Combining landscape genetics, radio-tracking and long-term monitoring to derive management implications for Natterjack toads in agricultural landscapes
Manuel Frei, WSL Swiss Federal Research Institute and ETH Zurich
The Natterjack toad (Epidalea calamita) is a pond-breeding amphibian of open terrains with sparse vegetation. Through the loss of pioneer habitats such as floodplains or extensively managed gravel pits, but also through the intensification of agriculture, the Natterjack toad has become a highly threatened species. In Switzerland, more than 60 % of the known Natterjack-toad occurrences have disappeared in the past 30 years.
Understanding the influence of landscape properties and structure on movement and connectivity of potentially isolated populations is essential for successful conservation management, especially in intensively managed agricultural landscapes. This study assessed the movement, genetic structure and population dynamics of the Natterjack toad in an agricultural landscape of the Suhre valley. Analyses were done by using genetic data derived from microsatellites, radio-tracking data (Schweizer 2013) and census data of population sizes collected within a long-term amphibian monitoring program. In the Suhre valley, situated on the Central Plateau of Switzerland, exist about 20 breeding ponds of the Natterjack toad. Some of them are – surprisingly – located in intensively managed agricultural areas. Are these populations viable or are agricultural areas «ecological traps», i.e. attract toads which then die as a consequence of unsuitable habitat? In other words: should managers invest in extended construction of breeding ponds in agricultural areas to sustain a threatened species?
In total, 19 migration events between populations were detected. The assessment of first-generation migrants from assignment tests uncovered eleven migrants, six migrants could be detected through the unintentional repeated genetic sampling of some individuals and two by radio-tracking. At least five migrants crossed the canalised Suhre river, and seven migration events were observed to cross main roads. So, neither the canalised Suhre river nor main roads seem to be strong barriers for migrating Natterjack toads. The mean distance covered by the 19 migration events was 1500 m with a minimum of 280 m and a maximum of 4200 m.
Accordingly, none of the toad breeding sites was genetically isolated, although there was a trend that built-up areas and roads fostered genetic differentiation in addition to geographical distance. A slight genetic gradient from North to South indicates, that Natterjack toads migrate from the two largest populations in the North and South to smaller breeding ponds in the agriculture area. Analysing the toads as a spatial network supported the inferences that the two largest populations were sources, suggesting that population size may drive the movement pattern of the toad.
The results suggest that the Natterjack toad is well established in the intensely managed agricultural landscape of the Suhre valley. Following conclusions for management are drawn:
- Natterjack toads are able to build viable populations in agricultural landscapes and it is not likely that the investigated agricultural landscape is an ecological trap for Natterjack toads, at least not in situations with abundant ephemeral ponds.
- Conservation priority should also be given to currently large and well-established source populations as the investigated toad network is presumably driven by two populations in the North and South of the study area.