Ecological theory and biological control
Successful classical biological control in long-lived ecosystems occurs when an imported natural enemy keeps the density of the alien pest insect below the density at which it causes economic damage. It has been generally accepted for some 75 years that such enemies establish a low stable equilibrium pest density and maintain it by imposing density-dependent mortality on the pest. In recent years it has been suggested that success typically involves aggregation by the enemy, perhaps to those patches that contain more pests. This theory is embodied in Nicholson-Bailey models of the parasitoid-host interaction.
This chapter reviews this body of theory and compares it with real systems. It appears, at least in some cases, that the basic premise of the theory — a stable interaction on the small scale — does not hold. In the few examples studied, it also appears that the mechanisms proposed in the models, including aggregation, do not account for control. In the one apparently stable system studied — red scale on citrus controlled by the parasitoid Aphytis — a refuge may be the key stabilizing factor; otherwise this interaction also has features implying instability. In another case the maintenance of a pest weed (ragwort) in the face of enemies driving it extinct may depend upon an invulnerable pest stage (the seed bank).
Few real systems have been analyzed and it should not be assumed that the above results hold in general. Stability may be common in some circumstances, and it seems likely that aggregation to local pest density will be important in some instances.
There is a strong trade-off between stability and degree of pest suppression in the Nicholson-Bailey models discussed. The trade-off is associated with the lack of within-generation dynamics in these models: stability arises from density dependence in the parasitoid (decreasing efficiency with increasing parasitoid density) rather than in the pest. This process operates and the trade-off is severe even when the parasitoid aggregates to patches that (initially) contain more pests. By contrast, such aggregation in a model that allows the parasitoids to redistribute themselves in response to the continually-changing pest distribution results in better control, but can also be destabilizing.
The consequences of these results for biological control in theory and practice are discussed. More work is needed to expose the mechanisms operating in real examples of successful control. For systems in which control involves local instability, further modeling of ensemble dynamics should help pinpoint possible key features that may allow both regional persistence of the system and consistently low pest densities.
KeywordsBiological Control Natural Enemy Density Dependence Ecological Theory Pest Population
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