Population trends are determined by gains through reproduction and immigration, and losses through mortality and emigration. These demographic quantities and resulting population dynamics are affected by different external and internal drivers. We examined how these demographic quantities were affected by weather, research-induced disturbance, local density, colony site and year in a metapopulation of 17 sociable weaver (Philetairus socius) colonies over 17 years of study (4 years for reproduction). Most colonies declined, but at different rates. The four demographic quantities were related to different drivers. Survival strongly varied among years and colonies and was positively related to rainfall and negatively related to extreme temperature (together explaining 30 % of variation) and disturbance (measured as number of captures conducted at a colony; 7 %). There was a trend for a positive relationship between reproduction and rainfall (50 %). Movement was mainly related to local density: individuals were more likely to emigrate from small to large colonies and from colonies that were either well below or above their long-term mean. They were more likely to immigrate into colonies that were nearby, and below their mean size. We then quantified the effects of these relationships on metapopulation dynamics using a multi-site matrix projection model. Rainfall was potentially a strong driver of metapopulation dynamics. In addition, field-work disturbance might have contributed to the decline of this metapopulation but could not explain its full magnitude. Hence, through a combination of analytical methods we were able to obtain information on the main drivers affecting dynamics in a declining metapopulation.
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We thank E. Herrmann, D. J. Schaefer, R. Visagie and numerous volunteers for help with field work, M. A. du Plessis for guidance and help with funding, R. Green, H.D. Oschadleus and C. Brown for comments on the manuscript, and De Beers Consolidated Mines for access to Benfontein Game Reserve. Funding for fieldwork was provided by the DST/NRF and the Centre of Excellence at the Percy FitzPatrick Institute. R. A. was supported by the National Research Foundation of South Africa (NRF; grant 85802). C. D. was funded by the CNRS, C. N. S. by the University of Cape Town and St John’s College, Cambridge, and R. C. by Program Ciência 2008. C. D. and R. C. have received additional funding from the Interchange Program Pessoa (France and Portugal). The NRF accepts no liability for opinions, findings and conclusions or recommendations expressed in this publication.
Communicated by Chris Whelan.
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Altwegg, R., Doutrelant, C., Anderson, M.D. et al. Climate, social factors and research disturbance influence population dynamics in a declining sociable weaver metapopulation. Oecologia 174, 413–425 (2014). https://doi.org/10.1007/s00442-013-2768-7
- Allee effect
- Colonial bird
- Environmental stochasticity
- Metapopulation model