Rain and rodents: complex dynamics of desert consumers

Water is the lifeblood of the desert. It comes in rains that are typically scant and sporadic, but can be so intense as to cause flooding. Because water is the resource in shortest supply, the amount and timing of precipitation directly limits plant growth and primary production. Seasons of exceptionally heavy and frequent rains produce the spectacular desert blooms shown in nature films and magazines. Seasons of exceptionally high rainfall are also thought to cause increases in rodent populations and outbreaks of rodent-borne diseases such as hantavirus and plague. El Niño is supposed to cause exceptionally heavy winter rainfall in the deserts of southwestern North America, leading in turn to plant growth, abundant seeds and insects, high populations of small mammals, density-dependent increases in parasites and diseases, and increased contact between rodents, their pathogens, and humans, resulting in disease epidemics. Thus, the outbreak of the Sin Nombre strain of hantavirus that killed 27 people in the Four Corners region of the southwestern United States in the summer of 1993 was attributed to the rains, plant production, and rodent increases triggered by the El Niño events of 1991–1992 and 1992–1993 (Harper and Meyer 1999). Ecologists have long been interested in these kinds of complicated pathways of interactions and particularly in how relationships between resources and consumers affect the structure and dynamics of ecosystems. The “bottom-up” pattern of regulation described above occurs when pulsatile resource inputs are transmitted up food chains, causing increases first in plants and then in successively higher trophic levels. This contrasts with “top-down” regulation, in which the feeding activities of top carnivores cascade down food chains to affect successively lower trophic levels (e.g., Hairston et al. 1960, Oksanen et al. 1981, Carpenter and Kitchell 1988). But ecological systems are complex, and there is reason to believe that resource–consumer relationships can exhibit chaotic or other forms of complicated nonlinear dynamics (e.g., Schaffer and Kot 1985, Hanski et al. 1993, Hastings et al. 1993, Lima et al. 1999). Long-term ecological studies provide unique opportunities to study resource–consumer relationships in realistically complex natural settings. Since 1977 we have been monitoring the weather, plants, and rodents in the Chihuahuan Desert near Portal, Arizona (figure 1; Brown 1998, Ernest et al. 2000). The resulting data allow us to evaluate the relationship between El Niño events and rainfall, the dependence of plants on precipitation, and the ways in which episodic rains affect desert rodent populations. After 23 years of study, we are far from understanding the dynamics of this ecosystem. One thing that is clear, however, is that simple bottom-up regulation does not occur. The responses of desert consumers to precipitation are complex and nonlinear