title: Invasive rodents have multiple indirect effects on seabird island invertebrate food web structure.
abstract: Burrowing seabirds that nest on islands transfer nutrients from the sea, disturb the soil through burrowing, damage tree foliage when landing, and thereby modify the surface litter. However, seabirds are in decline worldwide, as are their community- and ecosystem-level impacts, primarily due to invasive predatory mammals. The direct and indirect effects of seabird decline on communities and ecosystems are inherently complex. Here we employed network analysis, as a means of simplifying ecological complexity, to better understand the effects seabird loss may have on island invertebrate communities. Using data on leaf litter communities, we constructed invertebrate food webs for each of 18 offshore oceanic islands in northeastern New Zealand, nine of which have high seabird densities and nine of were invaded by rats. Ten network topological metrics (including entropy, generality, and vulnerability) were compared between rat-invaded and uninvaded (seabird-dominant) islands. We found that, on rat-invaded islands, the invertebrate food webs were smaller and less complex than on their seabird-dominated counterparts, which may be due to the suppression of seabird-derived nutrients and consequent effects on trophic cascades. This decreased complexity of food webs due to the presence of rats is indicative of lower ecosystem resistance via lower trophic redundancy. Our results show that rat effects on island ecosystems are manifested throughout entire food webs, and demonstrate how network analysis may be useful to assess ecosystem recovery status as these invaded islands are restored.
authors.affiliation: Institute for Applied Ecology, Auckland University of Technology, 33 Symonds Street, Auckland, New Zealand.
authors.forename: Joshua J
authors.identifier: 0000-0002-8653-7110
authors.initials: JJ
authors.lastname: Thoresen
authors.affiliation: Institute for Applied Ecology, Auckland University of Technology, 33 Symonds Street, Auckland, New Zealand.|Department of Conservation, Private Bag 68-908, Auckland, New Zealand.
authors.forename: David
authors.initials: D
authors.lastname: Towns
authors.affiliation: Institute for Applied Ecology, Auckland University of Technology, 33 Symonds Street, Auckland, New Zealand.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Leuzinger
authors.affiliation: Department of Biology and Wildlife and Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska, 99775, USA.|Department of Biology, Rhodes College, 2000 North Parkway, Memphis, Tennessee, 38112, USA.
authors.forename: Mel
authors.initials: M
authors.lastname: Durrett
authors.affiliation: Department of Biology and Wildlife and Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska, 99775, USA.
authors.forename: Christa P H
authors.initials: CPH
authors.lastname: Mulder
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE901-83, Umeå, Sweden.
authors.forename: David A
authors.initials: DA
authors.lastname: Wardle
mesh_terms: D000818:Animals; D001717:Birds; D020387:Food Chain; D058865:Introduced Species; D007448:Invertebrates; D062312:Islands; D008954:Models, Biological; D009520:New Zealand; D011235:Predatory Behavior; D051381:Rats
keywords: ecosystem restoration; food webs; invasive mammals; invertebrates; network analysis; rats; seabirds