Invasive predators alter food webs by simplifying and eliminating trophic cascades

Trophic cascades represent one of food web ecology’s most exciting concepts because it explains how predators indirectly change landscapes.  While this concept describes many landscapes, debate continues regarding the concept’s generality.  Much of this debate concerns the relative importance of two mechanisms: top predators indirectly benefiting basal prey by consuming intermediate predators (density-mechanism) or by causing intermediate predators to hide rather than hunt (trait-mechanism).  Absent from this debate, however, are the impacts of invasive predators.  Analogous invasive and native food webs in a California estuary provide an ideal system to test how invasive predators affect trophic cascades.  Here we demonstrate that invasive predators alter trophic cascades by changing the strength of density- and trait-mediated interactions.  In the native food web, crabs (Cancer antennarius), whelks (Acanthina spirata), and oysters (Ostreola conchaphila) create a trophic cascade comprising density- and trait-mechanisms that maintain abundant living oysters.  But the invasive food web contains mostly dead native oysters because invasive crabs (Carcinus maenas) and whelks (Urosalpinx cinera) fail to generate density- or trait-mediated cascades.  Eliminating these mechanisms has impacted the estuary because invasive whelks have depleted approximately 50 percent of the landscape’s oysters.  In addition, a simplified trophic cascade containing only a density-mechanism may provide the landscape’s remaining oysters with biotic resistance: invasive whelks do not behaviorally respond to native crabs, allowing native crabs to consume invasive whelks where they co-occur and indirectly benefit oysters.  By simplifying and eliminating native trophic cascades, invasive predators may be predictably altering the organization of coastal landscapes.