Ecology, 90(2), 2009, pp. 452–464 Ó 2009 by the Ecological Society of America Indirect effects of invasive predators on litter decomposition and nutrient resorption on seabird-dominated islands DAVID A. WARDLE, 1,2,4 PETER J. BELLINGHAM, 2 KAREN I. BONNER, 2 AND CHRISTA P. H. MULDER 3 1 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE901 83, Umea ˚, Sweden 2 Landcare Research, P.O. Box 40, Lincoln 7640 New Zealand 3 Department of Biology and Wildlife and Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775 USA Abstract. Despite recent interest in the ecosystem impacts of invasive aboveground organisms, most work in this area has focused on effects of invasive plants, and the effects of invasive herbivores and predators remain poorly understood. We studied 18 forested, offshore islands in northern New Zealand. Nine of these host high densities of burrowing seabirds that serve as ecosystem drivers by transporting nutrients from the ocean to land. The other nine have been invaded over the past 50–150 years by rat species introduced from Europe which serve as predators of seabird eggs and chicks and severely reduce their densities. We collected fully expanded leaves and fresh leaf litter from invaded and uninvaded islands for each of 12 perennial plant species that represent a wide spectrum of life forms from ground dwelling to emergent canopy species. We found that, across these species, invasion by rats significantly reduced nitrogen (N) but not phosphorus (P) concentrations of foliage and litter, promoted N but not P resorption from leaves before litter fall, and reduced the release of N but not P from decomposing litter. Rat invasion also negatively affected litter decomposability but had no overall effects on litter quality variables other than N. Our results provide evidence that rat invasion causes more conservative cycling of N but not P through foliage and litter and limitation of ecological processes by N but not P. We found few instances in which the effects of rat invasion on response variables varied significantly across plant species, meaning that invasion had similar effects for species that varied greatly in growth form and foliar and litter quality. Further, correlation analyses across the 12 species showed that foliar and litter quality traits were poor predictors of how invasion effects on resorption and decomposition variables varied among species. Our results show that the effects of invasive predators on native prey can have substantial indirect effects on variables relevant for ecosystem functioning. These types of effects are probably widespread, especially given the role of seabirds in improving soil fertility in many coastal ecosystems worldwide and the wide global distribution of predators of seabirds. Key words: biological invasion; invasive predators; litter decomposition; New Zealand; nutrient resorption; nutrient subsidies; rats; seabirds; trophic cascade. INTRODUCTION Invasive species are increasingly being recognized as important drivers of community- and ecosystem-level properties, especially when the fundamental attributes of the invaders are unrepresented in the native biota (Vitousek and Walker 1989, Sanders et al. 2003, Funk and Vitousek 2007). As such, there is increasing recognition that invasive plant species can significantly alter the decomposer subsystem, and a growing number of studies have provided evidence of plant invasions altering the quality of litter input to ecosystems (Allison and Vitousek 2004, Standish et al. 2004), decomposer community structure (Van der Putten et al. 2007), and soil fertility (Ehrenfeld 2003). Although it is well known that animal community structure can also exert large indirect effects on the quantity and quality of resources entering the soil (Bardgett and Wardle 2003), surpris- ingly few studies have considered the effects of aboveground invasive consumers on the decomposer subsystem. However, a handful of recent studies have provided evidence of decomposer-related processes being influenced by invasive herbivores such as deer (Wardle et al. 2001) and moths (Lovett et al. 2006) and invasive predators such as ants (O’Dowd et al. 2003), flatworms (Boag and Yeates 2001), foxes (Croll et al. 2005), and rats (Fukami et al. 2006). Indirect effects of predators on the decomposer subsystem are consistent with a small but growing body of terrestrial literature pointing to trophic cascades as potentially important drivers of community and ecosystem processes (Letour- neau and Dyer 1998, Wardle et al. 2005, Schmitz 2006). Seabirds are major ecosystem drivers in many coastal regions worldwide, especially during nesting. These birds forage on marine resources and transport these resources from the ocean to the land, frequently greatly Manuscript received 7 January 2008; revised 18 June 2008; accepted 20 June 2008. Corresponding Editor: J. B. Yavitt. 4 E-mail: david.wardle@svek.slu.se 452