Kategorie: News
The bullfrog: A problem for the fauna of the Rhine floodplain landscape near Karlsruhe
Invasive (and also introduced) species are animal species that have established themselves with human influence in an area where they were not native before and have built up new populations there. Mostly, these species have arrived in foreign regions with the active help of humans. Sometimes, however, they have also escaped unintentionally from aquariums, terrariums or animal farms.
A good food supply and sometimes higher temperatures are ideal conditions to establish themselves in a foreign environment. The mink for example, originating from North America, was released from fur farms in Europe and the muskrat was introduced to Central Europe by fur traders.
More often, however, it happens that animal and plant species (neophytes) enter foreign territories by transport routes or through canals. 150 neozoa have been recorded in German waters to date, such as the Chinese mitten crab, which was introduced from Asia, or the basket clam, which has now made it as far as Lake Constance.
The introduced species can both enrich and threaten local biodiversity. For decades, alien species have been increasing worldwide. And not only that: the increases have even become stronger in recent years. This is gradually becoming a problem, because neozoa can also cause considerable damage. They displace native species or cause economic and health-related consequential costs.
One example is the rather unknown American bullfrog. The amphibian, which was introduced from North America, now poses quite a problem for the wildlife of the Rhine floodplain landscape near Karlsruhe.
A giant threatens the local wildlife
But why is the gigantic frog, which has a body size of about 60 cm and weighs about one kilo, such a big problem? And what is to be done now? "Exactly this question, I asked a biologist when I was able to be present at a tadpole collection action" explains 46-year-old Martin Hess from Böblingen. He has been dealing with this topic for more than 3 years and published a self-made documentary about the problem with these amphibians some time ago.
"The biologist then explained to me that bullfrogs have a very good metabolism and are not really picky when it comes to finding food. They threaten native amphibians and salamanders. But the biggest problem is: they have no natural enemies!" he explains.
To contain the problem and relieve population pressure, so-called collection campaigns are taking place in the Karlsruhe area. Here, the tadpoles are collected by divers before they transform into frogs. These actions are approved and always take place in the presence of a biologist.
"Bullfrogs feel very comfortable here with us.... That's why, they reproduce so rapidly. Up to 25,000 tadpoles hatch from a single spawning. That's what makes it particularly difficult for us to contain the problem" said Martin Hess.
"Right now, only five lakes here in the area are affected. If you didn't collect now, and we just know from Corona what it means when a population grows very quickly, you could have bullfrogs spreading across all the lakes and the species numbers virtually exploding."
Whether the problem with the unloved croaking migrants can now be eliminated for good in this way is rather unlikely.
"I personally also said it in my documentary, I don't think you can still get this problem solved with collection actions. However, we can try to slow down the population pressure with these actions. This way, nature can better adapt to it. We have to give nature the time to come to terms with it."
The "Rana catesbeiana" is among the largest frog species in the world. Probably for commercial interests, it was once brought to Europe from America and settled in northern Italy and western France.
Authorities suspect that a pet shop owner "dumped" the slippery glutton on a branch of the Rhine. That's how it got into the Baden area and started spreading. Now there is nothing left to do but to make sure that the spreading curve is slowed down.
Can we divers help?
"At the moment there is an appeal" explains Martin. "Divers who see bullfrogs should report them!" Sightings and reports from divers best via: neobiota.info/sb_Ochsenfrosch.
Own initiatives should not happen in any case to protect the environment. "The tadpoles are huge, but confusion still often happens. The collection events take place in lakes, reeds and in the shore area. If hordes were to run out now to collect the tadpoles, they would do more harm to the environment than help."
"Data is obtained during each collection event to help manage the problem. Every single animal is measured and documented. However, the most important thing is that the animals do not suffer in any way.... That is why a biologist is always present. By law, it is forbidden to catch amphibians just like that."
When Karlsruhe authorities first encountered bullfrogs in the late 1990s, initially only one small lake was affected. A short time later, bullfrog populations were detected in ten different bodies of water.
"What has been achieved so far is that the problem is still very local," he said. "Hannelore Brandt, the president of the Baden Diving Association said that in the first few years around two thousand specimens were collected. Now we are at twenty thousand per year. As far as we know so far, the problem only exists in the Karlsruhe area."
"It is a success for the official initiative that no other areas have been affected so far!" said Martin Hess.