title: Burrowing seabird effects on invertebrate communities in soil and litter are dominated by ecosystem engineering rather than nutrient addition.
abstract: Vertebrate consumers can be important drivers of the structure and functioning of ecosystems, including the soil and litter invertebrate communities that drive many ecosystem processes. Burrowing seabirds, as prevalent vertebrate consumers, have the potential to impact consumptive effects via adding marine nutrients to soil (i.e. resource subsidies) and non-consumptive effects via soil disturbance associated with excavating burrows (i.e. ecosystem engineering). However, the exact mechanisms by which they influence invertebrates are poorly understood. We examined how soil chemistry and plant and invertebrate communities changed across a gradient of seabird burrow density on two islands in northern New Zealand. Increasing seabird burrow density was associated with increased soil nutrient availability and changes in plant community structure and the abundance of nearly all the measured invertebrate groups. Increasing seabird densities had a negative effect on invertebrates that were strongly influenced by soil-surface litter, a positive effect on fungal-feeding invertebrates, and variable effects on invertebrate groups with diverse feeding strategies. Gastropoda and Araneae species richness and composition were also influenced by seabird activity. Generalized multilevel path analysis revealed that invertebrate responses were strongly driven by seabird engineering effects, via increased soil disturbance, reduced soil-surface litter, and changes in trophic interactions. Almost no significant effects of resource subsidies were detected. Our results show that seabirds, and in particular their non-consumptive effects, were significant drivers of invertebrate food web structure. Reductions in seabird populations, due to predation and human activity, may therefore have far-reaching consequences for the functioning of these ecosystems.
authors.affiliation: Landcare Research, PO Box 69040, Lincoln, 7640, New Zealand. kateorwin@hotmail.com.
authors.forename: Kate H
authors.initials: KH
authors.lastname: Orwin
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, 901-83, Umeå, Sweden.
authors.forename: David A
authors.initials: DA
authors.lastname: Wardle
authors.affiliation: Department of Conservation, Newton, PO Box 68 908, Auckland, 1145, New Zealand.|Institute for Applied Ecology New Zealand, Auckland University of Technology, Auckland, 1142, New Zealand.
authors.forename: David R
authors.initials: DR
authors.lastname: Towns
authors.affiliation: Landcare Research, PO Box 69040, Lincoln, 7640, New Zealand.|Agriculture and Agri-Food Canada, KW Neatby Building, 960 Carling Ave, Ottawa, ON, K2W 1A7, Canada.
authors.forename: Mark G
authors.initials: MG
authors.lastname: St John
authors.affiliation: Landcare Research, PO Box 69040, Lincoln, 7640, New Zealand.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Bellingham
authors.affiliation: Landcare Research, PO Box 69040, Lincoln, 7640, New Zealand.
authors.forename: Chris
authors.initials: C
authors.lastname: Jones
authors.affiliation: Department of Entomology, Te Papa Tongarewa Museum of New Zealand, Wellington, 6140, New Zealand.
authors.forename: Brian M
authors.initials: BM
authors.lastname: Fitzgerald
authors.affiliation: 15 Willowbrook, Pukekohe, 2120, New Zealand.
authors.forename: Richard G
authors.initials: RG
authors.lastname: Parrish
authors.affiliation: Landcare Research, PO Box 69040, Lincoln, 7640, New Zealand.
authors.forename: Phil O'B
authors.initials: PO
authors.lastname: Lyver
mesh_terms: D000818:Animals; D001717:Birds; D017753:Ecosystem; D005247:Feeding Behavior; D020387:Food Chain; D007448:Invertebrates; D062312:Islands; D009520:New Zealand; D010944:Plants; D011157:Population Dynamics; D011235:Predatory Behavior; D012987:Soil / Q000737:chemistry*
chemical_list: D012987:Soil
keywords: Consumptive; Food webs; Generalized multilevel path models; Non-consumptive; Resource subsidy