Plant responses to pocket-gopher disturbances across pastures and topography

Pocket gophers are important disturbance agents in rangelands, yet little i s known about how plant responses to gopher disturbances vary with grazing and topography. We measured the spatial distribution of soil mounds created by the northern pocket gopher (Thomomys talpoides attenuatus Hall and Mont ague) in shortgrass steppe, and sampled plant cover and species composition on gopher mounds at 3 topographic positions within 2 pastures that were li ghtly and heavily grazed by cattle. Measurements were taken during 1996 and 1997 in each pasture along a 75 x 900-m transect that spanned the same top ographic gradient: a south-facing slope, a north-facing slope, and an uplan d plain. Pocket-gopher mounds were more numerous in the lightly grazed past ure but mounds were larger in the heavily grazed pasture. An estimated 1-6% of the total area was disturbed on uplands and south-facing slopes, and <1 % was disturbed on north-facing slopes. Plant cover on mounds was generally higher in the heavily grazed than in the lightly grazed pasture, primarily due to a greater cover of the dominant perennial grass, blue grama (Boutel oua gracilis [H.B.K.] Lag ex Griffiths). Detrended correspondence analysis also showed that pasture had a greater effect on plant species composition on mounds than topography or yearly variation. Our results demonstrate that topography affected the spatial distribution of pocket-gopher disturbances , and pasture influenced the pool of plant species colonizing mounds. It is therefore important to assess animal-disturbance effects and plant respons es to disturbances on rangelands within the broader context of topography a nd grazing.