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Natural Control
The process by which densities of populations are maintained in nature is referred to as "natural control." Biological control has contributed significantly to the theoretical understanding of natural control, and understanding the major concepts of natural control is key to understanding how natural enemies control pests and how they can be used in biological control programs. In this overview, we define relevant terms and review the major theories of natural control. Although a full treatment of the concepts, theories and models of natural control is beyond the scope of the present work, the interested reader is directed to the Huffaker et al. (1984) paper cited in the references section.
Population Dynamics:
A population is a group of interbreeding individuals of the same species located in a defined area. The early history of the field of ecology reflected the interest of ecologists in determining a theoretical structure to explain the observed patterns of population dynamics, particularly with reference to identifying the relative role of factors responsible for causing population change. The understanding of natural control to which the majority of biological control specialists subscribe is that populations exists at a characteristic abundance -- which is defined as the long-term expected numbers of individuals in a population. Paraphrasing the famous ecologist E. P. Odum, "an acre of forest may have 10,000,000 aphids and 150 oak trees -- but not the reverse." The presence of a characteristic abundance suggested that populations were being maintained around a given level (density) through the actions of factors found in the local environment. Evidence from biological control, particularly the reduction and maintenance of introduced pest populations following introduction of "exotic" natural enemies was seen as confirming both the existence of a characteristic abundance and the role natural enemies play in maintaining insect population densities.
In Figure 1, natural control is illustrated, although the population number is fluctuating over time (purple line), it is bounded within a range, indicated by the blue lines. The yellow line represents this population's "characteristic abundance," the long term expected number of individuals in the population.
Figure 1
Density Dependence:
Maintaining population density around a characteristic abundance required the action of factors that behaved in a density dependent fashion. The tendency for population to be maintained around a characteristic abundance via action of density dependent factor(s) is referred to as population regulation. (In contrast, the term "control" implies containment of a population within (broad) limits of fluctuation.). A factor that acts in a density-dependent fashion increases its impact on the affected population as the density of the population increases (see Figure 2). Thus, natural enemies whose percentage attack rate increases in response to host (prey) density increases are said to be acting in a density-dependent fashion. (In contrast, if the number (not percentage) attacked increase as a function of host (prey) density, this is not density dependent). The need to have density-dependent factor(s) regulating populations around a characteristic abundance was seen as necessary to counter the potential exponential growth rate that all populations possess. Without a factor(s) that acted in a density-dependent fashion, populations would eventually grow to the point where they consume their resource base and crash towards local extinction. The persistence of populations -- and the relative lack of data for local extinctions -- was seen as confirming evidence for the existence of density-dependence factors and the regulation of populations in nature. Other than natural enemies, factors that act in a density-dependent fashion are intra-and inter-specific competition and territoriality.
Figure 2
Alternative Theories:
Not surprising, alternative views exist. Historically, the major challenge to the "density-dependent school" came from those who felt that the evidence for population stability in nature was not proved, and that the numbers of individuals in a population was largely determined by the time available for population growth. Thus, population control can be accounted for via vagaries in environmental limits that are not related to density per se. Whereas followers of the density-dependent school saw populations existing in a characteristic abundance, those of the density-independent school saw populations in flux, with extinctions common and the long-term expected number of a population only a statistical -- not biological -- reality. The abundance and distribution of populations reflected adaptation to local conditions that are limited as to the nature, magnitude and direction of change.
A synthesis of views that followed the thinking of W. R. Thompson was given by A. Milne and largely adopted by Huffaker (see Huffaker et al. 1984). In this view, populations track environmental change, expanding in favorable times and contracting during unfavorable periods. Population control was seen to have elements of maintenance within boundaries ("control" of the density-dependent school) and return to equilibrium ("regulation" sensu density-dependent school). Extinction of populations happen and the imposition of density dependence occurs for only relatively short time periods. For the most part, it is a combination of so-called "imperfect density-dependent" factors (including natural enemies) and density-independent factors (primarily weather) that influence population dynamics. (Huffaker termed these "conditioning factors," which, uninfluenced by density, will control or set the framework of environment upon which density dependent factors act). At lower densities, either density-independent factors "relax" or the population goes extinct. At higher densities, the only "perfect" density-dependent factor, intra-specific competition prevents continued population growth and causes the population to decline to lower levels (The terms "perfect" vs. "imperfect" refer to the time lag associated with the factor: imperfect acting with a time-lag, and perfect acting "instantaneously").
What does this means for biological control?
Our understanding of the role of natural enemies in the natural control of insect populations has evolved over time. Originally seen as acting as so-called "perfect" density-dependent agents regulating populations, more synthetic theories place the impact of natural enemies within a context of overall environmental impact on population dynamics. The empirical evidence that natural enemies can significantly lower pest populations, and that permanent reduction of pest densities below economically important levels has been observed in hundreds of case histories, has not been altered.
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