Plant responses to pocket-gopher disturbances across pastures and topography.

Abstract

Pocket gophers are important disturbance agents in rangelands, yet little is 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 Montague) in shortgrass steppe, and sampled plant cover and species composition on gopher mounds at 3 topographic positions within 2 pastures that were lightly 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 topographic gradient: a south-facing slope, a north-facing slope, and an upland plain. Pocket-gopher mounds were more numerous in the lightly grazed pasture 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 (Bouteloua 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 responses to disturbances on rangelands within the broader context of topography and grazing.