Vertebrate Pest management: Understanding populations and communities

I recently caught up with Charles Darwin University researchers Clive McMahon and Mike Lawes to talk about pest management. We discussed the importance of knowing the biology and density of a pest and how it interacts with its environment (ecology).

Figure. S-shaped population growth curve (Tutorvista.com).

A population is a group of individuals of one species in a certain area. Population growth tends to follow a pattern (see figure above):

1.   Lag phase – individuals slowly adapt to the new environment

2.   Accelerating growth phase – finding resources and mates becomes easier

3.   Exponential growth phase – plentiful mates and resources

4.   Deccelarating growth phase – competition for resources limits growth

5.   Stationary phase – carrying capacity is reached.

If population size can be reduced back to the lag phase, the population will be slow to bounce back (reduced mates and potential inbreeding) and the manager will have a good chance of maintaining control.

If a control program only manages to reduce the population to a size where exponential growth occurs, the population will respond quickly to return back towards carrying capacity. This outcome may benefit the pest because it improves resource availability for survivors who can still easily find mates, and reduced populations may reduce the incidence of disease.

Knowing how many animals need to be removed to reach the lag phase requires knowledge of how many animals are in the population, and its annual rate of growth.

We also need to know how far animals move across the landscape and whether these animals might immigrate into the control area. More mobile animals from populations near carrying capacity are likely to immigrate to access more resources. If immigration is likely, a regional approach may be required.

Unfortunately there is a lack of knowledge about pest populations in the NT, or tools to rapidly assess population density. Without knowing this information, managers are taking a risk allocating resources to pest management. In some cases the impacts that a pest causes, or societal expectations for control, means that managers may accept the risk and take action. The key here is that if the numbers culled does not reduce per unit of effort, you are probably not affecting the population and may require more information or a regional approach before proceeding.

The desire for quick results before gaining a full understanding of species interaction can also lead to perverse outcomes. Clive spoke about the control of cats on Macquarie Island that resulted in an increase in rabbits, which destroyed the islands vegetation. In the NT, anecdotal evidence suggests that buffalo control may result in increased pig numbers, so it may be worth thinking about which pest impacts you the most.
 

Alternatively, multi-species control may be appropriate. For example, if dog control reduced predation on cat or rabbit populations and led to increased impacts to small mammals or vegetation cover, it may be worth thinking about controlling cat or rabbit populations as well. At the very least, understanding how communities of different animals interact will help to monitor for potentially perverse outcomes.

For information on pest species, click here.