats (and conserve their island prey), but as a simple model

in turn causes another competitor to decline. Or the removal of

a predator might increase the abundance of a prey species that

developed by Courchamp et al. (1999) explains, the programs may

not have the desired effect, especially where, as is often the case,

is competitively superior to another, leading to a decrease in the

rats have also been allowed to colonize the island (Figure 20.1).

density of the latter. In a survey of more than 100 experimental

The rats (‘mesopredators’) typically both compete with and

studies of predation, more than 90% demonstrated statistically

significant results, and of these about one in three showed

prey upon the birds. Hence, removal of the cats (‘superpredators’),

unexpected effects (Sih et al., 1985).

which normally prey upon the rats as well as the birds, is likely

These indirect effects are brought especially into focus when

to increase not decrease the threat to the birds once predation

the initial removal is carried out for some managerial reason

pressure on the mesopredators is removed. Thus, introduced

cats on Stewart Island, New Zealand preyed upon an endangered

– either the biological control of a pest (Cory & Myers, 2000) or

the eradication of an exotic, invader species (Zavaleta et al., 2001)

flightless parrot, the kakapo, Strigops habroptilus (Karl & Best, 1982);

c

Superpredator

Mesopredator

Prey

Figure 20.1 (a) Schematic representation

of a model of an interaction in which a

‘superpredator’ (such as a cat) preys both

on ‘mesopredators’ (such as rats, for which

it shows a preference) at a per capita rate

µ

, and on prey (such as birds) at a per

capita rate µ

, while the mesopredator

also attacks prey at a per capita rate η

.

Each species also recruits to its own

population at net per capita rates r

, r

and r

. (b) The output of the model with

realistic parameter values: with all three

species present, the superpredator keeps

the mesopredator in check and all three

species coexist (left); but in the absence

of the superpredator, the mesopredator

Time→

drives the prey to extinction (right).

(After Courchamp et al., 1999.)

but controlling cats alone would have been risky, since their pre-

ferred prey are three species of introduced rats, which, unchecked,

250

could pose far more of a threat to the kakapo. In fact, Stewart

200

150

Island’s kakapo population was translocated to smaller offshore

100

islands where exotic mammalian predators (like rats) were absent

50

0

or had been eradicated.

Further indirect effects, though not really ‘unexpected’, have

60

occurred following the release of the weevil, Rhinocyllus conicus,

40

as a biological control agent of exotic thistles, Carduus spp., in the

20

USA (Louda et al., 1997). The beetle also attacks native thistles in

the genus Cirsium and reduces the abundance of a native picture-