Gopher-mound and casting disturbances contribute to plant community structure in a Cascade Range meadow complex

Background/Question/Methods

Pocket gophers are a major agent of disturbance in grasslands throughout North America.  Gophers bury vegetation under mounds of excavated soil, which by various mechanisms can influence aspects of plant community structure including species diversity, the relative abundance of growth forms, and spatial heterogeneity of species composition.  However, in mountain meadows that experience deep winter snow pack, gophers also create soil castings, smaller tube-shaped deposits, which have been previously ignored in studies of plant-gopher disturbance relationships.  We studied the influences of the Mazama pocket gopher (Thomomys mazama) in montane meadows at Bunchgrass Ridge (1350 m elevation) in the Oregon Cascades. We quantified relationships between gopher disturbance and community structure at larger spatial scales than previously studied in this system, and we also considered, for the first time, effects of both mounds and castings.  We measured cover of mounds, castings, and individual graminoid and forb species in contiguous 20x20 cm quadrats along twenty, 5-m transects in each of four plots with differing species composition. We used linear mixed-effects models to explore relationships between community structure (species richness, evenness, growth-form dominance, compositional heterogeneity) and disturbance (cover of mounds, castings, and mounds plus castings).  

Results/Conclusions

Roughly 60-70% of quadrats contained some form of disturbance.  Cover averaged 12-21% for mounds and 9-14% for castings, among plots. Total plant cover was negatively correlated with cover of both mounds and castings, demonstrating for the first time the importance of castings in this system.  However, only mounds influenced growth-form dominance, reducing graminoid cover and increasing the forb/graminoid ratio—effects attributable to the greater volume and longevity of mounds.  Forb-disturbance relationships were not consistent among plots, likely due to variation in meadow species composition, i.e., differences in the abundance of species with varying tolerance of burial.  Transect-scale richness and compositional heterogeneity (variation in species composition among quadrats within transects) increased with disturbance, but evenness was unaffected.  Frequent, small-scale disturbances create a shifting mosaic of vegetation states, reducing local dominance of graminoids and enhancing species richness and heterogeneity at larger spatial scales.