Population Fluctuation

Insect populations can fluctuate dramatically over time. If environmental conditions change in a way that favors insect population growth, the population will increase until regulatory factors reduce and finally stop population growth rate. Some populations can vary in density as much as 105-fold (Mason 1996, Mason and Luck 1978, Royama 1984, Schell and Lockwood 1997), but most populations vary less than this (Berryman 1981, D. Strong et al. 1984). The amplitude and frequency of population fluctuations can be used to describe three general patterns. Stable populations fluctuate relatively little over time, whereas irruptive and cyclic populations show wide fluctuations.

Irruptive populations sporadically increase to peak numbers followed by a decline. Certain combinations of life history traits may be conducive to irruptive fluctuation. Larsson et al. (1993) and Nothnagle and Schultz (1987) reported that comparison of irruptive and nonirruptive species of sawflies and Lepidoptera from European and North American forests indicated differences in attributes between these two groups. Irruptive species generally are controlled by only one or a few factors, whereas populations of nonirruptive species are controlled by many factors. In addition, irruptive Lepidoptera and sawfly species tend to be gregarious, have a single generation per year, and are sensitive to changes in quality or availability of their particular resources, whereas nonirruptive species do not share this combination of traits.

Cyclic populations oscillate at regular intervals. Cyclic patterns of population fluctuation have generated the greatest interest among ecologists. Cyclic patterns can be seen over different time scales and may reflect a variety of interacting factors.

Strongly seasonal cycles of abundance can be seen for multivoltine species such as aphids and mosquitoes. Aphid population size is correlated with periods of active nutrient translocation by host plants (Dixon 1985). Hence, populations of most species peak in the spring when nutrients are being translocated to new growth, and populations of many species (especially those feeding on deciduous hosts) peak again in the fall when nutrients are being resorbed from senescing foliage. This pattern can be altered by disturbance. Schowalter and Crossley (1988) reported that sustained growth of early successional vegetation following clearcutting of a deciduous forest supported continuous growth of aphid populations during the summer (Fig. 6.1). Seven dominant mosquito species in Florida during 1998-2000 showed peak abundances at different times of the year, but the interannual pattern varied as a result of particular environmental conditions, including flooding (Zhong et al. 2003).

Longer-term cycles are apparent for many species. Several forest Lepidoptera exhibit cycles with periods of ca. 10 years, 20 years, 30 years, or 40 years (Berry-man 1981, Mason and Luck 1978, Price 1997, Royama 1992, Swetnam and Lynch 1993) or combinations of cycles (Speer et al. 2001). For example, spruce budworm,

Population Fluctuation Spruce Budworm

Seasonal trends in aphid biomass in an undisturbed (dotted line) and an early successional (solid line) mixed-hardwood forest in North Carolina. The early successional forest was clearcut in 1976-1977. Peak abundances in spring and fall on the undisturbed watershed reflect nutrient translocation during periods of foliage growth and senescence; continued aphid population growth during the summer on the disturbed watershed reflects the continued production of foliage by regenerating plants. From Schowalter (1985).

Seasonal trends in aphid biomass in an undisturbed (dotted line) and an early successional (solid line) mixed-hardwood forest in North Carolina. The early successional forest was clearcut in 1976-1977. Peak abundances in spring and fall on the undisturbed watershed reflect nutrient translocation during periods of foliage growth and senescence; continued aphid population growth during the summer on the disturbed watershed reflects the continued production of foliage by regenerating plants. From Schowalter (1985).

Choristoneura fumiferana, populations have peaked at approximately 25-30-year intervals over a 250-year period in eastern North America (Fig. 6.2), whereas Pandora moth, Coloradia pandora, populations have shown a combination of 20-and 40-year cycles over a 622-year period in western North America (Fig. 6.3). In many cases, population cycles are synchronized over large areas, suggesting the influence of a common widespread trigger such as climate, sunspot, lunar, or ozone cycles (W. Clark 1979, Price 1997, Royama 1984,1992, Speer et al. 2003). Alternatively, P. Moran (1953) suggested, and Royama (1992) demonstrated (using models), that synchronized cycles could result from correlations among controlling factors. Hence, the cause of synchrony can be independent of the cause of the cyclic pattern of fluctuation. Generally, peak abundances are maintained only for a few (2-3) years, followed by relatively precipitous declines (see Figs. 6.2 and 6.3).

Explanations for cyclic population dynamics include climatic cycles and changes in insect gene frequencies or behavior, food quality, or susceptibility to

Cyclic Population Growth

Spruce budworm population cycles in New Brunswick and Quebec over the past 200 years, from sampling data since 1945, from historical records between 1978 and 1945, and from radial growth-ring analysis of surviving trees prior to 1878. Arrows indicate the years of first evidence of reduced ring growth. Data since 1945 fit the log scale, but the amplitude of cycles prior to 1945 are arbitrary. From Royama (1984) with permission from the Ecological Society of America.

Spruce budworm population cycles in New Brunswick and Quebec over the past 200 years, from sampling data since 1945, from historical records between 1978 and 1945, and from radial growth-ring analysis of surviving trees prior to 1878. Arrows indicate the years of first evidence of reduced ring growth. Data since 1945 fit the log scale, but the amplitude of cycles prior to 1945 are arbitrary. From Royama (1984) with permission from the Ecological Society of America.

Pandora Moths Central Oregon

2000

1600 1700 Year

Percentage of ponderosa pine trees recording outbreaks of pandora moth in old-growth stands in central Oregon, United States. From Speer et al. (2001) with permission from the Ecological Society of America. Please see extended permission list pg 570.

1600 1700 Year

2000

Percentage of ponderosa pine trees recording outbreaks of pandora moth in old-growth stands in central Oregon, United States. From Speer et al. (2001) with permission from the Ecological Society of America. Please see extended permission list pg 570.

disease that occur during large changes in insect abundance (J. Myers 1988). Climatic cycles may trigger insect population cycles directly through changes in mortality or indirectly through changes in host condition or susceptibility to pathogens. Changes in gene frequencies or behavior may permit rapid population growth during a period of reduced selection. In particular, reduced selection under conditions favorable for rapid population growth may permit increased frequencies of deleterious alleles that become targets of intense negative selection when conditions become less favorable. Depletion of food resources during an outbreak may impose a time lag for recovery of depleted resources to levels capable of sustaining renewed population growth (e.g.,W. Clark 1979). Epizootics of entomopathogens may occur only above threshold densities. Sparse populations near their extinction threshold (see the next section) may require several years to recover sufficient numbers for rapid population growth. Berryman (1996), Royama (1992), and Turchin (1990) have demonstrated the importance of delayed effects (time lags) of regulatory factors (especially predation or parasitism) to the generation of cyclic pattern.

Changes in population size can be described by four distinct phases (Mason and Luck 1978). The endemic phase is the low population level maintained between outbreaks. The beginning of an outbreak cycle is triggered by a disturbance or other environmental change that allows the population to increase in size above its release threshold. This threshold represents a population size at which reproductive momentum results in escape of at least a portion of the population from normal regulatory factors, such as predation. Despite the importance of this threshold to population outbreaks, few studies have established its size for any insect species. Schowalter et al. (1981b) reported that local outbreaks of southern pine beetle, Dendroctonus frontalis, occurred when demes reached a critical size of about 100,000 beetles by early June. Above the release threshold, survival is relatively high and population growth continues uncontrolled during the release phase. During this period, emigration peaks and the population spreads to other suitable habitat patches (see Chapter 7). Resources eventually become limiting, as a result of depletion by the growing population, and predators and pathogens respond to increased prey or host density and stress. Population growth slows and abundance reaches a peak. Competition, predation, and pathogen epizootics initiate and accelerate population decline. Intraspecific competition and predation rates then decline as the population reenters the endemic phase.

Outbreaks of some insect populations have become more frequent and intense in crop systems or natural monocultures where food resources are relatively unlimited or where manipulation of disturbance frequency has created favorable conditions (e.g., Kareiva 1983, Wickman 1992). In other cases, the frequency of recent outbreaks has remained within ranges for frequencies of historic outbreaks, but the extent or severity has increased as a result of anthropogenic changes in vegetation structure or disturbance regime (Speer et al. 2001). However, populations of many species fluctuate at amplitudes that are insufficient to cause economic damage and, therefore, do not attract attention. Some of these species may experience more conspicuous outbreaks under changing environmental conditions (e.g., introduction into new habitats or large-scale conversion of natural ecosystems to managed ecosystems).

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Responses

  • isto
    Why might a population show a cyclic pattern of growth?
    6 years ago

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