Oceans and coasts have been subject to human activity for centuries. But the effects of human activity on the oceans are now more widespread, with the resulting changes happening faster than ever.
It’s hard to find a place in the ocean that hasn’t been invaded by an alien species. Maritime transport contributes greatly to this process, as more than 90% of world trade is carried out by sea.
The marine environment is also changing: temperatures, salinity, chemistry, sea level, ice content and ocean weather patterns are all being altered due to climate change. These changes in ocean environments are bound to affect biological invasions.
In a recent article, we explored the implications of climate change throughout the invasion process. We have found that climate change is likely to lead to more marine invasions because it will change which species are moved, how they are moved and where they are moved.
On top of that, climate change will alter where alien and native species are most likely to survive and spread. Climate change essentially makes marine invasions less predictable. The patterns and processes that underpinned invasions in the past cannot be transferred to future invasions.
And this should lead to a consequent decline in the ecosystem services that people currently receive from the ocean.
The main drivers
Navigation is the main invasion route for marine species. Climate change will affect shipping by altering weather patterns, sea conditions, melting ice, and more frequent extreme weather events. Some of the current shipping routes will become unviable in terms of safety or cost. This will change transport routes, destinations and transit times. This will ultimately impact how, when and where alien species are transported and introduced by ships.
For example, the melting of the Arctic ice cap will allow about 5% of world trade to use new shipping routes through the North Pole. These new shipping routes will increase connectivity between Europe and Asia and subsequently reduce transit times by up to 40%.
The implications for alien species are twofold: first, there will be greater mixing of European and Asian marine species; and second, shortened transit times will likely increase the survival of transported organisms.
Changes in global trade, industries and tourism are also expected to affect traffic volumes and therefore the volume of alien species that are unintentionally transported and introduced. For example, with increased traffic in the Arctic, fewer ships will use other routes. This will displace the volume of alien species that are transported and introduced to different regions.
It is difficult to know exactly how this will play out in Africa as research on the implications of climate change for African shipping routes and the consequential effects on the volumes of alien species transported and introduced into African ports is lacking.
Marine agriculture is another area we have identified as a source of major change. Many exotic marine species are intentionally imported for cultivation. As climates change, sea conditions may become less optimal for traditionally farmed species. Operations will likely shift to using new species that are productive under changed environmental conditions. Or industries may move to new locations. The oyster industry along the west coast of North America is a prime example.
Ocean acidification in this area has caused such high mortalities of Pacific oyster larvae (Crassostrea gigas) that nearly all oyster farming in the Pacific Northwest has failed. Many farmers have now moved their operations to Hawaii, where conditions are more favorable. While this is good for oyster production, it has increased the risk of invasion Hawaii faces.
At present, there is not enough information available to see if similar situations could occur along African coasts. However, as the livelihoods of many Africans depend on marine agriculture, it is essential to improve our understanding of how ocean conditions may change in mariculture hotspots and what the implications may be for communities. cultivated species.
The third driver we identified was habitat modification.
To become a successful invader, an alien species must survive and establish a population in the new environment. Then the species must spread to the new region. The ability of introduced species to do these things is influenced by both the environment and the interactions between exotic and native species.
As climate change continues to alter ocean temperatures and chemistry (eg ocean acidification), previously unsuitable habitats are expected to become suitable for newly arriving, established or expanding alien species.
Alternatively, changing ocean conditions may become less optimal for some native species. For example, the native brown mussel (perna perna) has retracted its range along the southern coast of South Africa in response to declining seawater temperatures.
It is still very difficult to anticipate how a particular species (exotic or native) may be affected by an environmental change. Indeed, each species will respond based on its ability to tolerate or adapt to new conditions.
The effects of environmental changes will affect interactions between exotic and native species. This is problematic because sometimes native species out-compete and out-predict exotic species, preventing them from becoming invasive. Alternatively, the absence of predation and competition by native species can help exotic species to establish and spread. For example, invasions of the Mediterranean mussel (Mytilus galloprovincialis), the bisexual mussel (Semimytilus patagonicus) and the common acorn barnacle (balanus glandule) in South Africa are all unhindered due to predators such as the Ringed Dogwhelk (Cingulate trochia) exerting low predation pressure on them.
There are still serious knowledge gaps that prevent a better understanding of how climate change will affect biological invasions. These shortcomings are evident, for example, in:
Taxonomy – cryptic invasions often go unrecognized when an alien species is misidentified as a native species.
Natural history – the life history traits of alien species are rarely quantified.
Ecology – species ranges are often not georeferenced or routinely monitored.
Invasion biology – sometimes we don’t know if a species is native or alien.
Physiological tolerances of native and exotic species.
Basic environmental data in many regions.
These uncertainties make it increasingly difficult for managers, conservationists and policy makers to anticipate and therefore prevent invasions.
Our ability to effectively manage invasive species will depend on the proactive and adaptive nature of our prevention, eradication, containment and mitigation measures. These need to consider how climate change affects the movement of alien species, fitness at the species level, and understand how climate change affects interactions between groups of species.