Alan A. Berryman

What causes population cycles of forest Lepidoptera

Hypotheses for the causes of regular cycles in populations of forest Lepidoptera have invoked pathogen-insect or foliage-insect interactions. However, the available data suggest that forest caterpillar cycles are more likely to be the result of interactions with insect parasitoids, an old argument that seems to have been neglected in recent years.

Deciphering the Effects of Climate on Animal Populations: Diagnostic Analysis Provides New Interpretation of Soay Sheep Dynamics

Soay sheep on the island of Hirta exhibit periodic population collapses that have been proposed to result from nonlinear interactions between weather, population density, and age structure. Here we employ a diagnostic approach to reanalyze the data from 1985 to 2004 and find that climate mainly affects the equilibrium population size, thus acting as a lateral perturbation. From this, we derive a simple energetic model for a population interacting with its food supply in the presence of variable winter weather. This model explains the strong nonlinearity in the Soay sheep population regulation function and provides a framework for evaluating climatic perturbations. We examined two integrative climatic indexes, one representing effects on forage production and the other representing the severity of winter weather. Results suggest that the latter has the main effect on Soay sheep population dynamics. Models incorporating this variable provided fairly accurate predictions of Soay sheep population fluctuations. The diagnostic approach offers an objective way to develop simple, nonstructured population models that are useful for understanding the causes of population fluctuations and predicting population changes, provided they are based on a careful consideration of the underlying biological and/or ecological mechanisms.

Population Dynamics of Forest Insects

Insects do not normally cause serious damage to forests unless their numbers increase, for some reason or another, to very high densities. The area of ecology that deals with changes in the density of organisms over time and that attempts to explain the causes of these changes is known as population dynamics. This chapter explores the concepts of population dynamics as they relate to forest insects, in order to better understand the causes of forest pest outbreaks. This basic understanding is essential for both forest managers and insect pest managers, for without it they will often find themselves groping in the dark for solutions to their pest problems and may even make decisions that compound rather than solve the problems.

Forest Insect Pests

The term pest describes those organisms that have a negative impact on human survival or well-being, either acting as parasites; transmitting pathogens; competing with humans for food, fiber, or other useful resources; or just plain annoying them. The term pest, therefore, is highly subjective and reflects the human viewpoint. A truly objective and impartial view (i.e., a strictly scientific one) may lead to the conclusion that the concept of a pest is inappropriate, for all organisms play important roles in the development and maintenance of ecological communities (see Chapter 3). From this viewpoint we may even come to the conclusion that only one organism really threatens the stability and persistence of living systems on this planet—that Homo sapiens is the real pest!

Assessing the Risk of Insect Outbreaks

In the preceding chapter we discussed methods for monitoring ongoing forest insect infestations and for making short-term forecasts. These procedures address the manager’s concern with current conditions—where damage is occurring at present, how much damage is being done, and whether this damage will increase or decrease in the near future. By contrast, risk assessment methods are intended to predict the likelihood of insect problems in the more distant future—where and under what conditions insect outbreaks are likely to occur in the future.