Unexpected ecological impacts of alien species

Colossal Aggregations of Giant Alien Freshwater Fish as a Potential Biogeochemical Hotspot
Stéphanie Boulêtreau, Julien Cucherousset, Sébastien Villéger, Rémi Masson, Frédéric Santoul.
 PLoS ONE, 2011; 6 (10): e25732

Here is a very fascinating paper on the unexpected spin-offs from alien invasion. The researchers report consistent and previously undocumented occurences of aggregations of a giant alien freshwater fish, the Wels catfish (Silurus glanis) in Rhone river and its unexpected functional consequences in recipient ecosystems.

Evidence gathered by the researchers suggests that the mechanism responsible for the observed aggregations were not associated with schooling behavior, reproduction, and foraging or anti-predator behavior. Individuals were active, always swimming, but were not all pointing in the same direction as observed in polarized shoals. No synchronous movements were observed. In the case of schooling fish they maintain a minimum distance between conspecifics. Here the individuals were swimming while rubbing against each other. No mating behaviours were observed and groups occurred throughout the year at temperatures below the spawning threshold. The researchers contend that foraging behaviour was unlikely since no prey was captured and no foraging behaviour was displayed. All individuals were large enough to be released from any predation risk.

The fascinating observation is that defecation and excretion from dense aggregations of fish that rest over coral reefs provide important quantity of nitrogen and phosphorus that subsequently increase the growth rate of corals. The researchers say in some cases fish can translocate nutrients within the ecosystem by feeding in one location while defecating in another.  Heterogenous spatial distribution of fish can also create biogeochemical hotspots, i.e. places where nutrient release by animals exceeds the need of primary producers. The aggregations of alien fish studied by the researchers potentially represent the highest biogeochemical hotspots ever reported for freshwater ecosystems.

The researchers sign off like this "our study is unique in identifying unexpected ecological impacts of alien species. Our findings will be ground breaking news for many scientific fields including conservation biology, ecosystem ecology and behavioral ecology and anyone interested in biological invasion and the potential ecological impacts of alien species. Therefore, we believe that our manuscript will stimulate further research and discussion in these fields."