Population Dynamics of Bark Beetles
Annual Review of Entomology
The designation "bark beetle" includes several genera and many species of the family Scolytidae and usually refers to insects in this family that feed in the subcortical region of host trees. Several functional systems of classification have been proposed for bark beetles, which emphasize the condition of the breeding site, the physiological state of the host at colonization, and the degree of economic destructiveness (100). These systems have not proven particularly useful, because of the subjective nature of the definition of host condition and because of the ability of many species to exploit a variety of different host conditions. A prudent ecologist, interested in contributing to the theory of population dynamics, would not normally select a bark beetle species as an experimental animal, for several reasons. Bark beetles occur as members of diverse communities incorporating contemporaneous interactions of many associated organisms, are logistically difficult to sample, have population syStems characterized by extreme complexity and tremendous inherent variation, infest many different host tree species, and occur over broad geographic areas and physiographic types. Bark beetle population dynamics operates in a space and time framework ranging from square centimeters to square kilometers and from minutes to years. Notwithstanding this seemingly hopeless array of complicating variables, this fascinating group of insects has challenged the curiosity of entomologists and foresters for many years, and an extensive literature touching on many aspects of population dynamics now exists. This circumstance is fortuitous, because much of the theory of population dynamics of animals has been developed from the analysis and interpretation of relatively simple population systems, and it is reasonable to expect new discoveries from the scrutiny of the complex bark beetle systems. The goals of the following discussion are to define a structure for the population system of bark beetles, to describe the functioning of this system at various levels of complexity, and to examine the operation of the processes that modify the distribution and abundance of bark beetles through time and space. These processes have been classified in various ways, but generally they include climate (and weather), food supply, intra- and interspecific competition, parasitization, predation, and genetic responses. The format of this discussion is to consider the interaction of bark beetles and their hosts at three levels of organizational complexity: the tree, the stand, and the forest. The specific objectives are (0) to define a temporalspatial structure as the basic population unit of bark beetles, (b) to investigate the operation of the basic popUlation system at the infestation (stand) level of organization, and (c) to investigate the consequences of the operation of the infestation population system at the forest (ecosystem) level of organization. The primary emphasis in this discussion is directed at beetles of the genus Dendroctonus. Several of the 14 species of this genus (126) have been studied extensively for many years and a vast literature is available for scrutiny. Representatives of the genus have also been the focus of several large multidisciplinary research and development programs in North America during the last 10 years. There has been a concerted effort to develop mathematical models of the population systems of these insects. This modeling effort has provided a framework for organization of existing literature and concepts and has contributed to the development of technology required to measure and describe the complex population systems. A subject as broad-based in scope as population dynamics may be construed to encompass most aspects of the existing literature on bark beetles. Fortunately, the immense bulk of literature on bark beetles has been organized, reviewed, and summarized for a number of specific purposes. Complete citations for most of the contemporary literature pertinent to population dynamics of Dendroctonus bark beetles and many other facets of bark beetle ecology are catalogued in the following publications: literature reviews and synoptic bibliographies (6,9, 18,40,47,57,99, 104, 119), comprehensive or interpretative reviews of specific topics on bark beetles (3, 17,90, 100, 124, 125), comprehensive or interpretative reviews on bark beetle-fungi relationships (12, 54, 62, 105), and major monographs (7, 84, 1 13, 126).
Coulson, R.N. Population dynamics of bark beetles. Annu. Rev. Entomol. 1979, 24, 417-447