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

Abstract

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.

Patterns of reproduction in two sympatric gerbil species in arid Egypt

“…Results obtained by different authors suggest the occurrence of such a mechanism. The experimental addition of seeds, for example, increased the density of granivorous rodents (Brown & Munger 1985) and a reduction in the abundance of rodent pests stimulated them to recover rapidly (Brown & Tuan 2005). Compensatory mechanisms are strongest in non-isolated habitats, similar to the study area, where local communities are exposed to relatively unrestricted immigration from a regional pool .…”

Reorganization of an arid ecosystem in response to recent climate change

“…We counted all shrubs Ͼ1.0 m in diameter (primarily Acacia, Ephedra, Flourensia, and Prosopis). Populations of rodents and ants have been monitored on the experimental site since 1977, with rodents censused monthly and ant colonies counted annually (18,19).…”

“…Here, we use data from our long-term research in southeastern Arizona (18)(19)(20) to document large changes which have occurred since the late 1970s in three components in a Chihuahuan Desert ecosystem: precipitation, vegetation, and animal populations. Our 20-hectare study site, at 1330 m elevation in the San Simon Valley approximately 7 km east of Portal, Arizona (Fig.…”

Impact of an extreme climatic event on community assembly

“…To understand the impacts of the extrinsic abiotic drivers, it is essential to take account of the intrinsic competitive interactions among the rodent species. Most of the time, the seed-eating rodents at Portal are foodlimited, and competition is intense because they share a common resource (34)(35)(36). This is evidenced by (i) zero-sum dynamics, such that the collective rate of energy use by all rodent species remained relatively constant over the 30 years (31, 32); and (ii) consistent, positive responses of small species, especially pocket mice, to our kangaroo rat removal experiments (33,37).…”