When gray wolves were reintroduced to Yellowstone National Park, they sparked a resurgence of aspen trees. Young aspens had been devastated and all but disappeared, courtesy of elk, in what is known as a trophic cascade--an ecological process that begins at the peak of the food chain and ripples downward. At Yellowstone, that particular cascade goes from wolves to elk to aspen. The absence of the wolves, an apex predator, had triggered the process. Their return began to unwind it.
Yellowstone is a classic--but not singular --example of a predator-initiated trophic cascade. Other catalysts exist, including infectious agents, such as parasites and pathogens, and that is where former University of California, Santa Barbara, National Science Foundation postdoctoral research fellow Julia Buck--now assistant professor in the Department of Biology & Marine Biology at the University of North Carolina, Wilmington --comes in. Parasites and pathogens are her specialty. After a chance meeting at UCSB with William J. Ripple, Distinguished Professor of Ecology at Oregon State University--who described trophic cascades in Yellowstone soon after the wolves were reintroduced--Buck surveyed the literature for studies of cascades initiated by parasites and pathogens.
She curated 47 examples and categorized them into three different types, including a new paradigm not applicable to predator-prey cascades. The findings appeared in the journal Trends in Ecology & Evolution.
"Our analysis found a hybrid type of indirect effect: explains Buck. "The wolves initiated a consumptive density-mediated indirect effect on aspen by reducing the population of elk. They also caused a nonconsumptive trait-mediated indirect effect by scaring the elk so the ruminants ate less, which also contributed to tree recovery, but because infectious agents can be less than fully and immediately lethal, they can simultaneously consume their hosts and change their behavior, initiating what we call a consumptive traitmediated indirect effect."
Buck found this new category applied to 45% of the infectious agent case studies she discovered in her review. Case in point: larval trematodes --parasitic flatworms--that infect snails. Once infected, the snails lose their appetite and reduce their grazing, which, in turn, permits algae to flourish.
By far, the...