Paradoxically, a general environmental downside, such as the spread of invasive species, can bring certain benefits to some predators. This shift in the ecosystem has coincided with improvements in the otter population along Spanish rivers, in part due to its diet turning more toward American crayfish among increasingly exotic species in the country.
A study of otter feeding habits in the Valencian rivers shows the growing role of invasive American crayfish in the diet of this endangered mammal and notes that changes in its diet could influence the species’ recovery in recent years.
Aguas de Valencia recently presented the results of the report, following an agreement signed with the Limne Foundation in collaboration with the Climate Emergency Department that enables hundreds of volunteers to monitor otter populations in Valencia’s rivers and report project results.
Change in the feeding habits of the otter
The most recent studies, based on monitoring and analyzing the composition of this mammal’s pellets or feces, suggest that changes in otter feeding habits may have important implications for their survival.
The report by Adrián Lacomba and David Campos-Such opens the door to new research that could confirm or challenge this working hypothesis and its initial results. The authors argue that the widespread distribution of the American crayfish, procambarus clarkii, appears to be one of the factors contributing to the otter population’s increase. With rivers improving thanks to wastewater treatment upgrades that began in the 1990s, the authors note, this diversified diet may help otters expand their range.
However, the crayfish thrives in rivers with few environmental constraints, so it is found in degraded streams and unstable ecosystems. This broad presence could significantly expand the otter’s territory and open access to new basins.
Otter Expansion Areas
The described case shows that an invasive species can cause major imbalances and alter pre-existing ecosystems, yet a positive effect is observed for the otter in this particular situation.
Collaboration between the two foundations began in 2019 and enabled, among other actions, the collection and analysis of more than a hundred pellet samples, as well as the identification of new otter ranges in the Community of Valencia.
Notable improvements in water quality in Spanish rivers over the last three decades, largely due to widespread wastewater treatment plants, have supported a rapid rebound of otter populations (Lutra lutra), the only semi-aquatic mammal on the continent. After a near disappearance in the Iberian Peninsula during the 1970s, otter numbers have risen this century as riverbeds have cleaned up from pollution and as reservoir and hydraulic infrastructures have been managed more effectively.
As is often observed, not all outcomes are negative; another contributing factor to the otter recovery is the widespread presence of invasive species now found in 55% of Spanish river basins. Species such as carp, rainbow trout, and the American red crayfish have become additional food sources that help otters regulate native prey populations.
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