Population dynamics is the study of changes in the number of organisms in populations and the factors in?uencing these changes. It thus, by necessity, includes the study of the rates of loss and replacement of individuals and of those regulatory processes that can prevent excessive changes in those numbers. A wide variety of factors can affect the population dynamics of a particular species. These can be divided roughly into two categories. First, the extrinsic or environmental in?uences on populations, such as temperature, weather, food supply, competitors, natural enemies, diseases, and all possible combinations of the preceding; and second, the interactions between members of the same populations, be these direct or indirect, e.g., intraspeci?c competition, behavioral processes, and aggregation.
[...] There are, however, ways in which the host plant can inﬂuence the development of population cycles in forest insects. First, even if the nutritional quality of the host plant remained unchanged, the build-up of the herbivore population on the host plant can result in competition for resources, either through depletion of the food source or by the increase in the number of larvae feeding on a ﬁnite host plant. Second, the physiological state of trees (and other plants) is not static, and their susceptibility/suitability as food plants both within and between years can be changed. [...]
[...] Yet another effect of sublethal plant defenses is that the insect herbivore, feeding as it does on a suboptimal diet, is more likely to become stressed and more susceptible to infection by pathogens, e.g., fungal and viral diseases, although in some cases it is possible that the insect is able to sequester plant chemicals that inhibit virus infection. Population Cycles So how do these top-down and bottom-up forces interact with the insect herbivore to produce the population cycles seen in so many forest Lepidoptera? Populations that cycle are characterized by highs (peaks) and lows (troughs) in abundance. As foresters usually ﬁrst become aware of defoliating insects when they outbreak, it is appropriate to start our consideration on a peak, when the population is at its maximum. [...]
[...] As forest insects are of general and economic interest and generally occur in long-lived environments, there has been a tendency for ﬁeld data to be collected over many years. The resulting time series are often analyzed using sophisticated mathematical techniques. For example, autocorrelation analysis is used to describe the effects of a lagged population density and can also provide an indication of the periodicity of the time series. Partial auto- correlation, on the other hand, can provide an indication of the respective roles of direct and delayed density-dependent processes within a population. [...]
[...] Gypsy moth, western tent moth, and autumnal moth populations also show similar responses to the quality of their host plant in that host-mediated maternal effects affect the quality of their offspring and may generate cyclical population dynamics. There is, however, some debate as to the generality of these results and evidence of whether the maternal carry-over effects can generate the cycles on their own is equivocal. Multitrophic Interactions The situation becomes more complex when the top down forces meet those operating from the bottom up: the tritrophic or multitrophic interactions, between the predators, parasites, and other natural enemies, the herbivores and their host plants. [...]
[...] Parasitoids do have an important role in the population dynamics of forest insects and, in many cases, as in small ermine moths, appear to be the major cause of the cyclical crashes in population seen in these insects. The role of predators is less well supported. There is good evidence that predators have an effect on the population dynamics of forest insects, e.g., outbreaks of the pine beauty moth, Panolis ﬂammea, in northern Scotland are associated with a lack of generalist predators such as carabid beetles and spiders; other forest Lepidoptera, notably the Douglas-ﬁr tussock moth and the spruce budworm in North America, are subject to substantial predation by these agents. [...]
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