Population density is the number of individuals per unit geographic area (e.g., number per m2, per ha, or per km2). This variable affects a number of other pop ulation variables. For example, mean density determines population viability and the probability of colonizing vacant habitat patches. Density also affects population dispersion pattern (see the next section). A related measure, population intensity, is commonly used to describe insect population structure. Intensity is the number of individuals per habitat unit, such as number per leaf, per m branch length, per m2 leaf area or bark surface, per kg foliage or wood, etc. Mean intensity indicates the degree of resource exploitation; competition for space, food, or mates; and magnitude of effect on ecosystem processes. Intensity measures often can be converted to density measures if the density of habitat units is known (Southwood 1978).

Densities and intensities of insect populations can vary widely. Bark beetles, for example, often appear to be absent from a landscape (very low density) but, with sufficient examination, can be found at high intensities on widely scattered injured or diseased trees or in the dying tops of trees (Schowalter 1985). Under favorable conditions of climate and host abundance and condition, populations of these beetles can reach sizes of up to 105 individuals per tree over areas as large as 107 ha (Coulson 1979, Furniss and Carolin 1977). Schell and Lockwood (1995) reported that grasshopper population densities can increase an order of magnitude over areas of several thousand hectares within 1 year.

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