THE IMPACT OF FORAGING BY COLUMBIAN GROUND-SQUIRRELS, SPERMOPHILUS-COLUMBIANUS, ON VEGETATION GROWING ON PATCHES FERTILIZED WITH URINE

We investigated how urine fertilization (simulated urine patches) infl uenced the attributes of vegetation subjected to foraging, mainly by C olumbian Ground Squirrels, Spermophilus columbianus. This form of fert ilization appeared to stimulate increased foraging pressure from the g round squirrels on such patches, evident in a manifold increase in the amount of time spent foraging on the experimental plots after urine a pplication. The vegetation responded to the application of urine by sh owing an increase in its nutrient content (nitrogen, phosphorus and po tassium) and probably an increase in its palatability (water content). As an apparent consequence of the ground squirrel foraging activities , the vegetation on the fertilized plots changed both in canopy covera ge (species composition) in mid summer and in standing crop biomass at the end of the growing season. The coverage values for both graminoid s and forbs declined significantly, as did the standing crop biomass o n the fertilized plots, a phenomenon not recorded on the control plots .