Population Growth Confounds Phylogeographic Inference in Namaqua

The Namaqua sandgrouse, Pterocles namaqua, is a highly nomadic granivore of semiarid to arid habitats. As a result of nomadic movements in response to rainfall, the size of the breeding population in any one area fluctuates dramatically between breeding seasons. This high mobility in response to spatial and temporal abundance of food resources is expected to result in little population genetic structuring. Namaqua sandgrouse also shows a seasonally predictable partial migration between the southeast and northwest regions of South Africa, and a further possible north–south migration between southwestern South Africa and central Namibia. It is unclear whether birds migrating between these regions breed in only one or both regions. If populations breed in only one region of their migratory range, then population genetic structuring is predicted to occur. This study addresses Namaqua sandgrouse movements with the analysis of mitochondrial DNA control region sequences. In general, little population genetic structure was evident, yet strong signals of population growth were detected. Several populations have private alleles, which is in direct contradiction to the spatial genetic pattern expected under high levels of gene flow. We suggest that the inference of high levels of female gene flow could be an artifact of population growth and that additional loci will allow a greater understanding of Namaqua sandgrouse movements. The inference of the demographic processes of migration, admixture, isolation by distance, bottlenecks, and population growth has become a central challenge in phylogeographic studies (Emerson et al. 2001; Knowles and Maddison 2002). Such demographic processes, if acting in isolation, would each be expected to generate different spatial genetic patterns. However, each of these processes may occur during limited time periods, or multiple processes may occur simultaneously, and thus, the untangling of multiple historical factors contributing to spatial genetic pattern remains a major challenge (Wakeley and Hey 1998; Knowles 2004). Recent developments in coalescent approaches (Kuhner et al. 1998; Bahlo and Griffiths 2000; Beerli and Felsenstein 2001; Nielsen and Wakeley 2001) have, however, allowed the coestimation of demographic parameters of interest, such as population growth, effective population size, migration, time of isolation, among other parameters. In this article, we use such available computational methods to identify demographic processes that have contributed to observed spatial genetic patterns in the partially migratory and nomadic Namaqua sandgrouse (Pterocles namaqua). Namaqua sandgrouse inhabit arid to semiarid regions of southern Africa (Figure 1). As obligate granivores, they must drink regularly and are known to congregate in flocks during a synchronized flight to the nearest surface water each morning (Ward 1972). These flocks may cover a distance of more than 50 km a day in their movement between water, feeding, and roosting sites (Lloyd et al. 2000). Namaqua sandgrouse are highly nomadic over most of their distribution, exhibiting large interannual fluctuations in abundance in any one area (Lloyd, Little, Crowe, and Simmons 2001). In addition, this species has a partial migration, restricted to the southern African sub-region, between the southwestern and northeastern regions of its distribution (Malan et al. 1994; Lloyd, Little, Crowe, and Simmons 2001). Populations in the southwest (Namaqualand/Karoo) are generally larger in the summer months (September–April) and often disappear during the winter months (May–August), whereas populations in Journal of Heredity 2007:98(2):158–164 doi:10.1093/jhered/esl058 Advance Access publication December 7, 2006 a The American Genetic Association. 2006. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org.