The complex threats of exotic marine species

Ecosystems are changing worldwide as a result of the introduction of foreign species by human activities. In some cases, introduced species can become invasive, disrupting the ecosystem and even causing the extinction of some native species. Other introductions can be beneficial. This has ignited a contemporary debate around the overall impact of exotic species.

“We conducted this study to elucidate the magnitude and significance of the global effect of marine exotic species,” says ecologist Andrea Anton of King Abdullah University of Science and Technology in Saudi Arabia.

Anton and an international team of scientists collected data from more than 150 studies on 76 exotic marine species, representing six per cent of all listed exotics, and measured increasing or decreasing changes in ten ecological variables. 

Their analyses showed that introduced species caused an overall decrease in ecological responses that was modest in magnitude.

The majority of introduced species analysed (67 out of 76) did not cause significant changes to the established ecosystems. 

Analyses of species based on their position in the food chain showed that exotic predators caused the most disruptive impact, together with primary producers – species that live off inorganic sources of energy, such as light. 

Also, among 19 species that appeared in three or more studies, only two were consistently disruptive of the ecosystems they were introduced to: the European green crab Carcinus maenas and the seaweed Caulerpa cylindracea, a predator and a primary producer respectively. 

All in all, the team found that the introduction of foreign organisms caused changes in the behaviours of native species, such as altered mating and fear responses, declines in their abundance, and increased pollution by the heavy metals present in the tissues of the introduced, exotic species. Other ecological variables, including biodiversity, were not significantly affected.

The researchers say their findings could contribute towards meeting a UN target that aims to identify, and control or eradicate the worst invasive species by 2020.

“Distinguishing species that have minor effects from those with larger cascading impacts is critical for prioritizing management and prevention resources,” explains Belinda Gallardo of the University of Cambridge, UK, who was not involved in the study. 

“The impacts of invasive alien species in seas often go unnoticed by humans. Such rigorous integrated analyses are still rare, but highly needed for understanding and reducing the impacts of biological invasions,” adds Franz Essl of the University of Vienna, Austria, who was also not involved in the study.

Sign-up to receive our e-alert update every two weeks to keep up with everything new on the portal.