Experimental Manipulation of a Desert Rodent Community: Food Addition and Species Removal

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Since 1977 we have been conducting experiments in which we add supplemental seeds or remove certain combinations of species of seed—eating rodents and ants from 0.25—ha plots in the Chihuahuan Desert of southeastern Arizona. These experiments evaluate the extent to which food availability and interspecific competition influence rodent populations. Monitoring with live traps revealed that: (1) the addition of seed at the rate of 96 kg°plot—1°yr—1 resulted in an increased density of the largest granivorous rodent species (Dipodomys spectabilis), decreases in the densities of the two next—to—largest species (D. merriami and D. ordii), and no detectable changes in the densities of other rodents; (2) the removal of D. spectabilis, as well as other experimentally induced changes in the abundance of this species, resulted in reciprocal shifts in the densities of the two congeneric species, D. merriami and D. ordii, and no significant changes in densities of other rodents; and (3) the removal of all three Dipodomys species resulted in large increases in density of four of the five species of smaller seed—eating rodents, but had no effect on two species of insectivorous rodents. Taken together, these results indicate that limited food resources and interspecific competition pay major roles in regulating the density of rodent populations and determining the organization of desert rodent communities. However, the responses of the rodent populations to our manipulations were unexpectedly complex; they included long time lags, asymmetrical interactions, and little compensation in energy consumption. This indicates how much remains to be learned about the processes that determine the structure and function of even this relatively simple and well—studied community.