Assignment tests

This method can be used to investigate which individuals migrated into a population and from which population they originated from. Assignment is made based on the genetic similarity of individuals. If certain individuals are genetically different from all other individuals in a population, they are considered to be immigrants. Their origin is determined based on their genetic similarity to one of the other populations being studied. In this way, the actual dispersal distances can be recorded almost in real time.

Example: stepping stone ponds for European tree frogs in the Reuss River Valley

European tree frogs (Hyla arborea) are an endangered species in Switzerland. In many places there is a lack of well-connected spawning grounds to ensure the exchange of individuals and therefore of genes among populations. In the long term, this exchange is key to the survival of tree frogs because it maintains genetic diversity and averts inbreeding. Foto: Sonia Angelone

In recent decades, many new ponds have been created as “stepping-stone habitats” to assist European tree frog dispersal (e.g. along the Reuss River Valley in the canton of Aargau). However, these measures will only be successful if tree frogs reach and colonise the new ponds. It is therefore important to know the dispersal potential and possible barrier effects of intensively used landscapes, so that new ponds are planned in a way that they connect existing spawning grounds. In the Reuss River Valley, WSL investigated how fast European tree frogs settle in new ponds and where the frogs have migrated from.

The study found that in most cases new ponds were colonised within a year and that tree frogs can migrate up to 5 km across intensively used landscape. This distance is greater than previously assumed. The data show that the conservation measures implemented in the Reuss River Valley so far have been successful and can serve as a model for measures to promote European tree frogs in other parts of Switzerland.

Map of the Reuss River Valley below Bremgarten in the canton of Aargau, showing where European tree frogs migrated from to new stepping stone ponds and how far they travelled when migrating. Stars correspond to existing ponds and grey circles indicate new ponds. Arrows show the direction in which tree frogs migrated.