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Gradient in pine bark bug, Aradus cinnamomeus, densities with distance from the industrial complex (*) at Harjavalta, Finland. White circles = 0-0.50 bugs 100 cm-2, light brown circles = 0.51-1.75 bugs 100 cm-2, brown circles = 1.76-3.50 bugs 100 cm-2, and purple circles = 3.51-12.2 bugs 100 cm-2. From Heliovaara and Vaisanen (1986) by permission from Blackwell Wissenschafts-Verlag GmbH.

environmental conditions change. Two general types of spatial variation are represented by the expansion of growing populations and by the discontinuous pattern of fragmented populations, or metapopulations.

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