Invasive species represent one of the main threats to global biodiversity, affecting the function and structure of natural ecological communities and causing annual economic losses amounting to billions of dollars worldwide.

Since the publication of Charles Elton’s book “The ecology of invasions by animals and plants” in 1958, biological invasions have gained exponentially growing attention. The realization that most successful invasions are irreversible has motivated ecologists to attempt to identify the environmental factors influencing the spread of invaders in new ecosystems and to quantify and forecast the success of extant invasive processes.

Mytilus galloprovincialis is a mussel native to the Mediterranean and has an antitropical distribution, being recorded to date on the coasts of all continents except Antarctica. It possesses many features that make it a successful invasive species, including high fecundity, fast growth, competitive abilities and ease of transport of larvae and adults in ballast waters. This species is the most successful marine invader in southern Africa; the distribution there has expanded rapidly and extensively since the 1970s, however, over the last decade its spread has ceased.

We assessed the current invaded distribution of Mytilus galloprovincialis in southern Africa and evaluated what prevents further spread of this species. We combined broad scale field surveys, machine learning distribution modelling and biophysical modelling of oceanographic transport simulations.

We found that all environmentally suitable habitats in southern Africa have been occupied by the species. This includes rocky shores between Rocky Point in Namibia and East London in South Africa (approx. 2800 km) and these limits coincide with the transition between the cooler and warmer water regimes; therefore the main factor limiting the northern spread on the southeast coast. However, biotic interactions with native fauna may also play an important role.