- As seas get warmer, tropical species are moving further from the equator
Tropical species are migrating from the equator to the poles as a result of climate change, whereas temperate species are receding. This enormous shift of marine life, known as tropicalization, is having a cascade effect on ecosystems and biodiversity, as well as having the potential to disrupt the world economy.
My colleagues and I recently selected and examined 215 scientific papers on tropicalization published between 2003 and 2023. Our research, which has just been published in the journal Trends in Ecology and Evolution, illustrates the breadth of this species movement and demonstrates how far-reaching its implications can be.
Tropicalization is a global trend driven by rising sea temperatures and heat waves caused by climate change. It is most noticeable in areas where strong currents move away from the equator. The Kuroshio Current in the Western Pacific, for example, has assisted certain corals and fish in migrating from the tropics to temperate Japanese waters.
However, tropicalization can occur in other parts of the planet where such currents do not exist. Mangrove trees, for example, are spreading northward along the coast of Florida as winter temperatures rise. These trees are normally vulnerable to freezing temperatures, but they can now live at higher latitudes, where they are replacing salt marshes.
Marine life in motion
Tropicalization affects a diverse spectrum of marine life, including corals, algae, and mangrove trees, as well as marine snails, reptiles, and even mammals, among many others. Nonetheless, the species involved share some characteristics.
Those that can go into warming seas, such as some marine snails or coral reef fishes, are good dispersers because they may travel further in quest of suitable habitat. Simultaneously, tropical species that are more generalist in their diet and behaviour may fare better in their new habitat. Coral reef fishes with broader diets, for example, are more likely to establish in a new range because they are more likely to discover a source of food.
Consequences for the environment and evolution
Tropicalization can have an effect on a single population, an entire species, or even an entire ecosystem. For example, as herbivorous fish migrate away from the equator, they consume a large amount of algae in their new environment. This provides more space for corals to dwell in the area, which contributes to further tropicalization. And, as tropical sea turtles and dugongs (a cousin of the manatee) spread southward along Australia’s west coast, their consumption is likely to significantly increase the pressure on already endangered seagrass meadows.
Some animals’ behaviour is changing. For example, as tropical damselfishes go farther south-eastern Australia away from the equator, they begin to join shoals with temperate species rather than keeping to their regular groupings of tropical peers. This behavioural change is assumed to be related to them living longer and getting larger.
Recent studies have just recently begun to reveal the genetic and evolutionary effects of tropicalization. For example, if established temperate species are pushed out, the population may lose its distinctive genetic variety, making it less able to adapt to future changes. Meanwhile, some temperate species have begun to adapt to their new tropical neighbours. Volcano barnacles, for example, have been spotted “bending” to fend off tropical predatory snails in temperate waters off the coast of Baja California, Mexico.
The socioeconomic ramifications
Tropicalization’s impacts will be favourable or harmful depending on the geographical region and players involved. The extensive movement of marine life is already having an impact on world fisheries, with higher harvests of tropical species in places like the western Pacific Ocean. However, tropicalization is also causing the extinction of commercially valuable temperate species and an increase in non-target species trapped in fishing nets.
Meanwhile, mangrove tree growth adds to increased carbon storage and sequestration as compared to the temperate saltmarshes they replace. Furthermore, the proliferation of charismatic tropical species like corals and the colourful marine life they support could benefit local economies through greater tourism.
As the newest UN climate summit develops, our changing environment necessitates greater research into Tropicalization and more informed responses to it. While previous research offers insight on its ecological repercussions, considerable gaps in understanding its evolutionary consequences and their interaction with complex socioeconomic impacts remain. Tropicalization is a worldwide issue that requires our attention right now.
