Flat hard disks are strictly protected in accordance with the European Habitats Directive, in both Natura 2000 areas and beyond. The sticky pond snail is very rare and listed as critically endangered on the Dutch Red List. Both live in lush underwater vegetation and are very sensitive to declining water quality. They are an indicator of quality par excellence. The sticky pond snail in particular is a very critical species.
Due to its protected status, the occurrence of flat disc hearing is inventoried (observed) on a regular basis. The sticky pond snail is based on this investigation. In the inventories, the composition of underwater vegetation is also analyzed and observed. It became clear that these two types of rare snails had fallen sharply in different areas of research. Significantly, especially in places where high densities of exotic crayfish were observed.
flat disc horn
Since 2004, ANEMOON has been carrying out inventories on a national scale in a six-year cycle at fixed observation sites. This is done within the framework of the Environmental Monitoring Network (NEM). In addition, additional research was carried out using the same systematic method on behalf of the counties of South Holland and Overijssel in the Natura 2000 regions of Nieuwkoopse Plassen, De Wieden, Weerribben and Olde Maten & Veerslootslanden. The numbers observed in the last period were compared with the results of research from six to ten years ago. This comparison showed that the Platt disc horns at Nieuwkoopse Plassen and De Wieden have sharply decreased in number. Crayfish are now quite a lot in these areas. In Weerribben and Olde Maten, on the other hand, the situation regarding the occurrence of flat disc antlers is stable. Few lobsters (so far) have been found in these last two areas in 2019. But that might just be a matter of time.
The national trend will be calculated by Statistics Netherlands in 2024 and will be published in the Habitat Report to the European Union.
snail sticky pool
Due to the strong index value of clean freshwater habitats with exceptionally high natural values, pond snails are consistently included in the observational study of the protected disc horn. As a result, it can be shown that this species was severely reduced in De Wieden and Nieuwkoopse Plassen. At Nieuwkoopse Plassen, the animals are in danger of disappearing completely. In this Natura 2000 area, it was found only in one of eight ancient sites. Only in Weerribben the species declined less severely. De Wieden and Nieuwkoopse Plassen formed primary core areas. (No sticky pool snails were found at Olde Maten & Veerslootslanden in both searches.)
Reducing snails and plants side by side
In the Natura 2000 areas examined, the receding of the Platt snail and the sticky pond snail is mainly associated with places where underwater vegetation has also declined. This concerns not only the decrease in the density of vegetation, but also the number of species of underwater vegetation. The average number of plant species also decreased sharply in a large part of the sites studied. For example, in different subregions, the vegetation of crab has largely or completely disappeared. The decline in the Natura 2000 areas examined is not limited to these two particular species. Decreasing vegetation, in terms of species richness and surface area, turns out to be bad for almost all species of aquatic snails. Only species that also thrive in vegetation-poor waters, such as the odd pointy antler and the native flat-tailed fish, increased. The loss of vegetation in different types of water in the Natura 2000 areas is particularly worrisome because it is precisely there that a great deal of biodiversity occurs and they constitute source areas for unprotected areas.
Strange lobsters as culprits
The decline in vegetation is almost certainly due mainly to the strong emergence of exotic vegetation-eating lobsters. Research in several European countries has shown that when crayfish are introduced, whole plants are consumed underwater. Areas of research mainly concern the highly opportunistic American red lobster. In addition to plants, lobsters also eat other living things, including mollusks whose shells they crack with their claws.
The effect of lobsters depends on the management
The study results indicate that the effect of crayfish is strongly influenced by the type of administration. What is remarkable is that in trenches that are only partially (in stages) cleaned every now and then, the submerged plants remain healthy enough, even if lobsters live there. Plant growth keeps up with the lobsters’ appetite as it were. But when the trenches are thoroughly cleaned and lots of vegetation removed, there is much less food for the lobsters, and the plant residues will undoubtedly be eaten. Subsequently, submerged plants can no longer recover, often resulting in only floating leaf plants such as white water lily and yellow lily remaining.
Set cleaning management
American Red Lobster fishing won’t be enough to stop this growing problem for Dutch underwater flora and fauna. Their reproduction is very successful and the population will continue to recover at lightning speed. Now that lobsters live en masse in many Dutch waters and are likely to grow much more, adjusting the management of cleaning trenches and other lobster-rich waters remains a solution. Only when more and more permanently rich vegetation-rich trenches are built again might groups of flat disc horn, pond snails and many other aquatic animals be able to recover.
The results of the research have now been published in the Nieuwkoopse Plassen district. The results regarding the Natura 2000 areas examined in Overijssel will be published during this year.
Text: Arno Boesveld and Adriaan Gmelig Meyling, both Stichting ANEMOON
Photos: Arno Boesveld (main photo: American red lobster under a small floating elevator in Wieden); Adrian Jamilig Melling