Predation by Norway rats in the intertidal zone of central Chile

The rocky intertidal zone of central Chile compnses a diverse array of invertebrate and vertebrate (i.e. birds, fish, humans) predators In this system, like in most temperate rocky shores, predation by small 'terrestrial' mammals such as mice and rats has been largely overlooked. This study documents predation on intertidal organisms by Norway rats Rattus norvegicus in a marine preserve in Las Cruces, central Chile. Five species of small mammals, 3 endemic and 2 introduced, were found in the littoral zone of the marine preserve. R. norvegicus was the most abundant of these species throughout the year-long mark-recapture study. Ten burrows of R. norvegicus containing remains of intertidal organisms were located in the littoral zone, 8 inside the marine preserve and 2 ca 200 m outside. Remains found in the burrows and stomach content analyses showed that rats prey on 40 d~fferent intertidal species, including both mobile and sessile organisms from the mid to the very low intertidal zone. The most numerous prey species in the burrows were keyhole limpets (490 individuals), mostly Fissurella crassa, followed by porcelanid crabs (187 individuals) and cancrid crabs of the genus Acanthoc}/clus (101 individuals). Prey composition in the burrows varied greatly. Some burrows were dominated by keyhole limpets and others by crabs, suggesting some degree of specialization by individual rats. Comparison between keyhole limpets found in burrows and on intertidal rocks inside the marine preserve showed that rats remove the smaller, less abundant sizes of keyhole limpets from the population, probably due to mechanical restrictions to dislodging larger individuals. The number of small keyhole limpets (<55 mm long) that Norway rats can potentially remove from the marine preserve in a year (over 8730) as compared to numbers of small limpets in the preserve, suggest that R. norvegicus may have important effects on the population structure and abundance of these species. Since humans deplete keyhole limpet populations outside the marine preserve, rats may compensate for part of the effects of humans harvesting inside the preserve. Preliminary data suggest that rats are less abundant in places distant from human populations and thus the proximity to human populations can be an important factor in deciding the location of marine preserves.