Destructive invaders: Calico crayfish threaten species diversity | DW Documentary

Key Concepts

• Calico Crayfish (Orconectes limosus): A non-native, invasive species in Germany known for its voracious appetite and rapid reproduction.
• Invasive Alien Species (IAS): Species that have been introduced outside their natural past or present distribution and whose introduction and/or spread threatens biological diversity.
• Biodiversity: The variety of life in the world or in a particular habitat or ecosystem.
• Biotope: A type of habitat, such as a pond or wetland, characterized by its specific flora and fauna.
• R-strategy: A reproductive strategy characterized by producing a large number of offspring, with little parental care, to ensure survival.
• Bait Bucket Introduction: The act of releasing live bait, such as crayfish, into a water body after fishing, leading to the introduction of invasive species.
• Crayfish Plague: A disease caused by an oomycete pathogen (Aphanomyces astaci) that is highly lethal to native European crayfish but often not to invasive species like the calico crayfish.
• Biocides: Chemical agents used to control or kill pests.
• Ecological Niche: The role and position a species has in its environment; how it meets its needs for food and shelter, how it survives, and how it reproduces.

Calico Crayfish Invasion in Germany: A Threat to Biodiversity

The calico crayfish (Orconectes limosus), a non-native species originating from North America, poses a significant threat to local wildlife and biodiversity in Germany due to its aggressive nature and exceptionally rapid reproduction rate. This invasive species has established dense populations, with tens of thousands found in single small ponds, leading to severe ecological disruption.

Ecological Impact and Voracious Appetite

Calico crayfish are omnivores, consuming a wide range of food sources including aquatic plants, insect larvae, and tadpoles. Their ability to survive in various water conditions and their voracious appetite mean they deplete local food sources. In some instances, after exhausting all other food, they resort to cannibalism, leaving behind "murky swamps" devoid of life. This behavior has been observed in small ponds near Rheinstetten, where a population of approximately 60,000 crayfish in late 2017 led to the complete devastation of the pond's ecosystem.

Rapid Reproduction and Competitive Advantage

A key factor in the calico crayfish's success is its accelerated reproductive cycle. They mature within three to four months of hatching, significantly faster than any native European crayfish species. A single female can carry up to 500 eggs. This reproductive advantage allows them to outcompete native species. Biologist Christoph Chucholl notes that native crayfish species no longer exist in the Upper Rhine floodplains, largely due to the calico crayfish's dominance.

Spread and Habitat Invasion

Calico crayfish are capable of spreading over land, enabling them to invade entire biotopes. They are making headlines in Germany with descriptions like "killing machine" and "catastrophe." Their spread is primarily along the Rhine river, extending northwards into North Rhine-Westphalia, and other European regions are also at risk. The species is not yet on the EU's list of invasive alien species, a status researchers believe is urgently needed.

Devastation of Conservation Projects and Native Species

The invasive nature of the calico crayfish has had a detrimental impact on conservation efforts aimed at reintroducing native species. A German-French research team is working to reintroduce the European pond turtle, considered extinct in the Rhine region. However, the crayfish constantly interfere with these projects, destroying habitats and consuming resources. The team reports spending more time dealing with invasive species than with native turtles. A pond created specifically for turtles, which was once a clear body of water with abundant plant and insect life, has been transformed into a murky swamp by the calico crayfish. This project, costing over a million euros, exemplifies how invasive species can undermine conservation initiatives.

Threat to Amphibians and Dragonflies

Amphibians are particularly vulnerable to the calico crayfish. Conservationist Paul Thomas highlights that their sanctuaries along the Rhine river are in danger. The crayfish devour amphibian eggs and tadpoles, leading to a potential decline to zero populations. In one observed biotope, hundreds of frogs were once found at night; now, only one remains, with the calico crayfish actively present. The crayfish's exoskeleton, shed during growth, is also a visible sign of their presence along the banks.

Dragonfly populations are also suffering. Biologist Jürgen Ott, who established a dragonfly oasis in Trippstadt, notes a significant decline in dragonfly populations in areas where calico crayfish have arrived, such as Kandel. The crayfish consume dragonfly eggs and larvae, depleting the local food chain and impacting species that prey on dragonflies, including birds, spiders, reptiles, and amphibians.

The Role of Crayfish Plague

The displacement of native crayfish species is exacerbated by the crayfish plague, a disease transmitted by invasive species like the calico crayfish. While the calico crayfish are often immune to the plague, it is almost always fatal to native European crayfish, typically within one to two weeks. This fungus-like pathogen paralyzes the afflicted animals and can cause limb loss.

Origin and Introduction into Germany

Calico crayfish originate from North America, where they employ an "r-strategy" of reproduction to ensure offspring survival amidst competition and predators. In Germany, they find a larger ecological niche with fewer natural competitors and predators. Their introduction to Germany is believed to have started around an artificial lake near Karlsruhe's international airport, possibly through the release of bait crayfish by Canadian soldiers stationed there until 1993. The practice of "bait bucket introduction," where unused bait crayfish are released into water bodies, is considered a more likely scenario for their initial spread.

Current Distribution and Human Introduction

Calico crayfish now inhabit approximately 80% of the floodplains along the Upper Rhine, with densities reaching up to 10 crayfish per square meter in some areas. They have spread northwards along the Rhine, with sightings reported in Wiesbaden, Düsseldorf, the Black Forest, and near Stuttgart. These further introductions are likely human-mediated, as the crayfish could not have migrated such distances independently.

Management and Control Strategies

1. Consumption: Fisherman Gilbert Zwick advocates for consuming calico crayfish, viewing them as a food source rather than just a pest. He has developed a taste for them and incorporates them into his diet. However, the scale of the problem suggests that widespread consumption alone is insufficient for eradication.

2. Biocides: In theory, biocides can be used to target invasive crayfish. Sweden, Norway, and the UK have models where biocides like pyrethrum have been tested on invasive signal crayfish. These treatments can be effective in eradicating crayfish in small test sites, but they also pose risks to other aquatic life, including fish, eggs, and larvae. Researchers consider biocides a "chemical emergency brake" that requires complete eradication of the target species. Along the Rhine, however, it is considered too late for biocides due to the widespread presence of calico crayfish.

3. Localized Protection Measures: On a local level, it is possible to prevent crayfish from invading and protect amphibian habitats. In Rheinstetten, a research team has implemented several measures: * Manual Removal: Fishing out tens of thousands of crayfish to protect endangered species. * Physical Barriers: Piling up gravel to prevent burrowing and collapsing tunnels, and encircling ponds with a 30-centimeter-high ring of tree trunks to act as an insurmountable obstacle. * Habitat Restoration: Ensuring that new conservation habitats are shielded from calico crayfish from the outset.

These localized efforts have shown success, with a pond in Rheinstetten, once overrun by crayfish, now clear and teeming with diverse life, including tree frogs and emperor dragonfly larvae. The Karlsruhe research team is collaborating with NABU to convert further biotopes using these methods.

Conclusion and Future Outlook

The consensus is that complete extermination of the calico crayfish in the Upper Rhine floodplains is no longer feasible; "that ship has sailed." However, containment and localized protection remain crucial. Broad, active measures are necessary to prevent further spread throughout Germany. The long-term consequences for the native underwater world are still unknown. The continued introduction of new crayfish, often from aquariums, highlights the critical role of human behavior in determining the extent of destruction caused by invasive species in unprepared ecosystems. The calico crayfish was never meant to be in Germany, and its presence underscores the need for greater awareness and responsible actions to protect native biodiversity.