When wolves aren’t enough: Revisiting trophic cascades in northern Wisconsin

Elimination of top predators in many terrestrial ecosystems has transformed food webs and ecosystem functions, and predator restoration is hoped to reverse the negative effects of this trophic downgrading. We use detailed wolf colonization records and vegetation surveys from the 1950s and 2000s to assess whether the return of wolves to northern Wisconsin in the 1970s restored a trophic cascade protecting native species from excessive deer browse. Surprisingly, understory plant diversity, richness, and frequency decreased more in areas with longer wolf presence, providing no evidence that wolves protected forests. Instead, our data suggest a “bottom-up” process wherein deer concentrate in areas where their preferred plant resources are abundant, and wolves track deer. Disentangling top-down from bottom-up trophic processes requires explicit tests of these competing hypotheses.