But how are these unique developments to be explained? “Lake Constance is a complex, dynamic system,” says Hudson, citing rapid changes in nutrient inputs, climate change and the many new, sometimes invasive species, such as ruffe or zebra and quagga mussels. Ecosystems are, however, also exposed to similar pressures in lakes where sticklebacks have not become invasive.
In a review published in Frontiers in Ecology and Evolution (open access), Hudson and his co‑authors from Eawag and the Universities of Bern and Basel attribute the invasiveness of the stickleback in Lake Constance to genetic factors, providing historical, morphological and ecological as well as genomic data to support their explanation.
The Baltic connection
Worldwide, over millions of years, marine sticklebacks have repeatedly colonised freshwater lakes and rivers. Populations in the Rhine and other European rivers are relatively young, resulting from migrations which only occurred after the last ice age. But sticklebacks in Swiss lakes are the descendants of aquarium fish, which were frequently released into the wild in the nineteenth century, often deriving from originally widely separated populations.
Established in Lake Geneva and the lakes of the Jura foothills were thus, in particular, sticklebacks from the Rhône. In Eastern Switzerland and Southern Germany, sticklebacks were documented first in tributaries of Lake Constance and later also in the lake itself. Here, during the period of eutrophication from the 1960s onwards, population explosions were observed, but also rapid collapses.
As demonstrated by Eawag studies published in 2019, the genetic material of Lake Constance sticklebacks derives from three lineages: from the Rhine, from the Rhône and – unlike elsewhere in Switzerland – predominantly from an East European lineage which, from the Baltic Sea, only colonised Polish and Baltic rivers after the last ice age.