Space Partitioning Without Territoriality in Gannets Ewan D
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REPORTS models, P = 0.002 and < 0.001; tables S2 and Space Partitioning Without S3]. Birds from colonies of all sizes divided their time equally between foraging and chick atten- Territoriality in Gannets dance (LME, P = 0.191; table S4), and the number of foraging trips per day was negatively depen- dent on N0.5 (LME, P = 0.024; table S5). Prey 1 † 2 † 2 3 Ewan D. Wakefield, * Thomas W. Bodey, * Stuart Bearhop, * Jez Blackburn, delivery rate, for which we assume trips per day Kendrew Colhoun,4 Rachel Davies,1 Ross G. Dwyer,2 Jonathan A. Green,5 David Grémillet,6,7 8 9 8 10 is a proxy, is therefore negatively dependent on Andrew L. Jackson, Mark J. Jessopp, Adam Kane, Rowena H. W. Langston, N0.5, supporting the prediction that colony size Amélie Lescroël,6,11 Stuart Murray,12 Mélanie Le Nuz,13 Samantha C. Patrick,14‡ 4 6 6 5 15 16 1 is limited by density-dependent competition ( , ). Clara Péron, Louise M. Soanes, Sarah Wanless, Stephen C. Votier, * Keith C. Hamer * Contrary to the hinterland model (3), we found no relationship between colony Voronoi polygon 2 Colonial breeding is widespread among animals. Some, such as eusocial insects, may use area and colony size (F1,35 < 0.01, P = 0.699, R < agonistic behavior to partition available foraging habitat into mutually exclusive territories; 0.01; fig. S5). others, such as breeding seabirds, do not. We found that northern gannets, satellite-tracked from Using empirical relationships between colony 12 neighboring colonies, nonetheless forage in largely mutually exclusive areas and that these size and foraging area, we devised a population- colony-specific home ranges are determined by density-dependent competition. This segregation level null model of the distribution of foraging may be enhanced by individual-level public information transfer, leading to cultural evolution gannets, assuming negligible competition between and divergence among colonies. birds from neighboring colonies (16). This suc- cessfully explains among-colony segregation when olonial animals are constrained by their polygons (Fig. 1A), as seen in some territorial colonies are far apart but predicts extensive over- colony locations, which are ultimately lim- animals (2). Food availability is assumed to be lap between several study colonies, particularly Cited by resource availability (1). However, proportional to polygon area, limiting colony in the Celtic Sea (Fig. 2A). However, observed within species, potential colony home ranges size. An alternative model proposes that density- UDs were largely mutually exclusive (fig. S2), often overlap, implying that competition among dependent competition among colony members overlapping markedly less than predicted (fig. colonies may also be limiting (2). In eusocial is limiting (4). As colonies grow, local prey de- S6). For example, the null population overlap central-place foragers, the spatial effects of di- pletion or disturbance requires birds to travel index (POI) [the number of potential pairwise on March 29, 2017 rect competition among colonies are well under- further to provision their young. However, this interactions between birds from adjacent colonies stood (2). By contrast, the spatial influences of model (“Ashmole’shalo”) does not consider in- (16)] for Little Skellig and Bull Rock (popula- indirect competition and information transfer on teractions among colonies and tacitly assumes tions ~29,700 and 3700 pairs; separation distance nonterritorial species (e.g., seals, swallows, and that adjacent colonies’ home ranges overlap (5). 27 km) was 105,000, whereas the empirical esti- seabirds), where levels of relatedness are much Indirect evidence exists to support both mod- mate was 6000, largely because foraging trips lower, remain conjectural. For example, the hinter- els (3, 6, 7), and recent tracking studies suggest were directed away from closely neighboring land model (3) predicts that breeding seabirds that seabirds and pinnipeds segregate along co- colonies (Fig. 1B). This pattern differs from the segregate along colonial lines, because of in- lonial lines (8–12). However, these studies proved hinterland model in two key respects: Segrega- equalities in travel costs from each colony. Pre- inconclusive on the causes and ubiquity of seg- tion was not absolute, and divisions between the dicted home ranges therefore comprise Voronoi regation, largely because few colonies were sam- UDs of unequally sized colonies were not equi- pled or tracking resolution was low. Here we used distant between the two (Fig. 1B and fig. S2) http://science.sciencemag.org/ high-resolution satellite tracks of the foraging but typically occurred closer to the smaller colony, 1School of Biology, University of Leeds, Leeds LS2 9JT, UK. 2 movements of 184 chick-rearing northern gan- a phenomenon also observed in penguins (9). Centre for Ecology and Conservation, University of Exeter, Morus bassanus Penryn, Cornwall TR10 9EZ, UK. 3British Trust for Ornithology, nets, (hereafter gannets), from Hence, the predictive performance of the hinter- The Nunnery, Thetford IP24 2PU, UK. 4The Royal Society for 12 of the 26 colonies fringing the British Isles land model (log-likelihood, L = –0.54, AIC the Protection of Birds, Northern Ireland Headquarters, Belvoir (median 17 birds/colony), representing ~80% of 3691; table S6) was poor in comparison to that 5 Park Forest, Belfast BT8 4QT, UK. School of Environmental the area’s breeding population (Fig. 1A and table of the null model [L = –0.30, Akaike informa- Sciences, University of Liverpool, Environmental Sciences, 6 S1), to test whether among-colony segregation oc- tion criterion (AIC) = 2231]. Liverpool L69 3GP, UK. CEFE-CNRS, 1919 Route de Mende, Downloaded from F-34293 Montpellier Cedex 5, France. 7FitzPatrick Institute, curs in a model colonial nonterritorial central-place Given the inability of existing models to ex- Department of Science and Technology–National Research Foun- forager. We then used population- and individual- plain gannet distribution when colonies are close dation Centre of Excellence, University of Cape Town, Rondebosch 8 level models to explore potential mechanisms together, we propose a multicolony extension of 7701, South Africa. Department of Zoology, School of Nat- ’ 4 ural Sciences, Trinity College Dublin, Dublin 2, Ireland. 9Coastal underlying spatial segregation. Ashmole shalo( ), which we term the density- and Marine Research Centre, University College Cork, Cork, Gannets are wide-ranging (maximum forag- dependent hinterland (DDH) model. As adjacent Ireland. 10The Royal Society for the Protection of Birds, Sandy, ing range ~700 km) pelagic seabirds that forage colonies grow, foraging ranges increase as a re- 11 Bedfordshire SG19 2DL, UK. Biodiversité et gestion des ter- in patches of enhanced production, primarily on sult of prey depletion or disturbance (6) until their ritoires, Université de Rennes 1–UMR 7204, Muséum National d’Histoire Naturelle, 35042 Rennes Cedex, France. 12Easter shoaling, mesotrophic fish and to a lesser extent, home ranges overlap. At low densities, birds from Craigie Dhu, Dunkeld, Perthshire PH8 0EY, UK. 13Réserve fisheries discards (13–15). In almost all cases, different colonies may forage together, but as prey naturelle des Sept-Iles, Ligue pour la Protection des Oiseaux, we tracked birds from adjacent colonies simul- availability decreases, populations respond by F-22560 Pleumeur Bodou, France. 14Marine Biology and Ecology taneously (16). Individual gannet tracks (Fig. 1B spreading down conspecific density gradients Research Centre, Plymouth University, Plymouth PL4 8AA, UK. 15 and fig. S1) and percentage utilization distribu- to the nearest areas subject to a lower rate of ex- Centre for Ecology and Hydrology, Penicuik EH26 0QB, UK. 6 16Environment and Sustainability Institute, University of Exeter, tions (UDs) (Fig. 2A and fig. S2) showed a dis- ploitation ( ). As a first approximation, we as- Penryn, Cornwall TR10 9EZ, UK. tinctive pattern of between-colony variation and sume a simple inverse relationship between the *Corresponding author. E-mail: [email protected] spatial segregation, within and across years (fig. exploitation by conspecifics from adjacent col- (E.D.W.); [email protected] (T.W.B.); [email protected]. S3). The size of 95% foraging UDs was strongly onies and the likelihood of new birds foraging uk (S.B.); [email protected] (S.C.V.); k.c.hamer@ 2 dependent (F1,8 = 149.7, P < 0.001, R =0.94; in an area (16). However, the trade-off between leeds.ac.uk (K.C.H.) N †These authors contributed equally to this work. fig. S4) on square-root colony size ( ). Likewise, transport and competition costs means that birds ‡Present address: Centre d’Etudes Biologiques de Chizé, CNRS- maximumforagingrangeandtripdurationwere favor areas close to their own colonies, so den- UPR1934, Villiers en Bois, France. dependent on N0.5 [linear mixed-effects (LME) sity declines with colony distance d (10). Hence, 68 5 JULY 2013 VOL 341 SCIENCE www.sciencemag.org REPORTS when colonies are large or close together, segre- proved a better fit to the tracking data (L = –0.58, 0.45) (16). The DDH model’s greater predictive gation between home ranges may become abso- AIC = 25440) than the null (L = –0.61, AIC = strength was most marked for colonies with close lute. Using these assumptions, we modeled the 27015; table S7; compare Fig. 2, A and B). Fur- neighbors (Fig. 2 and table S8). Notably, the DDH development of spatial segregation as colonies thermore, unlike the