just science day 5 kill and cure

Just Science day 5: Kill and cure?

The best way to protect an endangered species might be to ... cull it.

That's the conclusion reached by Michael Bode and Hugh Possingham in a

theoretical analysis published recently in Ecological Modelling. The

problem that they address is the tendency of some species to show

boom-and-bust population dynamics. This seems a particular feature of

tightly linked predator-prey systems: if predator is rare, prey booms,

predator has lots of food, predator booms, prey crashes, predator

starves, predator crashes, and so on. The Canadian lynx-hare system is

the most famous example.

At the bottom of the cycles, population sizes might be so small that

either predator or prey is highly vulnerable to extinction - by some

other event, such as disease or disaster.

Bode and Possingham model various options for damping these cycles by

removing some predators, prey, or both, from the system. "Remarkably,"

they say, "if the interventions are enacted at the appropriate time,

infrequent culling of a small number of individuals significantly

reduces the probability of extinction of the predator."

My first thought when I saw this, was that we probably rarely know

enough about a species' population dynamics to know when the best time

to cull is, and that such a move would be politically untenable. But,

as Bode points out, the most likely candidates for such a move are

large game-park species, such as elephants, about which we know quite

a lot. It's also heartening that their (as they say, simplistic)

simulation suggests that culling prey (which would be plants for

elephants, I suppose) is more likely to be effective than predators,

and that culling would be needed only rarely.

They conclude:

Most current methods of ecosystem control attempt to keep a single

species population below a certain threshold, or between fixed

bounds, leading to highly inefficient management. Furthermore,

unnecessarily high levels of culling can generate considerable

public opposition. Given the clumsy nature of such static controls,

this view may be reasonable. Our dynamic control solution has the

substantial benefit of minimising the preventable culling of

organisms, and being extremely cost-effective.

I swapped e-mails with Bode, and besides mentioning that, in parks

with very high elephant densities, such as Kruger, culling elephants

seems 'almost unavoidable' (the issue seems to have flared up most

recently in late 2005, judging by this BBC story), he also mentioned

that similar issues were arising in Australia regarding koalas and

kangaroos. As he says, why is it always the cure animals?

All I can say is I'm glad I don't have to make these decisions.

Culling animals is clearly a terrible business - particularly for

long-lived, highly social species such as elephants. But we now

effectively control the destiny of all large, slow-breeding

vertebrates. If the only places they can survive is in the parks we

set up, and if we take a truly utilitarian and, dread word, holistic

view of conservation, then we also have to take the responsibility of

'managing' their populations. Science isn't he only thing to guide us

on such decisions, but it's best ot take as informed and clever a

decision as possible, towards which Bode and Possingham's ideas can

only help.