Natural processes associated with vicariance, habitat dependence, local adaptation, and dispersal strategies (evolved to maximize fitness) promote the distribution of diversity among conspecific populations. These processes lead to the evolution of population structure over varying geographic scales and eventual speciation. At the same time, anthropogenic activities are often superimposed, and the impact on population structure will depend on how these different processes interact. Humans can cause fragmentation, population declines, expansions (e.g., when non-native species are introduced), and extirpation.
For populations that have been influenced by declines, recovery in population size can be rapid, but there may be a long-term impact on diversity (both through the initial loss of allelic diversity and through drift over time). Typically, population size recovery can occur as a species naturally disperses and recolonizes formerly occupied areas. This process of natural recolonization usually occurs when degraded habitats are restored and dispersal corridors are available (see Hochkirch et al. 2007). Populations may also be restored through human intervention using translocations. There are various complications associated with this process including the loss of diversity and distortion of allele frequencies if the founder population size used for the translocation is small (e.g., Ralls et al. 2000), and the possibility that interbreeding between introduced and native populations may result in reduced fitness if too dissimilar (e.g., Rhymer and Simberloff 1996). At the same time, translocations are often proposed as a means of “genetic rescue” introducing new genes into depauperate threatened or endangered populations (e.g., Pimm et al. 2006; cf. Creel 2006).
Introduced populations may remain relatively isolated with low diversity following small reintroductions. This was found to be the case for the Alpine ibex, Capra ibex ibex where founder populations had mostly been serially bottlenecked (Biebach and Keller 2009), and the white-tailed deer, Odocoileus virginianus, where multiple source populations introduced into an area where the species had been extirpated retained founder signatures and genetic structure (DeYoung et al. 2003). However, introduced populations that rapidly expand may retain more variation and integrate with native populations more readily (e.g., Zenger et al. 2003). This should also depend on the interaction between population density, dispersal behavior and range, and proximity to native populations (see Latch and Rhodes 2005). In this study, we investigate the effects of historical population declines followed by more recent recolonization via natural and non-natural dispersal among populations of the British roe deer (Capreolus capreolus). This set of population histories provides an opportunity to investigate the implications of the interaction between natural processes associated with population expansion and philopatry, and the influence of anthropogenic impacts on population size and distributions.
The only deer species indigenous to the United Kingdom are the roe deer and red deer (Cervus elaphus). The first postglacial records of roe deer date back to between 10,050 and 9600 YBP from a site found at Thatcham in Berkshire (Yalden 1999). During the late medieval period, British roe deer populations were severely reduced, probably as a result of overhunting and deforestation. Historical documents indicate that these declines were so severe that roe deer were confined to parts of Scotland and possibly some of the northern English border counties (Whitehead 1964). In most of the midlands and southern English counties, roe deer were reportedly absent by the 16th century (Ritson 1933). During the 1800s, roe deer populations began to recover and, since then, this recovery has been remarkable (see Ward 2005). Recovery in northern parts of the United Kingdom can generally be attributed to natural expansion of remnant populations into formerly occupied areas following afforestation (Taylor 1948). In southern parts of the United Kingdom, all populations are believed to have descended from reintroduction events (see Table 1 and Whitehead 1964). All populations in the United Kingdom have now expanded substantially in size, and numbers are currently controlled in culls managed by either independent landowners or local collaboratively run Deer Management Groups (Phillips et al. 2010). The series of independent (from European founders) and compounded bottlenecks from translocated populations within the United Kingdom, together with the overall impact of the medieval bottleneck, could be expected to have reduced genetic diversity. Depending on factors associated with dispersal behavior and population dynamics, as indicated above, more or less isolation and structure may have evolved. One previous study examined roe deer population genetics in the United Kingdom using allozyme markers, and showed that roe deer exhibited polymorphism at only one locus, consistent with the expectation of reduced diversity. The one polymorphic locus indicated evidence of an east/west cline in southern populations, which was described as consistent with the reintroduction records of roe deer (Hewison 1995). However, the resolution of that study was low, given the small number of markers applied, and their relatively low diversity levels.
|Site of introduction||Date||Site of origin||Number released|
|Milton Abbas, Dorset||1800||Perth, Scotland||4|
|Windsor Great Park, Berks||1825||Dorset||4|
|Windsor Great Park, Berks||1850||Petworth|
|Epping Forest, Essex||1883||Dorset||6|
|Epping Forest, Essex||1884||Unknown||8|
|Thetford, Norfolk||1884||Württemberg, Germany||12|
|Maybole, Strathyclyde, Scotland||1820||Unknown|
|Annandale, Dumfries, Scotland||1854||Unknown|
|Drumlanrig, Dumfries, Scotland||1860||Unknown|
In this study, we investigate roe deer populations sampled from areas in the northern and southern United Kigndom (see Table 2) using 16 polymorphic microsatellite DNA markers together with 744 bp sequence data from the mtDNA control region. These areas were chosen as to best represent the differing population histories (as described above). We test the hypotheses that admixture among native and introduced populations will have resulted in recovered diversity following the historical bottleneck, and minimized population structure among rapidly expanding populations over the restricted geographic range represented by mainland Britain. These are realistic expectations, given the known histories and rate of expansion. The very different reality of high levels of structure and insularity may depend on strong philopatric behavior and the way in which natural populations are founded. We consider the results in the context of how the contemporary processes of philopatry and dispersal have interacted with historical processes to generate the current pattern of differentiation.
|Region||Area||Mitochondrial samples (n)||Microsatellite samples (n)|