Abstract
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.
Highlights
Facilitation occurs when one species enhances the environment or performance of another species [1]
The best-fitting mortality risk model for recently founded colonies accurately described spatial patterns of surviving colonies and indicated that survivorship increased with decreasing distance from a kangaroo rat mound (Fig. 3A)
These spatial patterns and colony dynamics may be related to the benefits of being close to areas of high kangaroo rat activity
Summary
Facilitation occurs when one species enhances the environment or performance of another species [1]. This positive interaction can arise directly through ecosystem engineering of the abiotic environment (e.g. solar radiation, water, or soil nutrients) or indirectly through effects on secondary species (e.g. suppressing a competitor or increasing abundance of prey) [2]. The role of facilitative interactions in shaping animal communities is less well understood [1]. Species interactions, such as facilitation, are usually scale dependent reflecting the spatial heterogeneity of abiotic and biotic factors [5,6]. Along with other scaledependent processes, affect population dynamics, which in turn influence the distribution of individuals across the landscape
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