Introduced rats indirectly change marine rocky intertidal communities from algae- to invertebrate-dominated

Abstract

It is widely recognized that trophic interactions structure ecological communities, but their effects are usually only demonstrated on a small scale. As a result, landscape-level documentations of trophic cascades that alter entire communities are scarce. Islands invaded by animals provide natural experiment opportunities both to measure general trophic effects across large spatial scales and to determine the trophic roles of invasive species within native ecosystems. Studies addressing the trophic interactions of invasive species most often focus on their direct effects. To investigate both the presence of a landscape-level trophic cascade and the direct and indirect effects of an invasive species, we examined the impacts of Norway rats (Rattus norvegicus) introduced to the Aleutian Islands on marine bird densities and marine rocky intertidal community structures through surveys conducted on invaded and rat-free islands throughout the entire 1,900-km archipelago. Densities of birds that forage in the intertidal were higher on islands without rats. Marine intertidal invertebrates were more abundant on islands with rats, whereas fleshy algal cover was reduced. Our results demonstrate that invasive rats directly reduce bird densities through predation and significantly affect invertebrate and marine algal abundance in the rocky intertidal indirectly via a cross-community trophic cascade, unexpectedly changing the intertidal community structure from an algae- to an invertebrate-dominated system.

Fig. 1.

The Aleutian archipelago with sample islands indicated in red (rat-infested, n = 8) and blue (rat-free, n = 15).

Fig. 2.

Mean (±SE) values for parameters sampled on (n = 8) rat-infested (red) and (n = 15) rat-free (blue) islands in the Aleutian Islands. * indicates a significant difference at the P ≤ 0.05 level. Bird densities (birds·km⁻¹ of shoreline) were estimated from population counts made by the U.S. Fish and Wildlife Service of rat-infested (n = 8) and rat-free (n = 89) islands. Invertebrate densities (invertebrates·m⁻²) were estimated from total counts of individuals from 480 cm² photo quadrats. Aggregating invertebrate and algal densities (percent cover of rocky intertidal) were estimated from point counts of species from 480 cm² photo quadrats. (A) Densities of Glaucous-winged gulls. (B) Densities of Black Oystercatchers. (C) Densities of algal grazing invertebrates that are known bird dietary items. (D) Percent cover of fleshy algae. (E) Densities of nongrazing invertebrates that are known bird dietary items. (F) Percent cover of barnacles, nongrazing invertebrates, and known bird dietary items. (G) Densities of sea anemones, nongrazing invertebrates that are not bird dietary items. (H) Percent cover of nongrazing aggregating invertebrates that are not bird dietary items.

Fig. 3.

Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands through direct predation on birds that forage in the intertidal. Dotted arrows indicate indirect effects, whereas solid arrows indicate direct effects. Rats keep Glaucous-winged gull and Black Oystercatcher numbers low, which releases intertidal invertebrates such as barnacles and herbivorous snails and limpets from foraging pressure. Greater numbers of grazing invertebrates leads to a significant decrease in algal cover, which allows more settling space for sessile invertebrates. The marine rocky intertidal is altered from an algae- to an invertebrate-dominated system.