Space use of natterjack toads in agricultural areas
Esther Schweizer, Zürcher Hochschule für Angewandte Wissenschaften zhaw
Sand and gravel pits have been the typical secondary habitats for natterjack toads (Epidalea calamita). However, these habitats have become less suitable due to increased sand and gravel production. On the other hand, the endangered species has been observed with increasing frequency reproducing in seasonal ponds or wetlands in agricultural areas. For that reason the Canton of Aargau funded a study to find out if natterjack toads use agricultural areas also as summer and winter habitats. Another goal of the study was to capture the location and extent of the toads’ home ranges as well as to identify the types of habitats and shelters the toads occupy during the different seasons.
A telemetry-study was conducted in the upper valley of the Suhre in the Canton of Aargau, an area dominated by intensive agriculture on clayey soil. A sample of 50 adult male natterjack toads was tagged with external radio transmitters. The location of the toads was checked and recorded, initially daily and then several times a week. A survey of the agricultural area was conducted to identify the available habitats for the toads with regards to type of soil cultivation, vegetation, and their spatial relation.
The study showed that adult natterjack toads indeed use agricultural areas all year round. The summer- and winter habitats were far away from the place of reproduction, at least for the observed population. Their home ranges were identified as areas where the cultivation fields were rather small or formed long stripes. The toads preferred grassland and meadows in spring and fall, yet in summer they tended to prefer ploughed fields. They preferred wheat or corn fields early in the summer and later vegetable- and potato fields. Preferred structures for shelter were mouse holes, crevasses or even small depressions in the soil, tufts of grass or other patchy vegetation. Some toads were found dug in up to 30 cm deep in the soft soil of vegetable or potato fields on hot summer days. For hibernation they chose sunny slopes with deep mouse holes. No casualties or injuries due to agricultural measures have been observed.
Implication for natterjack toad conservation management
The collected data allows the conclusion that agricultural areas are suitable as summer and winter habitat for adult natterjack toads. The species prefers areas with small field sizes and different cultivation fields in close proximity. They avoid entering fields wider than twenty meters. Therefore artificial wetlands introduced as conservation measures should be placed at the convergence of different cultivation fields offering various habitat types for summer residence with nearby sunny slopes where toads can find frost-free soil for hibernation.