A monitoring study of vertebrate community ecology in the northern Sonoran Desert, Arizona

Abstract

I synthesized monitoring results for vertebrates at Organ Pipe Cactus National Monument (ORPI), 1987-1998. Small mammals, lizards, and predators were studied using ongoing Ecological Monitoring Program (EMP) protocols (trap grids; transects) and other methods (drift-fences, visual encounter, road-cruising). EMP protocols for rodents and lizards performed well, but some recalibration is needed. Populations declined to observed minima during a 1989-1990 drought, and increased with strong rains during 1990-1995. Small rodents (pocket mice) increased fastest, but declined first, after 1992. The medium-sized Merriam's kangaroo rat: increased to a 1994 peak and then also collapsed. The larger packrat increased most slowly, not declining until after 1995. These temporal differences are consistent with a tradeoff of capacity for increase with resistance to predation pressure. After post-drought increases, most lizard populations declined when predator pressure became high, after 1992, and then increased during dry years after 1995, while predators declined. Endothermic predators were monitored by simple daily record-keeping. They increased 3-4 fold from 1989-1995, with subsequent declines. A literature review showed two subguilds: a small-prey group, which increased rapidly in 1991-1993, and a larger-prey group, which increased more slowly. For most snakes, population fluctuations during 1989-1998 did not appear dramatic. Western diamondback rattlesnakes and coachwhips were the most important mammal- and reptile-eating snakes, respectively. Large cohorts of young rattlesnakes were produced during 1992 and 1993. The western diamondback approximately doubled by 1995, and the coachwhip increased during 1991-1993. I summarized results for prey taxa, using bivariate correlation and path analysis. I used precipitation as a proxy for food productivity, and constructed predation pressure indices that combined snakes and endotherms. Conspecific density was the most consistent (negative) correlate of population growth. Predation (negative) and productivity (positive) also had relatively consistent associations with annual prey population growth. Lizard population growth was positively correlated with summer rain, whereas some rodents and endothermic predators had positive correlations with winter rain. The analysis supported a competitive effect of Merriam's kangaroo rat on pocket mice. I recommend adding predator monitoring to the EMP, and propose that resource management and academic ecology may develop a beneficial collaboration in the context of monitoring programs.