Wetlands cover less than one percent of Canton Zurich – around 1850 the area was ten times as large. The reason for this decrease is that many wetlands have since been drained. They are now used for agricultural purposes or have been built over. When this happens, it is clear that the plant species adapted to wetlands disappear. Several wetland-specialised species in the Canton have already become extinct, and numerous other species have become much rarer.
In her master’s thesis, the environmental scientist Anine Jamin investigated whether more plant species can be expected to disappear. She compared the number of species in wetlands that have lost less than half their area since 1850 with those in wetlands that have greatly shrunk. She found that there are currently more plant species in severely shrunken wetlands than you would expect on the basis of their small size. However, this is only seemingly a good sign: the observed surplus of these species indicates an ‘extinction debt’. The biologist, Ariel Bergamini, who supervised the master’s thesis, explains: “When the habitat shrinks and becomes fragmented, many plants survive for a while. But the populations in the remaining isolated patches are small. Over time, such species are lost because they can no longer, for example, reproduce successfully.”
Opportunity for nature conservation
The consequence for the Zurich wetlands is that, even if the wetland area no longer decreases further, more plants will die out until the number of species is in line with the size of the area. To ensure these plants can be preserved, Ariel and Anine call for urgent measures to promote the wetlands and their flora: the existing network of wetlands should be supplemented and expanded by rewetting parts of wetlands that have been drained. To improve the quality of the remaining wetlands, existing drainage ditches must be closed and sufficient buffer zones set up. As paradoxical as this may sound, Ariel also sees extinction debt as an opportunity for nature conservation: “The species are still present locally and can be saved.”
(Birgit Ottmer, Diagonal 2/19)