Over boomkikkers en mensen. Deel 3: (her)introducties

About tree frogs and man. Part 3: Introductions and re-introductions. In the previous article on the historical relationships between humans and tree frogs, it was suggested that the presence of populations southwest of the rivers Rhine and Meuse may be the result of introductions by humans. In this third and last article, this option is explored. Whether tree frogs were more widely distributed during warmer periods is not clear from late medieval and early modern sources. The supposition that tree frogs take advantage of warmer climatic conditions was derived from the historical relationship between rising temperatures and lower ages of maturation of tree frogs since the 18th century. Written sources from the 19th and 20th centuries indicate that there was a lively international trade in tree frogs, and that they often escaped or were released from captivity. In the 20th century, tree frogs were introduced into several nature reserves in the Netherlands in the context of ecological restoration; whether these should be regarded as introductions or as re-introductions is not clear. Thus, it also remains unclear to what extent existing populations can be regarded as ‘natural’. Documented attempts to introduce, or possibly re-introduce, tree frogs, particularly south of the large rivers, leave ample room for doubt about the authenticity of other tree frog populations; most introduction efforts were never properly recorded. Besides the creation of habitat by opening up landscape for agricultural purposes, introductions may have been an important additional factor of human origin in extending the tree frog’s distribution area in Northwestern Europe.