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Indirect effects of invasive predators on litter decomposition and nutrient resorption on seabird-dominated islands

TitleIndirect effects of invasive predators on litter decomposition and nutrient resorption on seabird-dominated islands
Publication TypeJournal Article
Year of Publication2009
AuthorsWardle, DA, Bellingham, PJ, Bonner, KI, Mulder, CPH

Despite recent interest in the ecosystem impacts of invasive abovegroundorganisms, most work in this area has focused on effects of invasive plants, and the effects ofinvasive herbivores and predators remain poorly understood. We studied 18 forested, offshoreislands in northern New Zealand. Nine of these host high densities of burrowing seabirds thatserve as ecosystem drivers by transporting nutrients from the ocean to land. The other ninehave been invaded over the past 50–150 years by rat species introduced from Europe whichserve as predators of seabird eggs and chicks and severely reduce their densities. We collectedfully expanded leaves and fresh leaf litter from invaded and uninvaded islands for each of 12perennial plant species that represent a wide spectrum of life forms from ground dwelling toemergent canopy species. We found that, across these species, invasion by rats significantlyreduced nitrogen (N) but not phosphorus (P) concentrations of foliage and litter, promoted Nbut not P resorption from leaves before litter fall, and reduced the release of N but not P fromdecomposing litter. Rat invasion also negatively affected litter decomposability but had nooverall effects on litter quality variables other than N. Our results provide evidence that ratinvasion causes more conservative cycling of N but not P through foliage and litter andlimitation of ecological processes by N but not P. We found few instances in which the effectsof rat invasion on response variables varied significantly across plant species, meaning thatinvasion had similar effects for species that varied greatly in growth form and foliar and litterquality. Further, correlation analyses across the 12 species showed that foliar and litter qualitytraits were poor predictors of how invasion effects on resorption and decomposition variablesvaried among species. Our results show that the effects of invasive predators on native preycan have substantial indirect effects on variables relevant for ecosystem functioning. Thesetypes of effects are probably widespread, especially given the role of seabirds in improving soilfertility in many coastal ecosystems worldwide and the wide global distribution of predators ofseabirds.

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