Our objective was to evaluate the effects of kangaroo rat mounds on species diversity and composition at a semiarid-arid grassland ecotone. We expected that source populations of plants occurring on kangaroo rat mounds have important influences on the species composition of vegetation at the landscape scale, and that these influences differ by grassland type. Our study was conducted at the Sevilleta LTER in New Mexico, where a grassland type dominated by Bouteloua gracilis, a shortgrass steppe species, and a grassland type dominated by B. eriopoda, a desert grassland species, meet to form patches across the landscape.
Four 0.4 ha plots were sampled for species diversity and composition in a regular 7m x 7m grid in each grassland type. Kangaroo rat mounds were also mapped and sampled for vegetation measures in four areas of 1.6 ha in each type. The landscape scale abundance of many subordinate species was increased significantly by populations occurring on kangaroo rat mounds in both grassland types. However, the area affected by the burrowing activity of kangaroo rats was twice as large in the B. eriopoda dominated grassland type. Furthermore, dominant plants on mounds in the B. eriopoda type were also abundant in off-mound areas whereas dominant plants on mounds in the B. gracilis type were not as abundant off-mound. These results indicate that the presence of mounds in the B. gracilis dominated type is creating islands of plant communities that are distinct from the rest of the grassland. Therefore, the occurrence of certain plant species in this grassland type may be intimately associated with the disturbance regime at this ecotone. This study demonstrates that effects of small burrowing animals may facilitate the coexistence of species at this ecotone.
Experimental Design - We selected eight stands, four dominated by Bouteloua eriopoda and four by Bouteloua gracilis (stands were marked with A1, A2, A3, A4 in the data set for the Bouteloua eriopoda type, and U1, U2, U3, U4 for Bouteloua gracilis type). Study areas were selected using aerial photos to ensure that they were located in well-defined black grama- or blue grama-dominated belts. Study areas were 126 m x 126 m in size and were placed so that they contained a randomly selected, average-sized, and recently abandoned kangaroo rat mound in the center.
Field Methods - In each patch type, we mapped all kangaroo-rat mounds and measured their size within the four 1.6 ha (126 m x 126 m) areas (total areas = 8). We classified each mound as active, recently abandoned, or old according to the level of small mammal activity. We noted the dominant and co-dominant plant species on each mound (i.e. species with at least 5% relative cover). In the center of each 1.6 ha plot, we estimated canopy cover (visual cover estimation) by species in 100 2m2 quadrats arranged in a regular 7 m x 7 m grid centered on a randomly selected and recently abandoned mound. Each grid covered a 63 m x 63 m area in the centre of the 1,6 ha stands. For each quadrat, we noted its location as being on a mound proper, at the edge of a mound, or in the off-mound vegetation.