Post-fragmentation population structure in a cooperative breeding Afrotropical cloud forest bird: emergence of a source-sink population network M. HUSEMANN,* 1 L. COUSSEAU, † 1 T. CALLENS, † E. MATTHYSEN, ‡ C. VANGESTEL, †§ C. HALLMANN ¶ and L. LENS † ** *General Zoology, Institute of Biology, Martin-Luther University Halle-Wittenberg, Halle (Saale), Germany, †Terrestrial Ecology Unit, Department of Biology, Ghent University, Ghent, Belgium, ‡Evolutionary Ecology Group, Department of Biology, University of Antwerp, Antwerp, Belgium, §Entomology Department, Royal Belgian Institute of Natural Sciences, Brussels, Belgium, ¶Sovon, Dutch Centre for Field Ornithology, PO Box 6521, 6503 GA, Nijmegen, Netherlands, **Ornithology Section, Department of Zoology, National Museums of Kenya, Nairobi, Kenya Abstract The impact of demographic parameters on the genetic population structure and viabil- ity of organisms is a long-standing issue in the study of fragmented populations. Demographic and genetic tools are now readily available to estimate census and effec- tive population sizes and migration and gene flow rates with increasing precision. Here we analysed the demography and genetic population structure over a recent 15- year time span in five remnant populations of Cabanis’s greenbul (Phyllastrephus ca- banisi), a cooperative breeding bird in a severely fragmented cloud forest habitat. Con- trary to our expectation, genetic admixture and effective population sizes slightly increased, rather than decreased between our two sampling periods. In spite of small effective population sizes in tiny forest remnants, none of the populations showed evi- dence of a recent population bottleneck. Approximate Bayesian modelling, however, suggested that differentiation of the populations coincided at least partially with an episode of habitat fragmentation. The ratio of meta-N e to meta-N c was relatively low for birds, which is expected for cooperative breeding species, while N e /N c ratios strongly varied among local populations. While the overall trend of increasing popula- tion sizes and genetic admixture may suggest that Cabanis’s greenbuls increasingly cope with fragmentation, the time period over which these trends were documented is rather short relative to the average longevity of tropical species. Furthermore, the criti- cally low N c in the small forest remnants keep the species prone to demographic and environmental stochasticity, and it remains open if, and to what extent, its cooperative breeding behaviour helps to buffer such effects. Keywords: approximate Bayesian computation, capture–recapture, census population size, demography, effective population size, gene flow, MARK, STRUCTURE, Taita Hills Received 1 December 2014; revision received 29 January 2015; accepted 4 February 2015 Introduction Population size and genetic connectivity largely deter- mine the vulnerability of populations to demographic and genetic stochasticity (Lande 1988; Ellstrand & Elam 1993; Frankham 1995; Young & Clarke 2000; Amos & Balmford 2001) and are therefore at the core of most conservation biological studies (Richards et al. 2003). The effective size of a population (N e ; Wright 1931), in particular, constitutes a key parameter as it estimates the rate at which genetic variance erodes due to sto- chastic events in small populations (Crow & Kimura 1970) and influences the rate of evolution (Lanfear et al. Correspondence: Martin Husemann, Fax: +49-345-5527428; E-mail: martinhusemann@yahoo.de 1 MH and LC contributed equally. © 2015 John Wiley & Sons Ltd Molecular Ecology (2015) 24, 1172–1187 doi: 10.1111/mec.13105