HABITAT PATCHINESS AND THE DISTRIBUTION, ABUNDANCE, AND POPULATION DYNAMICS OF AN INSECT HERBIVORE

Abstract

I examine how habitat patchiness influences the distribution, abundance, and spatial dynamics of the geometrid moth Itame andersoni in the Wrangell Mountains of Alaska. Itame inhabits a subdivided habitat created by patches of its host plant Dryas drummondii (Rosaceae) growing on bare ground. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. I surveyed five distinct sites to examine the relationships between plant patch size, patch isolation, Itame density, and pupal mass. I censused Itame caterpillar density in over 200 plant patches for 1-3 years and found that densities were highest in medium-sized Dryas patches and lowest in very large carpets of Dryas. Isolated and small patches had high variance in density leading to lower mean densities. In contrast to density, pupal masses were highest in the largest patches, indicating a positive effect of patch size on individual fecundity, which was highly correlated with pupal mass. Results from the survey, as well as experimental manipulations of density, showed that Itame density had a negative effect on pupal mass. By using observed caterpillar densities and pupal masses to predict egg densities, I determined that the variance in pupal masses between patches was not sufficient to explain the observed density-dependent population dynamics. Thus it appears that cat- erpillar density itself has more important population-level impacts than does pupal mass. Contrary to predictions in the literature, this leads to the conclusion that smaller plant patches and the subdivided habitat that they constitute are disproportionally important for Itame populations. Temporal population dynamics were fairly synchronous between Dryas patches within sites but asynchronous between sites separated by >0.5 km, suggesting that sites may form distinct populations of Itame. This work shows the direct impacts of even very small-scale habitat patchiness on species distribution and abundance and how effects at the small spatial scale may combine with effects of larger scale subdivision to determine population structure.