An otter at the mouth of a river Francisco Valverde
The otter was an endangered species in the Valencian community in the early 80’s. His fate seemed irreversible. However, since then the population of this aquatic marten has multiplied and its presence was discovered in unexpected places a century ago. Today it is an endangered animal but not critically endangered. The improvement in the quality of rivers through wastewater treatment processes from the 1990s and the adoption of protectionist policies are two reasons that usually explain this increase. Now another has been added that could be crucial: the spread of an invasive species, the American crab, or red crab, which research into its eating habits has shown has become the main diet of otters.
Analysis of more than a hundred otter deposits conducted since 2018 confirms that 75% of their diet comes from this invasive crab, explains Sales Tomás, Manager of the Limne Foundation, who prepared the study in collaboration with the Valencia Water Foundation. “The growing role of the invasive American crayfish in the endangered mammal’s diet suggests that changes in its diet may have had a significant impact on the species’ recovery in recent years,” says Tomás.
Gonzalo Juan, land warden who found the first otter tracks at the Mijares estuary, and Sales Tomás, environmentalist and director of the Limne Foundation, in a July 2020 picture. Mònica Torres
Introduced in the 1970s, this invasive crab significantly decimated the native crab population, which has recovered somewhat over the years as it transmits a disease that does not affect American crustaceans. But on the other hand, the ecosystem has taken on its presence and predators have incorporated it into their diet, giving life. “The ecosystem learns; Nor should we be apocalyptic with the so-called invasive or exotic species, explains Juan Jiménez, an otter researcher since the 1980s and a biologist from the Ministry of Agriculture, Rural Development, Climate Emergency and Ecological Transition, who was also involved in the research.
The case described would be a good example of how the introduction of an invasive species causes significant imbalances and can almost always negatively alter already existing ecosystems, although positive effects are described for the otter and in this particular case, emphasizes Adrián Lacomba and David Campos-Such , authors of the study presented last Friday at the headquarters of the Valencia Agencies.
So, an invasive species has been instrumental in saving from extinction, at least in the Valencian community. Jiménez adds that the change in diet has also been detected in the faeces in Castile-La Mancha. In any case, this research aims to open the door to new studies that can confirm or relativize this working hypothesis. Its authors claim that “the wide distribution of the American crab, Procambarus clarkii, appears to be one of the factors which, along with the improvement of rivers, has led to the increase in otter numbers.”
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Additionally, because the red crab doesn’t have too many environmental requirements, it’s also found in degraded rivers and unstable ecosystems, which “considerably expands,” a factor more than, the areas through which the otter can expand its territory and create new tanks its expansion
After the crab, the otters feed on local fish, barbel and, anecdotally, reptiles, says Sales Tomás, who exhibited at the presentation a plastic wrapper containing a fecal sample that gave off a strong “shellfish smell.”
This American crab resists very well the discontinuous, dry and torrential nature that characterizes the Valencian rivers of the Mediterranean coast. “They band together, they hide in the sand, and they last a long time,” says Jiménez. Thanks to this resistance, the otters find food and their population is increasing, although due to the difficulty of this type of calculation there is no estimate of their numbers, as it is a very elusive animal, experts say. If, based on the appearance and analysis of his excrement, there is evidence of his undoubted presence in many places where there was no evidence before.
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