Plant succession and gopher disturbance along an experimental gradient.

A field was disturbed via disking, divided into 36 plots, and fertilized with different ratios of N:Mg in 1980. High N:Mg supply ratios tended to favor the perennial grasses Agrostis scabra and Agropyron repens over annual plants. In 1981 the experimental field was invaded by a fossorial mammalian herbivore, the plains pocket gopher, Geomys bursarius. Geomys bursarius was significantly more active in plots dominated by Agrostis scabra and Agropyron repens. The gopher mounds it created were dominated by the annual grass Setaria glauca and the annual herb Polygonum convolvulus. Thus the direct effect of resource competition among these plants was counteracted by the indirect response of an herbivore to the treatments. Although nitrogen fertilization tended to increase the rate of successional replacement of annuals by perennials, the indirect response of gophers to nitrogen tended to slow the rate of succession. This suggests that herbivore responses to spatial variation in plant community composition and productivity may be an important determinant of the rate of succession and the species diversity of the community.