Mortality

Mortality is the population death rate (i.e., the per capita number of individuals dying per unit time). As with natality, we can distinguish a potential longevity or lifespan, resulting only from physiological senescence, from the realized longevity, resulting from the action of mortality factors. Hence, mortality can be viewed both as reducing the number of individuals in the population and as reducing survival. Both have importance consequences for population dynamics.

Organisms are vulnerable to a variety of mortality agents, including unsuitable habitat conditions (e.g., extreme temperature or water conditions), toxic or unavailable food resources, competition, predation (including cannibalism), parasitism, and disease (see Chapters 2-4). These factors are a focus of studies to enhance pest management efforts. Death can result from insufficient energy or nutrient acquisition to permit detoxification of, or continued search for, suitable resources. Life stages are affected differentially by these various mortality agents (e.g., Fox 1975b, Varley et al. 1973). For example, immature insects are particularly vulnerable to desiccation during molts, whereas flying insects are more vulnerable to predation by birds or bats. Many predators and parasites selectively attack certain life stages. Among parasitic Hymenoptera, species attacking the same host have different preferences for host egg, larval, or pupal stages. Predation also can be greater on hosts feeding on particular plant species, compared to other plant species, based on differential toxin sequestration, or predator attraction to plant volatiles (Stamp 1992, Traugott and Stamp 1996, Turlings et al. 1990,1995).

In general, mortality resulting from predation tends to peak at intermediate population densities, when density is sufficient for a high rate of encounter with predators and parasites, but prior to predator satiation (Fig. 5.3) (Southwood 1975, 1977, see Chapter 8). Mortality resulting from competition and canni-

Chromatographic Peaking

Log population density

| Relationship between population density, natality, and mortality caused by predators and parasites (peaking at lower population density) and interspecific competition (peaking at higher population density). From Southwood (1975). Please see extended permission list pg 570.

Log population density

| Relationship between population density, natality, and mortality caused by predators and parasites (peaking at lower population density) and interspecific competition (peaking at higher population density). From Southwood (1975). Please see extended permission list pg 570.

balism increases at higher population densities (see Fig. 5.3) (Fox 1975a, b, Southwood 1975, 1977). Competition may cause mortality through starvation, cannibalism, increased disease among stressed individuals, displacement of individuals from optimal habitats, and increased exposure and vulnerability to predation as a result of displacement or delayed development.

Survival rate represents the number of individuals still living in relation to time. These individuals continue to feed and reproduce, thereby contributing most to population size as well as to genetic and ecological processes. Hence, survival rate is an important measure in studies of populations.

Survivorship curves reflect patterns of mortality and can be used to compare the effect of mortality in different populations. Lotka (1925) pioneered the comparison of survivorship curves among populations by plotting the log of number or percent of living individuals against time. Pearl (1928) later identified three types of survivorship curves based on the log of individual survival through time

Type Survivorship

| Three generalized types of survivorship curves. Type 1 represents species with high survival rates maintained through the potential life span. Type 2 represents species with relatively constant survivorship with age. Type 3 represents species with low survival rates during early stages but relatively high survival of individuals reaching more advanced ages.

| Three generalized types of survivorship curves. Type 1 represents species with high survival rates maintained through the potential life span. Type 2 represents species with relatively constant survivorship with age. Type 3 represents species with low survival rates during early stages but relatively high survival of individuals reaching more advanced ages.

(Fig. 5.4). Type 1 curves represent species, including most large mammals, but also starved Drosophila (Price 1997), in which mortality is concentrated near the end of the maximum life span. Type 2 curves represent species in which the probability of death is relatively constant with age, leading to a linear decline in survivorship. Many birds and reptiles approach the Type 2 curve. Type 3 curves are seen for most insects, as well as many other invertebrates and fish, which have high rates of mortality during early life stages but relatively low mortality during later life stages (Begon and Mortimer 1981, Pianka 1974). Species representing Type 3 survivorship must have very high rates of natality to ensure that some offspring reach reproductive age, compared to Type 1 species, which have a high probability of reaching reproductive age.

The form of the survivorship curve can change during population growth. Mason and Luck (1978) showed that survivorship curves for the Douglas-fir tussock moth, Orgyia pseudotsugata, changed with population growth from stable to increasing, then decreasing. Survivorship decreased less steeply during population growth and decreased more steeply during population decline, compared to stable populations.

As described for natality, mortality integrates the differential survival among various genotypes, the basis for evolution. Survivors live longer and have greater capacity to reproduce. Hence, selective mortality can alter gene frequencies rapidly in insect populations.

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Responses

  • crispus
    Is the douglas fir type 3 survivorship?
    6 years ago

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