Dispersion is the spatial pattern of distribution of individuals. Dispersion is an important characteristic of populations that affects spatial patterns of resource use and population effect on community and ecosystem attributes. Dispersion pattern can be regular, random, or aggregated.
A regular (uniform) dispersion pattern is seen when individuals space themselves at regular intervals within the habitat. This dispersion pattern is typical of species that contest resource use, especially territorial species. For example, bark beetles attacking a tree show a regular dispersion pattern (Fig. 5.1). Such spacing reduces competition for resources. From a sampling perspective, the occurrence of one individual in a sample unit reduces the probability that other individuals will occur in the same sample unit. Variability in mean density is low, and sample densities tend to be normally distributed. Hence, regularly dispersed populations are most easily monitored because a relatively small number of samples provides the same estimates of mean and variance in population density as does a larger number of samples.
In a randomly dispersed population, individuals neither space themselves apart nor are attracted to each other. The occurrence of one individual in a sample unit has no effect on the probability that other individuals will occur in the same sample unit (see Fig. 5.1). Sample densities show a skewed (Poisson) distribution.
Aggregated (or clumped) dispersion results from grouping behavior or restriction to particular habitat patches. Aggregation is typical of species that occur in herds, flocks, schools, etc. (see Fig. 5.1), for enhancement of resource
| Dispersion patterns and their frequency distributions. A: Regular dispersion of Douglas-fir beetle entrances (marked by the small piles of reddish phloem fragments) through bark on a fallen Douglas-fir tree. B: Random dispersion of aphids on an oak leaf. C: Aggregated dispersion of overwintering ladybird beetles on a small shrub in a forest clearing.
exploitation or protection from predators (see Chapter 3). Gregarious sawfly larvae and tent caterpillars are examples of aggregated dispersion resulting from tendency of individuals to form groups (see Fig. 2.12). Filter-feeding aquatic insects tend to be aggregated in riffles or other zones of higher flow rate within the stream continuum (e.g., Fig. 2.14), whereas predators that hide in benthic detritus, such as dragonfly larvae or water scorpions, are aggregated in pools as a result of their habitat preferences. Aphids may be aggregated as a result of rapid, parthenogenic reproduction, as well as host and habitat preferences. Massonnet et al. (2002) found that the aphid Macrosiphoniella tanacetaria, a specialist on tansy, Tanacetum vulgare, can be aggregated at the level of individual shoots, plants, and sites.
For sampling purposes, the occurrence of an individual in a sample unit increases the probability that additional individuals occur in that sample unit. Sample densities are distributed as a negative binomial function, and variance tends to be high. Populations with this dispersion pattern require the greatest number of samples and attention to experimental design. A large number of samples is necessary to minimize the obviously high variance in numbers of indi viduals among sample units and to ensure adequate representation of aggregations. A stratified experimental design can facilitate adequate representation with smaller sample sizes if the distribution of aggregations among different habitat types is known.
Dispersion pattern can change during insect development, during change in population density, or across spatial scales. For example, larval stages of tent caterpillars and gregarious sawflies are aggregated at the plant branch level, but adults are randomly dispersed at this scale (Fitzgerald 1995, McCullough and Wagner 1993). Many host-specific insects are aggregated on particular hosts in diverse communities but are more regularly or randomly dispersed in more homogeneous communities dominated by hosts. Some insects, such as the western ladybird beetle, Hippodamia convergens, aggregate for overwintering purposes and redisperse in the spring. Aphids are randomly dispersed at low population densities but become more aggregated as scattered colonies increase in size (Dixon 1985). Bark beetles show a regular dispersion pattern on a tree bole, as a result of spacing behavior, but are aggregated on injured or diseased trees (Coulson 1979).
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