Journal of Animal Ecology 2003 72, 873 – 887 © 2003 British Ecological Society Blackwell Publishing Ltd. Nutrient exchanges between marine and terrestrial ecosystems: the case of the Galapagos sea lion Zalophus wollebaecki J. M. FARIÑA*†, S. SALAZAR‡, K. P. WALLEM*, J. D. WITMAN† and J. C. ELLIS† *Center for Advanced Studies in Ecology and Biodiversity (CASEB), Pontificia Universidad Católica de Chile, Santiago, Chile; †Department of Ecology and Evolutionary Biology, Brown University, Providence, USA; and ‡Marine Research and Conservation Department, Charles Darwin Research Station (CDRS), Puerto Ayora, Galápagos, Ecuador Summary 1. The movement of materials and organisms between ecosystems is a common process in nature. 2. In the present study we investigate the hypothesis that the transport of nutrients by low-mobility species and their effect on terrestrial ecosystems depends on habitat topography. Specifically, we hypothesized that the influence of a marine organism with low mobility on terrestrial environments would be spatially restricted. 3. To address this hypothesis we analysed the distribution (both geographical and local scales) of Galapagos sea lion colonies, and quantified the spatial extent of their influ- ence on terrestrial ecosystems (soil and plants). 4. Our results showed that the influence of Z. wollebaecki on Galapagos terrestrial habitats is restricted to shorelines with low elevations, but that it is it is geographically ubiquitous across the Archipelago. 5. Our study demonstrated that Z. wollebaecki is an effective vector for the transport of marine nutrients to terrestrial ecosystems. Transported nutrients occur in high concen- trations in the soils and are used by shoreline plants. These effects are spatially restricted to the areas where seals occur and the most parsimonious explanatory variable for these patterns is the islands’ topography (or elevation). Key-words: Cryptocarpus, isotopic enrichment, sea lion, Sesuvium, soil ammonia and nitrate, topography. Journal of Animal Ecology (2003) 72, 873 – 887 Introduction The movement of materials and organisms between ecosystems is a common process in nature (e.g. Likens & Borman 1975; Nakano & Murakami 2001). Cross- ecosystem exchanges can be categorized into three major groups: (1) transport of nutrients and materials by physical agents such as water or wind; (2) transport of nutrients and materials by biotic agents such as vertebrates; and (3) movement of prey and consumers between habitats (Polis, Anderson & Holt 1997). Since Elton (1927), this process has been noted in both ter- restrial and aquatic communities. However, a theoret- ical framework for it has only recently been generated, mainly based on food web theory (DeAngelis 1980; Polis et al . 1997; Huxel & McCann 1998; Fagan, Cantrell & Cosner 1999; Huxel, McCann & Polis 2002). In this framework it is recognized that nutrients and materials are transported from more to less pro- ductive ecosystems, and that these spatial subsidies depend on the permeability of adjacent habitats, or on specific characteristics of physical–biotic vectors. For example, in the case of physical transport, the flow, speed and volume of water and /or wind could deter- mine the amount and kind of transported materials (Gosz 1991; Witman, Ellis & Anderson, in press). Mobility, behaviour and physiology are important characteristics Correspondence: J. M. Fariña, Center for Advanced Studies in Ecology and Biodiversity (CASEB), Pontificia Universidad Católica de Chile, Alameda 340, C.P. 6513677, Santiago, Chile. Tel. +56 2 6862610; Fax: +56 2 6862621; E-mail: jmfa- rina@bio.puc.cl