POPULATION CONSEQUENCES OF RESTRICTED DISPERSAL FOR AN INSECT HERBIVORE IN A SUBDIVIDED HABITAT

This paper addresses how dispersal ability of the geometrid moth Itame andersoni interacts with habitat subdivision to impact population structure, importance of different habitat types (e.g., patch sizes, habitat configurations), and regional distribution patterns in a subdivided habitat of patches of its host plant Dryas drummondii (Rosaceae). Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame distribution is patchy within areas colonized by Dryas. This research combines observational and experimental approaches to examine individual movement behavior, immigration, and emigration rates of both caterpillars and adult females at the scale of discrete Dryas patches. Itame caterpillars will cross bare ground separating plant patches; however, movement of caterpillars on bare ground was more restricted than predicted by a correlated random walk due to large turning angles which led to area‐restricted search. Caterpillar immigration into patches previously cleared of Itame is usually low and is negatively affected by patch isolation when local Itame densities are high. Caterpillar emigration is also significantly greater from patches with high Itame density. Immigration of females, which have reduced wings, was positively influenced by patch isolation and negatively influenced by the presence of a barrier preventing adult females from walking into patches. All females emigrate from their point of emergence and continue moving as they oviposit. Females lay eggs singly on foliage, in leaf litter, and on rocks under and surrounding Dryas patches. Significantly more eggs are laid off of Dryas patches than within Dryas patches, but this is not a result of concentration of searching effort off of Dryas patches. Females move greater distances than predicted by a correlated random walk. Movement parameters combined with oviposition parameters lead to the prediction that the average female will lay all of her eggs within 5–15 m of her starting point. Restricted movement appears to demographically isolate sites separated by distances of as little as 0.5 km. Limited between‐site dispersal may link these populations into metapopulations and may explain patchy regional patterns of distribution. Within sites, densities are likely to build up in smaller patches and subdivided habitat due to preferential oviposition on bare ground. These smaller patches may in turn be disproportionally important for population processes due to their contribution of dispersers through density‐dependent emigration within and between sites. Thus, this research demonstrates how limited dispersal can impact patterns of distribution, abundance, and population structure in a natural system.