Social Insects

Social insects pose some special problems for description of population structure. On the one hand, each individual requires resources and contributes to interactions with other organisms. On the other hand, colony member activity is centered on the nest, and collective foraging territory is defined by proximity to surrounding colonies. Furthermore, food transfer among nestmates (trophallaxis) supports a view of colonies as sharing a collective gut. Hence, each colony appears to function as an ecological unit, with colony size (number of members) determining its individual physiology and behavior. For some social insects, the number of colonies per ha may be a more useful measure of density than is number of individuals per ha.

However, defining colony boundaries and distinguishing between colonies may be problematic for many species, especially those with underground nests. Molecular techniques have proved to be a valuable tool for evaluating related-ness within and among colonies in an area.

Colonies of social Hymenoptera can be monogyne (having one queen) or polygyne (having multiple queens), with varying degrees of relatedness among queens and workers (Pamilo et al. 1997). Intracolonial relatedness can vary among colonies and among populations. In some ants, such as Solenopsis invicta and some Formica species, social polymorphism can be observed, with distinct monogynous (M type) and polygynous (P type) colonies. The two types generally show high relatedness to each other where they occur in the same area. However, gene flow is restricted in the polygynous type and between monogy-nous and polygynous types. Populations of polygynous colonies generally are more genetically differentiated than are those of monogynous colonies in the same area (Pamilo et al. 1997).

Polygyny may be advantageous in areas of intense competition, where the more rapid reproduction by multiple queens may confer an advantage, regardless of the relatedness of the queens. However, additional queens eventually may be eliminated, especially in ant species, with workers often favoring queens on the basis of size or condition rather than which queen is mother to most workers (Pamilo et al. 1997).

Similarly, termite colonies are cryptic and may have variable numbers of reproductive adults. Husseneder and Grace (2001b) and Husseneder et al. (1998) found DNA (deoxyribonucleic acid) fingerprinting to be more reliable than aggression tests or morphometry for distinguishing termites from different colonies or sites. As expected, genetic similarity is higher among termites within collection sites than between collection sites (Husseneder and Grace 2001a, Husseneder et al. 1998). Moderate inbreeding often is evident within termite colonies, but low levels of genetic differentiation at regional scales suggest that substantial dispersal of winged adults homogenizes population genetic structure (Husseneder et al. 2003). However, several species are polygynous and may show greater within-colony genetic variation, depending on the extent to which multiple reproductives are descended from a common parent (Vargo et al. 2003). Kaib et al. (1996) found that foraging termites tended to associate with close kin in polygynous and polyandrous colonies of Schedorhinotermes lamanianus, leading to greater genetic similarity among termites within foraging galleries than at the nest center.

Genetic studies have challenged the traditional view of the role of genetic relatedness in the evolution and maintenance of eusociality. Eusociality in the social Hymenoptera has been explained by the high degree of genetic relatedness among siblings, which share 75% of their genes as a result of haploid father and diploid mother, compared to only 50% genes shared with their mother (Hamilton 1964, See Chapter 15). However, this model does not apply to termites. Husseneder et al. (1999) and Thorne (1997) suggested that developmental and ecological factors, such as slow development, iteroparity, overlap of generations, food-rich environment, high risk of dispersal, and group defense, may be more important than genetics in the maintenance of termite eusociality, regardless of the factors that may have favored its original development. Myles (1999) reviewed the frequency of neoteny (reproduction by immature stages) among termite species and concluded that neoteny is a primitive element of the caste system that may have reduced the fitness cost of not dispersing, leading to further differentiation of castes and early evolution of eusociality.

II. POPULATION PROCESSES

The population variables described in the preceding section change as a result of variable reproduction, movement, and death of individuals. These individual contributions to population change are integrated as three population processes: natality (birth rate), mortality (death rate), and dispersal (rate of movement of individuals into or out of the population). For example, density can increase as a result of increased birth rate, immigration, or both; frequencies of various alleles change as a result of differential reproduction, survival, and dispersal. The rate of change in these processes determines the rate of population change, described in the next chapter. Therefore, these processes are fundamental to understanding population responses to changing environmental conditions.

A. Natality

Natality is the population birth rate (i.e., the per capita production of new individuals per unit time). Realized natality is a variable that approaches potential natality—the maximum reproductive capacity of the population—only under ideal environmental conditions. Natality is affected by factors that influence production of eggs (fecundity) or production of viable offspring (fertility) by individual insects. For example, resource quality can affect the numbers of eggs produced by female insects (R. Chapman 1982). Ohgushi (1995) reported that females of the herbivorous ladybird beetle, Henosepilachna niponica, feeding on the thistle, Cirsium kagamontanum, resorbed eggs in the ovary when leaf damage became high. Female blood-feeding mosquitoes often require a blood meal before first or subsequent oviposition can occur (R. Chapman 1982); the cerato-pogonid, Culicoides barbosai, produces eggs in proportion to the size of the blood meal (Linley 1966). Hence, poor quality or insufficient food resources can reduce natality. Inadequate numbers of males can reduce fertility in sparse populations. Similarly, availability of suitable oviposition sites also affects natality.

Natality usually is higher at intermediate population densities than at low or high densities. At low densities, difficulties in attracting mates may limit mating, or may limit necessary cooperation among individuals, as in the case of bark beetles that must aggregate to overcome host tree defenses prior to oviposition (Berryman 1981). At high densities, competition for food, mates, and oviposition sites reduces fecundity and fertility (e.g., Southwood 1975,1977). The influence of environmental conditions can be evaluated by comparing realized natality to potential natality (e.g., estimated under laboratory conditions).

Differences among individual fitnesses are integrated in natality. Differential reproduction among genotypes in the population determines the frequency of various alleles in the filial generation. As discussed earlier in this chapter, gene frequencies can change dramatically within a relatively short time, given strong selection and the short generation times and high reproductive capacity of insects.

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