When an invader may look beneficial
We all know about negative impacts caused by invasive alien species, but in recent times studies and observations have raised our attention on predation between invaders.
One case concerns Pomacea maculata, a large freshwater snail commonly known as the Spotted Apple Snail, native to South America. Introduced through aquarium trade, and regarded as a potentially serious agricultural and ecological pest, the snail has caused extensive damages and economic losses to rice cultivation in the Ebro delta, Spain.
The blue crab, Callinectes sapidus, native to the Atlantic Coast and to the Gulf of Mexico, is an important predator that has caused substantial impacts on the structure and function of benthic food webs and fisheries. This species is quite common along Spanish and Italian coasts, where it causes also economic losses to mussels and clams farms.
A novel interaction between these aquatic invaders, amongst the worst at global level, was recently reported (article).
It is assumed that the collapse of the apple snail population in the Ebro river is associated with over-predation by the blue crab, which rapidly colonized the river rod in 2018, sharing the same habitat with the apple snail.
The biological control exerted by the blue crab on the apple snail may be viewed as a positive outcome. However, the associated rise in blue crab populations may intensify the predatory pressure on other native species. This possibility, underscores the need for a comprehensive understanding of ecological interactions and potential cascading effects when managing invasive species.
Another example is offered by the African Sacred Ibis (Threskiornis aethiopicus) which has been observed feeding on the blue crab in the Venice Lagoon. The crab has lateral spurs that makes it difficult for other species to feed on it. Even though the Ibis might be seen as beneficial, the species is an invasive alien species of Union concern according to EU regulation 1143/2014, and potential cause of further imbalance in the ecosystem.
The interaction between invasive species, when an invasive predator benefits from a previously introduced and abundant prey, poses unanticipated challenges for the joint management of invaders, and needs to be considered carefully.