Burrowing seabird effects on invertebrate communities in soil and litter are dominated by ecosystem engineering rather than nutrient addition
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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.
KeywordsConsumptive Food webs Generalized multilevel path models Non-consumptive Resource subsidy
We thank members of the Ruamāhua Islands Trust, Hauraki Collective, Ngāti Hei Trust, and Ngāti Whānaunga, for their support and approval to access Ruamāhuanui and Korapuki islands. The scientific and logistical support of staff from the Department of Conservation Thames Area Office was appreciated. We thank Morgan Coleman, Keven Drew, Brian Karl, Caroline Thomson, Lindsay Smith, Karen Boot, Neil Fitzgerald, Jenny Hurst, Chris Morse and Kate Ladley (Landcare Research), Ewen Cameron (Auckland Museum), David Hamon and Frank Waitai (Hauraki) for field assistance. We acknowledge the service of our charter operators Waters Edge Charters (Whitianga) and Skyworks Helicopters. We also thank Duane Peltzer and Sarah Richardson (Landcare Research) for help with statistics. This project was funded primarily by New Zealand’s Ministry of Business, Innovation and Employment’s Mauriora ki nga Oi (Safe-guarding the life force of the grey-faced petrel—C09X0509) and Te Hiringa Tangata Ki Tai Pari Ki Tai Timu (Bicultural restoration of coastal forest ecosystems—C09X0908) projects. D.R.T. received funding assistance from the Department of Conservation and D.W. was supported by a Wallenberg Scholars award.
Author contribution statement
CJ, DAW, DRT, PJB, PO’BL formulated the idea. CJ, DAW, DRT, PJB, MGStJ, PO’BL designed the experiment. PJB, MGStJ, BMF, RGP, DRT collected the data. KHO analyzed the data. KHO, DAW, DRT, MGStJ and PJB wrote the manuscript; other authors provided editorial advice and expertise for invertebrate identification.
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