Andrew J. Edelman

SELECTION OF DREY SITES BY ABERT'S SQUIRRELS IN AN INTRODUCED POPULATION

Abstract Aberts squirrels (Sciurus aberti) are reported to depend on ponderosa pine (Pinus ponderosa) for food and nest sites. Introduced Aberts squirrels in the Pinaleño Mountains of Arizona, however, occupy mixed-conifer forests that contain almost no ponderosa pine (about 2%). We examined selection of drey sites in this introduced population. Dreys (i.e., spherical nests) were built adjacent to the trunk at 75% of the tree height. Dreys were found in 5 different conifer species and <2% were in ponderosa pine. Drey trees were larger and had more access routes than did random trees. Drey sites were steeper, had more large trees, Douglas-fir (Pseudotsuga menziesii), and southwestern white pine (Pinus strobiformis), and less corkbark fir (Abies lasiocarpa var. arizonica) than random sites. The structural characteristics of drey trees in the Pinaleños population also were very similar to drey trees used by natural populations of Aberts squirrels in ponderosa pine forests. Our results suggest that the dependence of Aberts squirrels on ponderosa pine is not as strong as previously reported. Structural features such as tree size and access routes appear to be more important to selection of drey sites than tree species.

DIET AND TREE USE OF ABERT'S SQUIRRELS (SCIURUS ABERTI) IN A MIXED-CONIFER FOREST

Abstract Aberts squirrels (Sciurus aberti) are reported to be dependent on ponderosa pine (Pinus ponderosa) forests for food, cover, and nest sites. Introduced Aberts squirrels in the Pinaleño Mountains of Arizona, however, occupy forests that contain little to no ponderosa pine. We documented diet and tree use of Aberts squirrels in mixed-conifer forests of the Pinaleño Mountains using observations of marked animals. Individuals ate similar food items as Aberts squirrels in ponderosa pine forests, including seeds, inner bark, buds, and fungi, but 5 conifer species were used as food sources. Douglas-fir (Pseudotsuga menziesii) and southwestern white pine (Pinus strobiformis) were the most frequently eaten conifer species. Aberts squirrels also were observed in all tree species. Our results suggest that the dependence of Aberts squirrels on ponderosa pine is not as strong as previously reported.

POTENTIAL FOR NEST SITE COMPETITION BETWEEN NATIVE AND EXOTIC TREE SQUIRRELS

Abstract In communities where strong interspecific competition between native species is lacking, exotic and native species often exhibit intense resource competition resulting in decline of native populations. We examined the potential for interspecific competition for nest sites between co-occurring native Mt. Graham red squirrels (Tamiasciurus hudsonicus grahamensis) and exotic Aberts squirrels (Sciurus aberti) in the Pinaleño Mountains of Arizona. Comparison of nest use between red and Aberts squirrels at different scales (nest, nest tree, and nest site) revealed contrasting results. Competition for nests and nest trees appears unlikely given the dissimilarity in use of nest types and tree characteristics. Aberts squirrels predominately used dreys, whereas red squirrels mostly used cavity nests. Neither squirrel species occupied a nest used by the other species. Nest trees differed in size and species between squirrels for dreys, but not for cavities. Aberts squirrel nest sites were found in a wider range of microhabitats including almost all microhabitats in which red squirrel nest sites were located. Although there was significant overlap, each species showed distinct trends in microhabitat. In general, red squirrel nest sites were characterized as more densely forested areas dominated by corkbark fir (Abies lasiocarpa var. arizonica), whereas Aberts squirrel nest sites were more open and contained greater tree species diversity. Overlap in microhabitat characteristics increases the likelihood of interspecific competition and could increase the vulnerability of red squirrels to extinction.

CHARACTERISTICS OF ABERT'S SQUIRREL (SCIURUS ABERTI) CAVITY NESTS

Abstract Cavity nests are rarely used by Aberts squirrels (Sciurus aberti) in ponderosa pine (Pinus ponderosa) forests. Introduced Aberts squirrels in the Pinaleño Mountains of Arizona, however, exhibit occasional use of cavity nests (ca. 10%). We examined characteristics of cavity nest sites in this introduced population. Cavity nests were located at two-thirds of the tree height and were found mainly in both dead and live aspen (Populus tremuloides). Aspen cavity trees were larger than aspen random trees. Aspen cavity sites had more large snags and less ponderosa pine and were less steep than aspen random sites. Aberts squirrels possibly use cavities in large aspen trees because thick trunks and older trees might allow larger cavities to form, might provide greater thermal and wind protection, or both. The greater use of cavity nests by the Aberts squirrels in our study could be due to the higher frequency of cavity-forming aspens or harsher winter conditions in mixed-conifer forests of the Pinaleño Mountains than in ponderosa pine forests where Aberts squirrels typically occur or both.

Positive Interactions between Desert Granivores: Localized Facilitation of Harvester Ants by Kangaroo Rats

Facilitation, when one species enhances the environment or performance of another species, can be highly localized in space. While facilitation in plant communities has been intensely studied, the role of facilitation in shaping animal communities is less well understood. In the Chihuahuan Desert, both kangaroo rats and harvester ants depend on the abundant seeds of annual plants. Kangaroo rats, however, are hypothesized to facilitate harvester ants through soil disturbance and selective seed predation rather than competing with them. I used a spatially explicit approach to examine whether a positive or negative interaction exists between banner-tailed kangaroo rat (Dipodomys spectabilis) mounds and rough harvester ant (Pogonomyrmex rugosus) colonies. The presence of a scale-dependent interaction between mounds and colonies was tested by comparing fitted spatial point process models with and without interspecific effects. Also, the effect of proximity to a mound on colony mortality and spatial patterns of surviving colonies was examined. The spatial pattern of kangaroo rat mounds and harvester ant colonies was consistent with a positive interspecific interaction at small scales (<10 m). Mortality risk of vulnerable, recently founded harvester ant colonies was lower when located close to a kangaroo rat mound and proximity to a mound partly predicted the spatial pattern of surviving colonies. My findings support localized facilitation of harvester ants by kangaroo rats, likely mediated through ecosystem engineering and foraging effects on plant cover and composition. The scale-dependent effect of kangaroo rats on abiotic and biotic factors appears to result in greater founding and survivorship of young colonies near mounds. These results suggest that soil disturbance and foraging by rodents can have subtle impacts on the distribution and demography of other species.