PARENT SESSION
Tuesday, August 8, 8:00-11:30 am
COS 20 - Food webs I: trophic interactions
Mississippi, Mezzanine Level, Cook Convention Center
Presiders: D Gruner

Introduced rats indirectly alter marine communities.

Kurle, Carolyn ^*,1, ¹ University of California Santa Cruz, Santa Cruz, CA, USA

ABSTRACT- The impacts of invasive species have been extensively documented. However, the vast majority of studies have demonstrated direct effects of predation and herbivory within the invaded ecosystem. Here I present data supporting the hypothesis that invasive rats (Rattus norvegicus) in the Aleutian Islands indirectly affect marine algal abundance in the rocky intertidal via a cross-community trophic interaction. Specifically, I found that rat predation significantly reduced ground nesting marine bird abundance and thus indirectly reduced predation intensity on intertidal invertebrates. I found significant differences in marine bird abundance on rat-infested (n=8) vs. rat-free (n=15) islands and this translated into significant differences in intertidal invertebrate abundance and algal cover on the two island types. Gull and oystercatcher abundances were an order of magnitude lower on islands with rats (Separate variance t-tests; t = 2.6, df = 99.9, P = 0.01 and t = 4.7, df = 105.9, P < 0.01, respectively). The species assemblages of invertebrates and algae were significantly different between islands with and without rats (MANOVAs: counts of individual invertebrates: Pillai Trace value = 0.858, F_7,15 = 12.940, P < 0.001; % cover of algae and aggregating invertebrates: Pillai Trace value = 0.593, F_6,15 = 3.878, P = 0.014). Densities of mussels, sea stars, sea anemones, and herbivorous snails and limpets were significantly higher on islands with rats than without (t = -2.13, df = 21, P = 0.045; t = 2.16, df = 21, P = 0.043; t = -2.06, df = 21, P = 0.052; t = -2.70, df = 21, P = 0.014 and t = -2.63, df = 21, P = 0.016, respectively). The percentage of rocky intertidal area covered by barnacles, tunicates, and sponges was also significantly greater on islands with rats (t = -3.41, df = 21, P = 0.003; t = -2.62, df = 21, P = 0.016; and t = -2.05, df = 21, P = 0.054). The differences in grazing invertebrate abundance resulted in a reduction by half in the percentage of area covered by fleshy algae on islands with rats (t = 3.24, df = 21, P = 0.004). My results illustrate the unexpected consequences of invasive animals and their potential to initiate indirect trophic cascades that can lead to large-scale ecological impacts on plant abundance and community structure.

Key words: trophic interactions, invasive species, marine islands