Alien species in the driving seat

Posted on July 25, 2012

Two of the most commonly cited threats to native aquatic species are the loss of habitat quality and invasions by alien species.  However, in situations where both of these potential threats coexist, it’s difficult to establish the relative importance of invasives and habitat loss.   This problem can block the development of effective strategies to conserve native species – for example, if  invasives do well in degraded habitats but don’t affect native species, then attempts to eradicate invasives can be a waste of time and money.  In heavily impacted regions such as the Mediterranean, where almost 70% of endemic freshwater fish species are threatened by extinction or are already extinct, there’s a pressing need for a better understanding of the causes of native decline.  At 170 locations on the Guadiana River in Spain and Portugal , electrofishing was used to collect fish abundance data, and the environment at each site was described by reference to 33 variables.  A total of 14 native and 12 invasive fish species were caught.  A variety of multivariate analyses were used to compare alternative models of the impacts of invasives and habitat change on native species.  The most satisfactory causal models were those where invasives were drivers, that is, where either (a) habitat change affects invasives, which affect natives, or (b) habitat change affects both invasives and natives, and invasives affect natives.  There was no support for the alternative possibility that invasives were “passengers” , i.e., that habitat change affected both invasives and natives but invasives didn’t affect natives.  The study found that the abundance of invasive species was highest in the vicinity of reservoirs.  Conservation recommendations emerging from this work included the eradication or control of invasives at key times of year, limiting the dispersal of invasives from reservoirs, and restoring natural flow regimes to favour native species.

Reference:  Hermoso, V., Clavero, M., Blanco-Garrido, F. & Prenda, J.  2011.  Invasive species and habitat degradation in Iberian streams: an analysis of their role in freshwater fish diversity loss.   Ecological Applications 21(1), 175–188.