Saving food webs by subtraction

Saving food webs by subtraction. An ecosystem is a dynamic, complex tangle of predators and prey in which the depletion of one species can trigger a cascade that leads to the extinction of several others. Some cascades are structural—they propagate when an extinction leaves some predators with no prey. Others are dynamic—they occur when an initial extinction leaves the system in an unstable state that, in mathematical parlance, is drawn by an attractor toward multispecies extinction, even though more favorable steady states might exist. Now, simulations by Adilson Motter and Sagar Sahasrabudhe (both at Northwestern University) suggest that many dynamic cascades can be mitigated with the strategic removal or suppression of a second species. Some rescues are intuitive, such as removing a second species that’s a predator or prey of the first. But the ideal rescue species need not share such a direct link with the cascade-instigating species; it can even be a species that would have gone extinct in the cascade anyway. Sometimes just partial suppression of a species can spare the entire food web. The team demonstrated how its approach might work in California’s Coachella Valley ecosystem, depicted as a network in the figure. Arrows point from each prey to its predator. The larger circles indicate species that, if removed, would most likely instigate a cascade; the yellow circles, those most likely to mitigate one. The results suggest that smart, sometimes counterintuitive population-control measures could be instrumental in conserving ecosystems. (S. Sahasrabudhe, A. E. Motter, Nat. Commun. 2, 170, 2011.)