Ole-Gunnar Støen Natal dispersal and social organization in in organization and social dispersal Natal brown bears Norwegian University of Life Sciences • Universitetet for miljø- og biovitenskap Department of Ecology and Natural Resource Management Philosophiae Doctor (PhD) Thesis 2006:6 Natal dispersal and social organization in brown bears Ole-Gunnar Støen PhD thesis Department of Ecology and Natural Resource Management Norwegian University of Life Sciences Ås, 2006 Contents Abstract ...................................................................................................................................... 1 Sammendrag............................................................................................................................... 2 List of papers.............................................................................................................................. 3 Introduction ................................................................................................................................ 4 Objectives of the study........................................................................................................... 6 Methods...................................................................................................................................... 6 The study areas and populations ............................................................................................ 6 Field and genetic methods...................................................................................................... 7 Main results................................................................................................................................ 8 Natal dispersal........................................................................................................................ 8 Inversely density-dependent natal dispersal (Paper I)........................................................ 8 Causes of natal dispersal (Paper II).................................................................................... 8 Spatial structure...................................................................................................................... 9 Home range size in subadults (Paper III)........................................................................... 9 Kin-related spatial structure (Paper IV) ............................................................................. 9 Sociality and reproduction ................................................................................................... 10 Delayed primiparity; a consequence of sociality? (Paper V)........................................... 10 Primiparity in Scandinavian brown bears (Paper VI) ...................................................... 11 Habitat selection................................................................................................................... 11 Preference for undisturbed rugged terrain (Paper VII) .................................................... 11 Discussion ................................................................................................................................ 12 Management implications .................................................................................................... 17 Acknowledgements .................................................................................................................. 19 References ................................................................................................................................ 20 Compilation of papers .............................................................................................................. 25 i Abstract Dispersal has important implications for population ecology, genetics and conservation of animal populations through the redistribution of individuals, whereas social organization has important implications for individual resource acquisition and reproduction. This study, which used radio-collared and shot brown bears in two areas in Scandinavia, revealed that the spatial distribution of bears was determined by the presence of other bears (i.e. population density), relatedness among adjacent individuals, and habitat quality. Natal dispersal and home range size were inversely density dependent, indicating some sort of territorial behavior in Scandinavian brown bears. The amount of home range overlap among adult females was determined by relatedness, and most related females formed matrilinear assemblages from which unrelated females were excluded. Almost all males and 40% of the females dispersed from their natal area. Inbreeding avoidance seemed to determine natal dispersal in males, whereas competition for philopatry was probably the reason for natal dispersal in females, even if staying close to relatives might entail reproductive suppression. The age of primiparity was higher in philopatric than in dispersing females, possibly due to behavioral reproductive suppression, but the youngest primiparious females (4-year-olds) lost most cubs. Matrilinear assemblages might lead to spatial variation in population dynamics, which implies that monitoring programs of population trends must have a large enough scale to even out this spatial heterogeneity. Harvest that changes the local density of bears may influence dispersal patterns and expansion of brown bear populations. Both males and females showed a strong preference for undisturbed rugged terrain, and because bears are sensitive to human disturbance, a coordination of ongoing recreational developments is needed to avoid further degradation of the brown bear habitat quality in Scandinavia. 1 Sammendrag Spredning av unge dyr fra fødestedet har en viktig innvirkning på arters populasjonsøkologi, genetikk og forvaltning, fordi dette fører til en romlig omfordelig av individene, mens sosial organisering påvirker individuell ressurservervelse og reproduksjon. Dette studiet, som benyttet radiomerkete og skutte brunbjørner i to studieområder i Skandinavia, fant at fordelingen av bjørn i terrenget avhenger av forekomsten av andre bjørner (dvs. bestandstetthet), slektskap mellom naboer, samt habitatkvaliteten. Spredningen og størrelsen på hjemmeområdet var omvendt tetthetsavhengig, noe som tyder på en form for territoriell adferd hos Skandinavisk brunbjørn. Beslektete voksne hunners leveområder overlappet mer enn ubeslektete, og de fleste beslektete hunner dannet ansamlinger hvorfra ubeslektede hunner ble utestengt. Nesten alle de unge hannene og 40% av hunnene vandret ut fra fødestedet. Den bakenforliggende årsaken til at hanner utvandret var for å unngå innavl, mens konkurranse om å få bli på fødestedet mellom kullsøstre antas å være årsaken til utvandring hos hunner, selvom det å bli på fødestedet kunne føre til reproduktiv undertrykkelse. Alder for førstefødsel var høyere hos gjenværende enn hos utvandrete hunner, muligens på grunn av reproduktiv undertrykkelse, men hunner som fødte første gang i ung alder (4 år gamle) mistet flest unger. Ansamlinger av beslektede hunner kan føre til en romlig variasjon i populasjonsdynamikken. Dette gjør at skalaen i overvåkning av populasjonstrender må være stor nok til å kunne utjevne denne hetrogeniteten. Jakt som endrer den lokale tettheten av bjørn kan påvirke spredningsmønstret og ekspansjonen av brunbjørnbestander. Både hanner og hunner foretrakk uforstyrret kupert terreng, og viser at bjørner er følsomme overfor menneskelig forstyrrelse. Derfor er det nødvendig med en koordinering av den pågående hytteutbyggingen for å unngå en ytterligere forringelse av kvaliteten på bjørnens habitat i Skandinavia. 2 List of papers Paper I Støen OG, Zedrosser A, Sæbø S, Swenson JE (2006) Inversely density-dependent dispersal in brown bears Ursus arctos. Oecologia (in press) Paper II Zedrosser A, Støen OG, Sæbø S, Swenson JE. Should I stay, or should I go? Natal dispersal in the brown bear. Submitted Paper III Dahle B, Støen OG, Swenson JE. Factors influencing home range size in subadult brown bears. J Mammal (2nd revision submitted) Paper IV Støen OG, Bellemain E, Sæbø S, Swenson JE (2005) Kin-related spatial structure in brown bears Ursus arctos. Behav Ecol Sociobiol 59: 191-197 Paper V Støen OG, Zedrosser A, Wegge P, Swenson JE. Socially induced delayed primiparity in brown bears Ursus arctos. Behav Ecol Sociobiol (2nd revision submitted) Paper VI Zedrosser A, Dahle B, Støen OG, Swenson JE. Primiparity, litter size and cub survival in a species with sexually selected infanticide, the brown bear. Submitted Paper VII Nellemann C, Støen OG, Kindberg J, Swenson JE, Vistnes I, Ericsson G, Katajisto J, Kalternborn B, Martin J. Terrain use by brown bears in relation to resorts and human settlements. Manuscript 3 Introduction Dispersal has important implications for population ecology, genetics and conservation through the redistribution of animals. Studies of dispersal are fundamental to many problems in theoretical and applied ecology, such as responses to habitat loss and fragmentation and re- establishment in species recovery programs etc. (Bullock et al. 2002). Natal dispersal is the most common mechanism of natural population redistribution, because the distance moved and proportion of both birds and mammals moving are commonly larger in subadults than in adults (Greenwood 1980, Dobson 1982, Gese and Mech 1991, Paradis et al. 1998, Nelson and Mech 1999). Intra-sexual competition and inbreeding avoidance has been proposed to explain the ultimate causes