Adriana Vergés, Erin McCosker, Mariana Mayer-Pinto, Melinda A. Coleman, Thomas Wernberg, Tracy Ainsworth, Peter D Steinberg
Welcome to the ‘Anthropocene’, a new geological era where humans are the dominant force of biogeochemical and ecological change across the planet. Overwhelming evidence shows how we are altering atmospheric, geological, hydrological and biospheric processes at increasingly faster rates. Even under the most optimistic scenarios, our atmosphere will warm by at least 2 ºC and our seas will rise by over half a meter by the end of the century. One consequence is species moving towards higher altitudes, deeper waters or towards the poles, to stay within their preferred temperature ranges.
However, not all species are responding equally to warming. Species move at different rates while some species stay within their original ranges. As a consequence, ecological communities are being re-organised into entirely novel ecosystems where new and unique combinations of species interact.
In this review, we focus on novel marine ecosystems that are emerging just outside tropical areas, where warm-water species like tropical fishes and corals are becoming increasingly dominant, and cool water species like kelp recede. This so-called ‘tropicalisation’ of our coastlines is already leading to extensive losses of underwater forests and the many species they support over hundreds of kilometres of coastline.
Although tropicalisation is a global and increasingly visible phenomenon, the overall consequences of these changes for coastal ecosystems and the many human activities that they support, like fishing or tourism, are poorly understood.
Here, we argue that the consequences will depend on which major habitat forming species end up dominating the seafloor. We put forward three potential tropicalisation scenarios, that differ in whether seaweeds, turf or corals become dominant.
Not all changes are necessarily for the worse, from the point of view of humans, as there may be some potential gains under some scenarios. For example, local fish productivity may increase in some tropicalised reefs dominated by corals, as the consumption of smaller, faster-growing algae surrounding the corals increases. However, this may be at the expense of other functions such as carbon export. So while productivity may increase at a local scale, there are likely to be losses in nearby systems that would have been supported by that exported carbon.
So, what are potential management solutions? One option is marine protected areas, which can be more resistant to invasion by shifting species than fished areas. Another alternative may be to develop new fisheries that specifically target range-expanding invaders.
More controversially, assisted evolution and migration strategies that facilitate the persistence of selected foundation species like corals or kelps under a changing environment may also be used. However this involves deciding what species we should aim to prioritise and which ones to let go, and could have unexpected ecological outcomes.
As technological innovations continue to develop, we argue that it is essential to understand what it is we are trying to conserve and consider the ethics surrounding increasingly interventionist approaches.