Modeling the “Unpredictable”
Many insect species - notably the gypsy moth, one of North America's most
devastating forest pests - produce periodic population surges, known as outbreaks, only to crash down to low levels. These outbreaks can devastate huge areas of forest, but they occur at long, irregular intervals and are hard to predict. A recently proposed model appears to untangle the factors that trigger such outbreaks, and that helps to explain why outbreaks are so unpredictable.
By combining elements from two prevailing but flawed insect- outbreak theories, and supplementing the combined theory with additional data, researchers produced a mathematical model that reproduces the unpredictable outbreaks of gypsy moth populations more accurately than any model yet. The model is quite general, and there are many insects beyond gypsy moths for which it applies. Approximately 80 species of butterfly and moth undergo outbreaks, as do some small mammals, including voles and lemmings. Ecologists who study insect outbreaks previously fell into two camps. One camp has focused on host-pathogen or host-parasite models, which presume that outbreaks primarily depend upon the presence or absence of disease among insect populations. These models can closely approximate the average length of time between outbreaks, but they fail to explain the relatively irregular timing with which outbreaks tend to occur. The other camp has focused on the role of "generalist" predators, such as spiders and birds. When the density of these generalists declines the insects they feed on quickly explode in number. The fusion of the two camps introduces the host-pathogen-plus-predator model, which combines the stabilizing effect of dependable predators with the effects of disease, thus accounting more accurately for the ebb and flow of forest life.
The refinement of the model is based on accounting for more variables. The authors considered elements like the variability of weather, the presence of more than one predator for each species and variability in the ability of insect larvae to resist disease. Taking into account so many dynamics yields a formula more complex - and more useful - than others that have been used to explain insect outbreaks. The model offers solutions for many of the insect-outbreak mysteries that have plagued ecologists for decades. For instance, scientists have sought models that explain why outbreaks adhere to such consistent timetables, but if you look at more insects, you see that most outbreaks are not regular.
Another mystery that appears solved is the question of why insect populations of the same species, even when they are thousands of miles away from each other, surge simultaneously. This is the only outbreak model that portrays what scientists call spatial synchrony. They suspect that the long duration between outbreaks ultimately limits the importance of the environmental distinctions between various regions. (Nature 7/15/04).
