Ecologists have identified that both "top-down" biotic interactions, such as competition or predation, and "bottom up" abiotic factors, such as precipitation and productivity, can play significant roles in the structure of communities. Now a long-term field experiment supported by Earthwatch in the semiarid Parque Nacional Bosque Fray Jorge, Chile, reveals that environmental change can shift the control of community structure between these two contrasting patterns.

The findings, presented by Dr. Peter Meserve (Northern Illinois University) and colleagues in a recent issue of BioScience, show that Fray Jorge's community structure can shift abruptly between years based on rainfall variation resulting from El Niño Southern Oscillation (ENSO). Their shifting control model demonstrates the importance of long-term studies and illustrates that community ecology can be subject to multiple controls.

"Some investigators want to categorize everything as either under biotic or abiotic control.," said Meserve, who in the early 1990s was principal investigator of Chile's Coastal Wildlife with coauthors Drs. Bryan Milstead (National Park Service) and Julio Gutiérrez (Universidad de La Serena). "The reality is that both types of control are important, and their relative importance ‘shifts' depending on the prevailing environmental conditions."

The authors' research involved monitoring populations of 10 species of native small mammals, their predators, and plant cover in a series of enclosures that determined the level of competition, herbivory, and/or predation. The researchers found that these "top-down" biotic interactions, particularly predation, had some impact on community structure, but their influence varied with ENSO-related changes in annual rainfall.

Predators had the most impact in ENSO years, when relatively high rainfall and plant productivity appeared to "release" consumers and their predators and allow "top-down" biotic control. During non-ENSO years, low rainfall limited plant productivity, resulting in more "bottom-up" control of the community structure.

For instance, during ENSO years when rain and hence plant food sources were abundant, predators had more "top down" impact on populations of squirrel-sized rodents called degus (Octodon degus). But during dry non-ENSO years, the lack of food had a greater impact on degu populations, overwhelming any predator effect with "bottom-up" control.

Over 13 years, species richness of small mammals in the experimental site oscillated from as many as 10 species, during high rainfall years, to as few as 1, demonstrating the shifting importance of abiotic controls. Meserve contrasts this pattern to other arid or semiarid areas, such as the Chihuahuan Desert of North America, where annual rainfall is less variable and biotic controls of community structure appear to be the rule.

"When we began this work, we really didn't have an appreciation for the importance of long-term environmental events," said Meserve. "Now that we have about 14 years under our belts, it is readily apparent that one cannot simply assume that what holds true for the functioning of semiarid systems in one part of the world will apply equally well in another."

Understanding how community structure is controlled is imperative for conserving biodiversity, especially in areas like north-central Chile that have been severely altered by fire, clearing, overgrazing, and other human disturbances. In addition, Meserve's findings suggest that peripheral habitats near the thorn scrub are important refuges for some small mammal species during prolonged drought periods, and deserve special attention from land managers.

For more information, see "Thirteen Years of Shifting Top-Down and Bottom-Up Control." Peter L. Meserve, et al. BioScience, July 2003, Vol. 53:7, pp. 633-646.

For a website detailing research at Parque Nacional Bosque Fray Jorge, go to http://www.bios.niu.edu/frayjorge/index.html

Earthwatch Institute is an international nonprofit organization which supports scientific field research worldwide by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to engage people worldwide in scientific field research and education to promote the understanding and action necessary for a sustainable environment.