Thirteen Years of Shifting Top-Down and Bottom-Up Control

For 13 years, we studied the role of biotic interactions, including predation, herbivory, and interspecific competition, in a semiarid thorn scrub community in north-central Chile. Using a large-scale field manipulation, we monitored changes in small mammals, plants, and vertebrate predators. We documented important “top-down” predation effects on some small-mammal species, and small-mammal effects on some plant species resulting from experimental exclusions. However, periodic El Niño Southern Oscillation (ENSO) events caused several high rainfall episodes during this interval, resulting in large “bottom-up” increases in both plants and animals. Therefore, we suggest that instead of exclusive top-down or bottom-up control, this system undergoes shifting control, with relatively greater importance of biotic interactions in wet years and of resource limitation in dry ones. Because intervals between ENSO events are long and responses are slow, long-term studies are essential for understanding such effects in semiarid or arid systems.