title: Density-dependent resource selection by a terrestrial herbivore in response to sea-to-land nutrient transfer by seals.
abstract: Sea-to-land nutrient transfers can connect marine food webs to those on land, creating a dependence on marine webs by opportunistic species. We show how nitrogen, imported by gray seals, Halichoerus grypus, and traced through stable isotope (δ15N) measurements in marram grass, Ammophila breviligulata, significantly alters foraging behavior of a free-roaming megaherbivore (feral horses, Equus ferus caballus) on Sable Island, Canada. Values of δ15N correlated with protein content of marram and strongly related to pupping-seal densities, and positively influenced selective foraging by horses. The latter was density dependent, consistent with optimal foraging theory. We present the first demonstration of how sea-to-land nutrient transfers can affect the behavioral process of resource selection (resource use relative to availability) of terrestrial consumers. We hypothesize that persistence of horses on Sable Island is being facilitated by N subsidies. Our results have relevance to advancing theory on trophic dynamics in island biogeography and metaecosystem ecology.
authors.affiliation: Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada.
authors.forename: Philip D
authors.initials: PD
authors.lastname: McLoughlin
authors.affiliation: Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada.
authors.forename: Kenton
authors.initials: K
authors.lastname: Lysak
authors.affiliation: Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada.
authors.forename: Lucie
authors.initials: L
authors.lastname: Debeffe
authors.affiliation: Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada.
authors.forename: Thomas
authors.initials: T
authors.lastname: Perry
authors.affiliation: Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan, S7N 3H5, Canada.|Department of Biology, University of Western Ontario, London, Ontario, N6A 5B7, Canada.
authors.forename: Keith A
authors.initials: KA
authors.lastname: Hobson
mesh_terms: D000818:Animals; D002170:Canada; D004463:Ecology; D020387:Food Chain; D060434:Herbivory; D006736:Horses; D062312:Islands; D046048:Seals, Earless
keywords: Sable Island; density-dependent resource selection; feral horse (Equus ferus caballus); food web; gray seal (Halichoerus grypus); isodar; marine-derived nutrient; marram grass (Ammophila breviligulata); stable isotope; trophic dynamics