© Marcel Burkhardt
A genetic study on the connectivity of woodpecker populations
Woodpeckers are generally considered to be poor dispersers. This appears to apply to the Middle Spotted Woodpecker as well, a species restricted to old oak-rich forests. Using genetic methods, we examined whether populations of this extreme habitat specialist are isolated and compared the findings with the patterns observed in the widespread Great Spotted Woodpecker.
Owing to the ongoing habitat destruction, many plant and animal species show a fragmented distribution. Whether individuals still move between the often small, local populations persisting in the fragments is unknown for many species. However, knowledge of the connectivity of local populations is important for the protection and promotion of endangered species. Several studies indicate that genetic diversity in small local populations is reduced compared to larger populations. Low genetic diversity may compromise the ability of species to adapt to changing environmental conditions and may also increase the risk of mating among close relatives and inbreeding depression. These mechanisms can affect the long-term persistence of small populations.
The Middle Spotted Woodpecker is one of 50 priority species of the Swiss species recovery programme for birds, which is carried out by the Swiss Ornithological Institute, BirdLife Switzerland and the Federal Office for the Environment FOEN. According to the national recovery plan for the Middle Spotted Woodpecker (in German or French, published in 2008), the dispersal capacities and hence the connectivity of populations need to be studied in detail. Our project focused on these knowledge gaps by studying the population genetics of the Middle Spotted Woodpecker and the Great Spotted Woodpecker. We hoped to lay the foundation for the effective promotion of the Middle Spotted Woodpecker, for example with respect to an appropriate choice of locations for oak plantations.
The project aimed to
- quantify the dispersal capacities of Middle and Great Spotted Woodpecker and thus the connectivity of woodpecker populations.
- assess the genetic diversity of Middle and Great Spotted Woodpecker populations.
- close knowledge gaps according to the recovery plan for the Middle Spotted Woodpecker.
- gain insights that will inform the establishment of oak plantations.
The study was conducted in those cantons that hosted the largest populations of Middle Spotted Woodpeckers in Switzerland. When the study began, these were the cantons Aargau, Basel-Landschaft, Neuchâtel, Schaffhausen, Thurgau, and Zurich. From 2009–2011, blood samples of both species were collected for genetic analyses in the strongholds of the Middle Spotted Woodpecker. These were supplemented with samples from Hessen (Germany), enlarging the study’s spatial scale.
Territories and breeding cavities were searched in April and May. A drop of blood was taken from two nestlings per brood for later genetic analyses. In total, between 14 and 19 broods of Middle Spotted Woodpecker and between 19 and 24 broods of Great Spotted Woodpecker were examined. In addition, we received feather samples from 16 Middle Spotted Woodpeckers and 21 Great Spotted Woodpeckers from Rolf Hennes in Bad Homburg, Germany.
The blood and feather samples were analysed in collaboration with Dr Glauco Camenisch and Prof. Lukas Keller from the Institute of Evolutionary Biology and Environmental Studies of the University of Zurich. Genetic diversity, genetic differentiation and connectivity of populations were quantified using 12 existing microsatellites developed for the Middle Spotted Woodpecker, six microsatellites for the White-backed Woodpecker, and seven newly developed microsatellites for the Great Spotted Woodpecker.
The Middle Spotted Woodpecker is red-listed in Switzerland and in many other European countries. This project aimed to improve our knowledge of aspects of the species' ecology that are of great relevance for the protection and promotion of this habitat specialist. Additionally, knowledge on the connectivity of Middle Spotted Woodpecker populations helps to determine which oak forests have the potential to be recolonised and hence should be managed in favour of the species, and where oak regenerations should be conducted to improve future connectivity. Protecting existing old oak forests and increasing connectivity among them is necessary for the long-term persistence of this highly specialised species in Switzerland.
Various measures of genetic diversity were smaller in the case of the specialised Middle Spotted Woodpecker when compared to the relatively undemanding Great Spotted Woodpecker. In addition, we found indications that the surveyed Middle Spotted Woodpecker populations, but not the Great Spotted Woodpecker populations, had experienced genetic bottlenecks fairly recently. Both findings can be explained by the significantly smaller population size of the Middle Spotted Woodpecker both in Switzerland overall and locally. In many species, small populations generally have a smaller genetic diversity than large populations.
The populations of both species were spatially structured, but in a different way. The Middle Spotted Woodpeckers from the seven populations were divided into different genetic groups: the first two groups consisted of individuals from Hessen and Neuchâtel, the third group of individuals from the cantons Schaffhausen, Thurgau and Zurich. Individuals from Aargau and Basel-Landschaft formed a fourth group, though this one was not as easy to delimit consistently. Overall, the genetic similarity decreased with increasing geographical distance between the populations. All of these findings suggest that the connectivity between populations is high when they are not too far apart. Still, Middle Spotted Woodpeckers are able to disperse across larger distances than hitherto assumed.
In the case of the Great Spotted Woodpecker, individuals from Aargau and Neuchâtel formed a genetic group each. Some but not all analyses supported a third genetic group consisting of Great Spotted Woodpeckers from the Canton of Zurich. Individuals from all the other populations could not be assigned to any of these groups nor to a separate one. Interestingly, genetic similarity neither decreased nor increased with growing geographical distance in the case of the Great Spotted Woodpecker. The results suggest that the Great Spotted Woodpecker has a strong capacity for dispersal, leading to the mixing of local populations at a large spatial scale. On the other hand, topographical features such as the Jura chains that rise up to 1500 m may affect contact between the population from Neuchâtel and the other populations included in the study, a hypothesis supported by the difference between individuals from Neuchâtel and Aargau, which is visible in the case of the Middle Spotted Woodpecker as well. Finally, ecological conditions could also be responsible for the separation of Great Spotted Woodpecker populations that are geographically close but that occupy different types of forest in Aargau (deciduous woods dominated by beech) and in Basel-Landschaft (oak forest).
Further information on the fieldwork can be found in an article of the magazine Schweizer Familie from 23 June 2011 (in German).
Lukas Keller & Glauco Camenisch, Institute of Evolutionary Biology and Environmental Studies, University of Zurich
Woodtli Baumpflege Ost AG
Basler Stiftung für biologische Forschung
Fachstelle Naturschutz Kanton Zürich
Hilfsfonds für die Schweizerische Vogelwarte Sempach
Natur- und Vogelschutz Möhlin
Swisslos-Fonds des Kantons Aargau
Swisslos-Fonds des Kantons Basel-Landschaft