Influence of distance and landscape composition on the dispersal of water frogs
Nagel, D., 2013: Influence of distance and landscape composition on the dispersal of water frogs. Master thesis. 34 p.
Masterarbeit der ETH Zürich, geleitet von Felix Gugerli, WSL Birmensdorf, und Janine Bolliger, WSL Birmensdorf
The recent loss in amphibian biodiversity calls for effective conservation measures. Remaining populations need to stay connected to prevent further local extinctions. However, species-specific knowledge about what is hindering and what is enhancing the maintenance of gene flow between populations is often lacking. Therefore, this study identified how distance and different landscape classes between ponds affect the successful dispersal of water frogs (P. lessonae - P. esculentus system). The study was carried out in 19 ponds near Aargau in Switzerland.
Firstly, genetic distance between ponds, calculated with microsatellite data of 13 markers from 396 frogs, was analyzed to check whether isolation by distance (IBD) applied. As estimates for genetic distance, Fst, Fst/(1-Fst), and Nei’s distance were used. Estimated Fst values ranged between -0.01 and 0.57 in the study area. A simple Mantel test showed that IBD is significantly contributing to genetic distance in water frogs. Secondly, the influence of landscape composition between locations was investigated with a corridor analysis using permuted multiple regression on distance matrices (MRM). MRM was carried out with two different measures for genetic diffetentiation (Fst/(1-Fst), and Nei’s distance), two different subjective landscape classifications, and three different corridor types (200 m - 1’000 m, 300 m, 400 m - 2’000 m). This resulted in twelve full models containing the regression coefficients of the landscape classes and the straight-line distance. In addition, backward elimination of variables was applied to obtain reduced models containing only significant predictive variables. The full models showed that open land, particularly agricultural areas, tended to enhance gene flow in contrast to most other landscape classes. In the reduced models, Euclidean distance together with waters and wetlands exerted a significant, negative effect on gene flow. Thereby, variable corridor widths depending on the distance between locations provided the best model fits. A possible explanation for waters and wetlands hindering gene flow is that ponds attract dispersing individuals and make longer travel routes unnecessary. The positive effect of open land on gene flow could originate from the enhanced mobility of frogs in such a landscape.
To counteract further amphibian decline, I suggest to constantly improving conservation measures like for example the creation of stepping-stone habitats between ponds. Thereby, the results of studies such as this one can be used to find an adequate definition of a real stepping-stone for different amphibians and make the measure thus more efficient. Additionally, this study identified options to further develop studies applying corridor analyses to investigate landscape effects on gene flow. I recommend using variable corridor widths depending on the distance between locations instead of fixed corridor widths. Additionally, comparing several landscape classifications may reduce subjectivity in the analysis.