An understanding of the four basic principles of population dynamics allows us to describe and explain the full spectrum of population dynamics to be expected in nature and, therefore, to develop schemes for classifying pest outbreaks.

Classification schemes attempt to organize things that we see or experience into groups or classes according to their common characteristics. In the case of forest pest management we are particularly interested in identifying the different kinds of outbreaks that we have to deal with, for different types of outbreaks may require different management approaches.

As we have seen, the basic patterns of population fluctuations is largely the result of feedbacks acting in response to population density. We saw that there are two basic kinds of feedbacks, destabilizing +feedback and stabilizing –feedback.

Unstable patterns of population dynamics result from the action of the 1^st and 2^nd principles. However, as all populations obey the 1^st principle without exception, there seems to be no point in employing this universal principle in a classification scheme. On the other hand, the 2^nd principle is only evoked when the members of the population cooperate in some way or another, and when this cooperation leads to higher per-capita birth and/or survival rates as population density rises. When this occurs, the R-function has a positive slope, and this can give rise to unstable extinction or escape threshold (see figure). The presence of escape thresholds creates the conditions for what have been called eruptive outbreaks. They are called eruptive because, once they start in a particular locality, called the epicenter of the outbreak, they tend to erupt or spread over large areas (Slide). The reason for this spreading out of the outbreak is that large numbers of pests produced in the epicenter tend to migrate into surrounding forests where they raise the local populations above their escape thresholds. Once this happens, these local populations grow rapidly, denude their food supplies, and continue the spread.

For the reasons discussed above, the first division in the classification of pest outbreaks will be into those species that exhibit eruptive dynamics and those that do not. Outbreaks of non-eruptive species will be called gradient outbreaks, for reasons that should become clear below.

Stable patterns of population dynamics result from the action of the 3^rd and 4^th principles, both of which create stabilizing –feedback. Two kinds of dynamics can result from these stabilizing forces: (1) high-frequency or "saw-toothed" oscillations resulting from the action of the 3^rd principle alone (here time delays in the operation of the –feedback are relatively short), and (2) low-frequency or "cyclical" oscillations resulting from the action of the 4^th principle (here time delays in the stabilizing –feedback are relatively long because of interactions between the population and its environment). Populations that are not affected by the 2^nd principle cannot escape from their stabilizing forces and, therefore, outbreaks can only be created by favorable changes in external environment. In other words, outbreaks of these species are a graded response to external factors and so are called gradient outbreaks. A distinguishing features of gradient outbreaks is that they are often restricted to certain locations (sites) where the environment is particularly favorable for the pest species. In addition, these outbreaks do not spread into adjacent unfavorable environments.

It is now possible to classify the dynamics of pest populations in the following way:

1. Sustained gradient outbreak. Permanent differences in the favorability of the environment for the pest in different localities leads to outbreaks on certain sites and not on others (see site A and B in the top figure). Sustained gradients can also occur following permanent or semi-permanent changes in the environment due to human actions (selective logging of certain species, plantation forestry, fire control, etc.). The white pine weevil an example of an insect that exhibits sustained outbreaks on certain dry sites but does not cause a problem on moist sites.
2. Pulse gradient outbreak. Short-term changes in the favorability of the environment for the pest lead to outbreaks of short duration (see site C in the top figure where D indicates an increase in environmental favorability and R a reduction back to normal). The fir engraver beetle, a non-aggressive bark beetle, often exhibits pulse gradient outbreaks after droughts or defoliation by the Douglas-fir tussock moth temporarily weakens large numbers of fir trees (see figure).
3. Cyclical gradient outbreak. In this case the operation of the 4^th principle causes high-amplitude cycles of abundance on very favorable sites while the cyclic dynamics are greatly reduced on less favorable sites. Here the 4^th principle is evoked by dense populations, while in the first 2 classes the 4^th principle does not operate. The larch budmoth is an insect that exhibits cyclical gradient outbreaks in the Swiss Alps (see figure) but remains at non-outbreak densities at lower elevations. Douglas-fir tussock moth outbreaks are also cyclical (see figure) and seem to be restricted to particular sites (see slide).
4. Sustained eruptive outbreaks. Here pest populations spread over large areas and remain at outbreak densities for many years at a particular locality, or site. Outbreaks such as these do not normally kill their host trees but usually return to low densities eventually (4^th principle is not evoked by dense populations). The gypsy moth may be an example of this kind of outbreak (see figure).
5. Pulse eruptive outbreaks. Here pest populations spread over large areas but return quickly to low densities at a particular locality, or site. Outbreaks such as these often kill or seriously impact their host trees, or induce outbreaks of natural enemies (parasitoids or pathogens) which force the pest population back to low densities (4^th principle is evoked by dense populations). The aggressive mountain pine beetle is an example of a species that exhibits eruptive dynamics.

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© 1998 Alan A. Berryman