More Predators, Healthier Prey Courtesy of Jon Eisenback, NemaPix
A new mathematical model is turning the conventional notion of predator-prey relationships on its
head. David Brown and colleagues at the University of California, Davis, have shown that prey
populations can benefit from a high predator density.1
Ecologists previously have shown that predators can have indirect positive effects on their prey
through nutrient cycling and mineralization. Brown's group, however, modeled predators' direct
positive effects on the prey population--a nematode eats soil-dwelling bacteria, moves to another
location, and then deposits live bacteria in new soil, where it can thrive. Belying the paradox of
the enrichment theory, which states that increasing resources at a food web base will eventually
cause populations to oscillate to the point of extinction, Brown's model shows that positive
feedback has a stabilizing effect. Using a simple food chain involving a resource, its consumer, and
the consumer's predator, researchers demonstrated that the consumer's feeding rate is proportional
to predator density, or to the number of predator-prey encounters.
While this model is based on the nematode-bacteria relationship, it eventually could serve as a
framework for modeling higher organisms, Brown suspects. At some level, "all models are wrong,"
but his is designed as "a simple framework to serve as a reference point for more system-
specific models."
Bill Longland, of the US Department of Agriculture's Research Service in Reno, Nev., acknowledges
that the model is directed toward microbial systems, but hopes it "will add new emphasis to the
importance of predator conservation in general."
--Maria W. Anderson
1. D.H. Brown et al., "Modeling direct positive feedback between predators and prey," Theor Popul
Biol, 65:143-52, March 2004.
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