Interspecific spatial patterns support indirect facilitation of harvester ants by kangaroo rats

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Andrew Edelman

Spatial patterns of structures (e.g., nests and burrows) in animal populations can provide insight into underlying ecological processes. Banner-tailed kangaroo rats (Dipodomys spectabilis) and harvester ants (Pogonomyrmex rugosus) are the largest and most dominant granivores found in rodent and ant communities of the northern Chihuahuan Desert. Both species build conspicuous, above-ground structures and are highly territorial. Kangaroo rats are hypothesized to indirectly facilitate (i.e., indirect positive effect on one species by another) harvester ants by foraging on large-seeded annuals, thereby reducing competition for small-seeded annuals that are utilized by ants. I tested the indirect facilitation hypothesis by examining inter-specific spacing patterns of mounds and colony discs built by coexisting banner-tailed kangaroo rats and harvester ants, respectively. Occupancy of mounds and colonies at the Sevilleta LTER, New Mexico were monitored over a 2 year period. I analyzed spatial association between colonies and mounds using Ripley’s K. In general, harvester ant colonies and kangaroo rat mounds were spatially associated with each other at fine scales (< 10 m) and independent at greater scales. Occupied mounds exhibited a stronger spatial association with harvester ant colonies than unoccupied mounds. New ant colonies were spatially independent of mounds, whereas older colonies were spatially associated with mounds. Spatial association between species supports the hypothesis of indirect facilitation of harvester ants by kangaroo rats. Kangaroo rats mounds do not appear to affect colony recruitment, but may decrease mortality of colonies near mounds through their local effects on soil and vegetation. Currently, I am fitting interaction models to interspecific spatial patterns to examine the underlying ecological processes.

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