untitled

The three most common causes of driven extinctions, roughly in order of import-

ance, are (i) contraction and modification of habitat; (ii) unsustainable harvesting by

humans; and (iii) introduction of a novel pathogen, predator, or competitor into the

environment (Hilton-Taylor 2000). Case studies are considered in some detail in Chapter

18. Before we consider the historical record, however, we briefly consider how the-
    oretical models can provide useful insight into each of these processes.

The first major way in which humans wreak havoc on threatened species is through

modification of trophic relationships within a pre-existing community. Often this is

via introduction of a competitor and/or predator for which an endemic species is poorly

prepared. This is particularly common for endemic species on islands that have evolved

for considerable periods without risk of predation. Such species are poorly equipped

to cope with a novel predator – the typical traits for wariness, stealthy lifestyle, and

inconspicuous coloration have not provided any selective advantage and may have

disappeared. This sets the scene for a brief, but sadly inevitable, slide into oblivion

once a novel predator has arrived.

The existing body of predator–prey theory is sometimes perfectly suitable for under-

standing such processes. If a predator is particularly efficient (high rates of capture

even at low prey densities, high efficiency of conversion of prey into new predators)

then prey densities would be expected to plunge suddenly to dangerously low levels,

for which there is a high probability of extinction due to demographic or environ-

mental stochasticity. A particularly graphic example is the brown tree snake (Boiga

irregularis), introduced onto the island of Guam in the 1950s (Savidge 1987). In the

course of two decades, this generalist predator has spread rapidly across the island.

This range expansion coincided with the rapid decline and (in some cases) dis-

appearance of 11 native species of forest birds on the island.

Special circumstances sometimes apply, however, that are far from obvious. For

example, introduction of rabbits onto a number of islands in the South Pacific has

apparently triggered collapse of endemic bird species which were previously well able

to withstand predation. A case in point is loss of an endemic parakeet and a banded

rail species from Macquarie Island (Taylor 1979). This has been attributed to hyper-

predation (see Chapter 21), a form of apparent competition by which an exotic prey

that is capable of withstanding predation subsidizes population growth by a resident

predator (Smith and Quin 1996). Endemic prey then decline because they are more

vulnerable to predation than the exotic species.

This scenario of asymmetric apparent competition (Holt 1977) induced via sub-

sidies to a common predator population was developed theoretically by Courchamp

et al. (2000b). They represented the hyperpredation process with the following

system of equations:

d

dt

Ct

Bt Rt Ct

Bt Rt

() BB RR Ct

[() ()]()

() ()

= ()

+

+

−

λμα λμ

α

ν

22

d

dt

Rt r Rt

Rt

K

Ct Rt

Rt

R R Rt Bt

() () R

()

() ()

()

() ()

=−max

⎡

⎣

⎢

⎤

⎦

(^1) ⎥− μ +α
d
dt
Bt r Bt
Bt
K
Ct Bt
Bt
B B B Rt Bt

() ()

()

() ()

()

max () ()

=−

⎡

⎣

⎢

⎤

⎦

(^1) ⎥− μ +
α
α
306 Chapter 17
17.8.1Extinction
threat due to
introduction of
exotic predators or
competitors