Places where tropicalization is detected all over the world. The map is based on data extracted from 138 peer-reviewed** articles published between 2004 and 2023. **Zarzyczny et al. (2023) Trends in Ecology and Evolution.
Climate change is causing tropical species in the ocean to migrate from the equator to the poles, while temperate species are retreating. This large-scale migration of marine organisms, known as tropicalization, is causing a cascade of consequences for ecosystems and biodiversity, with the potential to impact the global economy.
My colleagues and I recently identified and reviewed 215 scientific** related to tropicalization published between 2003 and 2023. Our findings, now published in the journal Trends in Ecology and Evolution, reveal the extent of this species movement and demonstrate the wide-ranging nature of its consequences.
Tropicalization is a global trend driven by rising sea temperatures and marine heatwaves caused by climate change. This is especially evident in areas where strong currents flow out of the equator. For example, the Kuroshio Current in the western Pacific Ocean has helped certain corals and fish migrate from the tropics to Japan's temperate waters.
However, other parts of the world that do not have such currents may also experience tropicalization. A notable example is the expansion of mangrove forests north along the Florida coast as winter temperatures rise. These trees are usually sensitive to freezing conditions, but can now survive at high latitudes, where they are replacing salt marshes.
Tropicalization involves a wide range of marine life, from large habitat formations such as corals, algae or mangroves, to marine snails, reptiles and even mammals. However, the species involved tend to have some things in common.
Those with access to warming oceans, such as some marine snails or reef fish, are good dispersers and are often able to go further afield in search of suitable habitats. At the same time, tropical species that are more generalist in what they eat and how they behave can perform better in their new range. For example, reef fish with a wider diet are more successful in establishing new ranges because they are more likely to find food**.
Tropicalization can affect individual populations, entire species, or even entire ecosystems. For example, when herbivorous fish move away from the equator, they eat the large amounts of algae found in their new homes. This creates additional space for corals to settle in the area, contributing to further tropicalization. As tropical sea turtles and dugongs (cousins of manatees) expand southward along Australia's west coast, they are expected to greatly increase pressure on already fragile seagrass meadows by eating them.
Some animals are adjusting their behavior. For example, when tropical damselflies move farther from the equator along southeastern Australia, they begin to form shallows with temperate species rather than sticking to the usual tropical peer groups. This change in behavior is thought to be associated with them living longer and bigger.
Recent investigations are only beginning to reveal the genetic and evolutionary consequences of tropicalization. For example, if established temperate species are expelled, it can lead to the loss of unique genetic diversity, making the population less able to adapt to future changes. At the same time, some temperate species are already adapting to their new tropical neighbors. For example, in temperate waters near Baja California, Mexico, volcanic barnacles have been observed to "bend" against tropical predatory snails.
Whether the consequences of tropicalization will be positive or negative will depend on the geographical region and the relevant stakeholders. The widespread movement of marine life has affected global fisheries, with increased catches of tropical species in regions such as the Western Pacific. However, tropicalization has also led to the loss of commercially important temperate species, as well as an increase in the accidental catch of non-target species during fishing.
At the same time, the proliferation of mangroves helps to enhance carbon storage and sequestration compared to the temperate salt marshes they replace. The expansion of attractive tropical species such as corals and the colorful marine life they possess can help the local economy by increasing tourism.
As the latest UN climate summit unfolds, our changing climate means we urgently need to investigate more about tropicalisation and take smarter action to tackle it. While existing research reveals its ecological impacts, significant gaps remain in understanding its evolutionary consequences and its interaction with complex socioeconomic impacts. Tropicalization is a worldwide phenomenon that is currently taking place, and it requires our attention.