Indirect effects of invasive rat removal result in recovery of island rocky intertidal community structure

Abstract

Eleven years after invasive Norway rats (Rattus norvegicus) were eradicated from Hawadax Island, in the Aleutian Islands, Alaska, the predicted three-level trophic cascade in the rocky intertidal, with native shorebirds as the apex predator, returned, leading to a community resembling those on rat-free islands with significant decreases in invertebrate species abundances and increases in fleshy algal cover. Rats had indirectly structured the intertidal community via their role as the apex predator in a four-level trophic cascade. Our results are an excellent example of an achievable and relatively short-term community-level recovery following removal of invasive animals. These conservation successes are especially important for islands as their disproportionately high levels of native biodiversity are excessively threatened by invasive mammals.

Figure 1

The (A) central Aleutian Archipelago, Alaska, USA contains the (B) Rat Islands Group, including Hawadax Island. The (C) intertidal plots and beach transects surveyed on Hawadax Island. Intertidal plot 6 was only surveyed in 2008 and 2013.

Figure 2

The presence of invasive rats on Aleutian Islands in Alaska creates a (A) four-level trophic cascade wherein rats negatively impact primary productivity, indirectly (dotted line) turning the rocky intertidal community into an invertebrate dominated system by depredating shorebirds and releasing intertidal grazers from bird predation pressure. On islands without rats, and presumably on islands in recovery after rat removal, such as Hawadax, the rocky intertidal becomes a (B) three-level trophic cascade wherein shorebirds depredate herbivorous invertebrates, thereby releasing algae from grazing pressure, and indirectly creating an algal dominated community. Birds also consume invertebrate non-grazers (e.g., mussels, anemones, seastars, and sponges), and their decreased abundances following rat removal may lead to increased availability of space in the rocky intertidal, further facilitating increases in algal cover. Figure modified from Kurle et al.^., Gena Bentall drew the images and C. Kurle created the figure.

Figure 3

The percent change in algal, barnacle, sponge, and tunicate percent cover and mean number per m² of invertebrates pre- vs. post-eradication (2008 vs. 2013 and 2019) measured from intertidal photo quadrats. * indicates significantly different data between 2008 and 2013 or 2008 and 2019, p < 0.05. The red diamonds indicate the percent change between islands with rats and without rats from Kurle et al.. No red diamond indicates an organism that was not measured in Kurle et al..

Figure 4

Example of digital photos of intertidal quadrats. A digital photo was taken first of the surface coverage of the plot (A). A second photo of the exact plot (B) was taken after the algal layer was removed to reveal the under-story community. Photos by J. Curl.