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.