When population size exceeds the number of individuals that can be supported by existing resources, competition and other factors reduce population size until it reaches levels in balance with resource supply. This equilibrium population size, which can be sustained indefinitely by resource availability, is termed the carrying capacity of the environment and is designated as K. Carrying capacity is not constant; it depends on factors that affect both the abundance and suitability of necessary resources, including the intensity of competition with other species that also use those particular resources.
Density-independent factors modify population size, but only density-dependent factors can regulate population size, in the sense of stabilizing abundance near carrying capacity. Regulation requires environmental feedback, such as through density-dependent mechanisms that reduce population growth at high densities but allow population growth at low densities (Isaev and Khlebopros 1979). Nicholson (1933, 1954a, b, 1958) first postulated that density-dependent biotic interactions are the primary factors determining population size. Andrewartha and Birch (1954) challenged this view, suggesting that density-dependent processes generally are of minor importance in determining abundance. This debate was resolved with recognition that regulation of population size requires density-dependent processes, but abundance is determined by all factors that affect the population (Begon and Mortimer 1981, Isaev and Khlebopros 1979). However, debate continues over the relative importances of competition and predation, the so-called "bottom-up" (or resource concentration/limitation) and "top-down" (or "trophic cascade") hypotheses, for regulating population sizes (see also Chapter 9).
Bottom-up regulation is accomplished through the dependence of populations on resource supply. Suitable food is most often invoked as the limiting resource, but suitable shelter and oviposition sites also may be limiting. As populations grow, these resources become the objects of intense competition, reducing natality and increasing mortality and dispersal (see Chapter 5), and eventually reducing population growth. As population size declines, resources become relatively more available and support population growth. Hence, a population should tend to fluctuate around the size (carrying capacity) that can be sustained by resource supply.
Top-down regulation is accomplished through the response of predators and parasites to increasing host population size. As prey abundance increases, predators and parasites encounter more prey. Predators respond functionally to increased abundance of a prey species by learning to acquire prey more efficiently and respond numerically by increasing population size as food supply increases. Increased intensity of predation reduces prey numbers. Reduced prey availability limits food supply for predators and reduces the intensity of predation. Hence a prey population should fluctuate around the size determined by intensity of predation.
A number of experiments have demonstrated the dependence of insect population growth on resource availability, especially the abundance of suitable food resources (e.g., M. Brown et al. 1987, Cappuccino 1992, Harrison 1994, Lunder-stadt 1981, Ohgushi and Sawada 1985, Polis and Strong 1996, Price 1997, Ritchie 2000, Schowalter and Turchin 1993,Schultz 1988,Scriber and Slansky 1981,Varley and Gradwell 1970). For example, Schowalter and Turchin (1993) demonstrated that growth of southern pine beetle populations, measured as number of host trees killed, was significant only under conditions of high host density and low nonhost density (Fig. 6.6). However, some populations appear not to be food limited (Wise 1975). Many exotic herbivores are generalists that are regulated poorly in the absence of coevolved predators, although this also could reflect poor defensive capacity by nonadapted plants.
Population regulation by predators has been supported by experiments demonstrating population growth following predator removal (Carpenter and Kitchell 1987, 1988, Dial and Roughgarden 1995, Marquis and Whelan 1994, Oksanen 1983). Manipulations in multiple-trophic-level systems have shown that a manipulated increase at one predator trophic level causes reduced abundance of the next lower trophic level and increased abundance at the second trophic level down (Carpenter and Kitchell 1987, 1988, Letourneau and Dyer 1998). However, in many cases, predators appear simply to respond to prey abundance without regulating prey populations (Parry et al. 1997), and the effect of predation and parasitism often is delayed and hence less obvious than the effects of resource supply.
Regulation by lateral factors does not involve other trophic levels. Interference competition, territoriality, cannibalism, and density-dependent dispersal have been considered to be lateral factors that may have a primary regulatory role (Harrison and Cappuccino 1995). For example, Fox (1975a) reviewed studies indicating that cannibalism is a predictable part of the life history of some species, acting as a population control mechanism that rapidly decreases the number of competitors, regardless of food supply. In the backswimmer, Notonecta hoff-manni, cannibalism of young nymphs by older nymphs occurred even when alter-
■ Low pine/low hardwood
□ Low pine/high hardwood
□ High pine/low hardwood
■ High pine/high hardwood
Effect of host (pine) and nonhost (hardwood) densities on population growth of the southern pine beetle, measured as pine mortality in 1989 (Mississippi) and 1990 (Louisiana). Low pine = 11-14 m2 ha-1 basal area; high pine = 23-29 m2 ha-1 basal area; low hardwood = 0-4 m2 ha-1 basal area; high hardwood = 9-14 m2 ha-1 basal area. Vertical lines indicate standard error of the mean. Bars under the same letter did not differ at an experimentwise error rate of P < 0.05 for data combined for the 2 years. Data from Schowalter and Turchin (1993).
b a a native prey were abundant (Fox 1975b). In other species, any exposed or unprotected individuals are attacked (Fox 1975a). However, competition clearly is affected by resource supply.
All populations probably are regulated simultaneously by bottom-up, top-down, and lateral factors. Some resources are more limiting than others for all species, but changing environmental conditions can affect the abundance or suitability of particular resources and directly or indirectly affect higher trophic levels (M. Hunter and Price 1992, Polis and Strong 1996, Power 1992). For example, environmental changes that stress vegetation can increase the suitability of a food plant without changing its abundance. Under such circumstances, the disruption of bottom-up regulation results in increased prey availability, and perhaps suitability (Stamp 1992, Traugott and Stamp 1996), for predators and parasites, resulting in increased abundance at that trophic level. Species often respond differentially to the same change in resources or predators. Ritchie (2000) reported that experimental fertilization (with nitrogen) of grassland plots resulted in increased non-grass quality for, and density of, polyphagous grasshop pers but did not affect grass quality and reduced density of grass-feeding grasshoppers. Density-dependent competition and dispersal, as well as increased predation, eventually cause population decline to levels at which these regulatory factors become less operative.
Harrison and Cappuccino (1995) compiled data from 60 studies in which bottom-up, top-down, or lateral density-dependent regulatory mechanisms were evaluated for populations of invertebrates, herbivorous insects, and vertebrates. They reported that bottom-up regulation was apparent in 89% of the studies, overall, compared to observation of top-down regulation in 39% and lateral regulation in 79% of the studies.
Top-down regulation was observed more frequently than bottom-up regulation only for the category that included fish, amphibians, and reptiles. Bottom-up regulation may predominate in (primarily terrestrial) systems where resource suitability is more limiting than is resource availability (i.e., resources are defended in some way [especially through incorporation of carbohydrates into indigestible lignin and cellulose]). Top-down regulation may predominate in (primarily aquatic) systems where resources are relatively undefended, or consumers are adapted to defenses, and production can compensate for consumption (D. Strong 1992, see also Chapter 12).
Whereas density dependence acts in a regulatory (stabilizing) manner through negative feedback (i.e., acting to slow or stop continued growth), inverse density dependence has been thought to act in a destabilizing manner. Allee (1931) first proposed that positive feedback creates unstable thresholds (i.e., an extinction threshold below which a population inevitably declines to extinction and the release threshold above which the population grows uncontrollably until resource depletion or epizootics decimate the population) (Begon and Mortimer 1981, Berryman 1996,1997, Isaev and Khlebopros 1979). Between these thresholds, density-dependent factors should maintain stable populations near K, a property known as the Allee effect. However, positive feedback may ensure population persistence at low densities and is counteracted, in most species, by the effects of crowding, resource depletion, and predation at higher densities
Clearly, conditions that bring populations near release or extinction thresholds are of particular interest to ecologists, as well as to resource managers. Bazykin et al. (1997), Berryman et al. (1987), and Turchin (1990) demonstrated the importance of time lags to the effectiveness of regulatory factors. They demonstrated that time lags weaken negative feedback and reduce the rigidity of population regulation. Hence, populations that are controlled primarily by factors that operate through delayed negative feedback should exhibit greater amplitude of population fluctuation, whereas populations that are controlled by factors with more immediate negative feedback should be more stable. J. Myers (1988) and Mason (1996) concluded that delayed effects of density-dependent factors can generate outbreak cycles with an interval of about 10 years. For irruptive and cyclic populations, decline to near or below local extinction thresholds may affect the time necessary for population recovery between outbreaks.
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