Roadside ditches as corridors for range expansion of the western harvester ant (Pogonomyrmex occidentalis Cresson)

The northeasternmost range extent of the western harvester ant (Pogonomyrmex occidentalis Cresson) occurs just east of the Missouri River in North Dakota. The earliest known records (1882) of this species place it in the same general position as existed at the time of the first exhaustive survey in 1966. A 1990 survey reveals substantial eastward and northward range expansion beyond locations known to be stable between 1966 and 1978. This suggests that this species' range has not yet stabilized with post quaternary climatic changes. Field observations show that, near the expanding edge of its range, a strong relationship exists between anthropogenic modification of the landscape and locational propensity. Specifically, land uses which periodically disrupt the soil, such as row cropping, show a nearly absolute lack of occupancy, while the well-drained, sheltered roadside ditches are heavily populated by P. occidentalis. The roads themselves closely resemble bare-soil, post-nuptial landing sites known to encourage P. occidentalis ant colonization. This strongly suggests that the roadside ditches act as corridors for range expansion of the species. The similarity between road network density within and beyond the species range, combined with severe drought conditions during 1988 and 1989 indicate that climate, as a regional scale variable, is the stimulus for range expansion, while landscape level queues provided by the roadside ditches, are the mechanism by which it is accomplished. Of the site level factors examined, only roadside ditch azimuths and soil texture showed statistical significance as possible locational factors, but no causal mechanism can be assumed.