The Effects of Rats on Rocky Intertidal Ecosystems in Northeastern New Zealand

Abstract

The rocky intertidal ecosystem is subject to a wide variety of physical and biological stresses. While predation is an important force within the littoral zone, predation by invasive rats (Rattus sp.) has generally been overlooked, with only a limited number of past studies referring to the possibility. This study devised a number of methods in which to test the hypothesis that invasive rats actively prey within the intertidal and cause significant compositional shifts in intertidal ecosystem assemblages within the Hauraki Gulf Marine Park in north eastern New Zealand. Using gut content analysis and stable isotope analysis, this study has shown that invasive rats prey on the intertidal with some individual rats having diets that consist almost entirely of marine derived nutrients. Using intertidal ecosystem surveys and timed crab surveys this study has also shown that, while effects are for the most part species specific, the combined effect of this predation is for intertidal platforms to move from being invertebrate-dominated to being algaldominated. Thus, the study documents a multi-tiered, landscape level trophic cascade. These findings support those of a previous study in Chile, which demonstrated intertidal predation by invasive Rattus norvegicus on a wide variety of taxa through burrow and gut content analysis. The findings further demonstrate that intertidal predation by Rattus sp. is an important biological force in shaping intertidal community structure. Rattus sp. are some of the most pervasive of invasive species the world over and have been shown by previous studies to have large negative effects on sea and terrestrial birds, reptiles, and terrestrial invertebrate populations. This study can therefore assist in island conservation and rodent eradication efforts by providing information on the feeding habits of Rattus sp.