The tropical water at the equator is distinguished for having the widest variety of marine life on the planet, with vibrant coral reefs and large aggregations of tunas, sea turtles, manta rays, and whale sharks. The number of marine species tapers off as you go towards the poles. Ecologists have found this global pattern to remain stable over recent centuries — until now.
New research found oceans around the equator have become too hot for most species to endure, and worldwide warming is accountable. Thus, the global pattern is quickly changing. And also, as species flee to cooler water towards the poles, it is likely to impose profound implications for marine ecosystems and human beings. A similar alteration happened 252 million years ago, and by the end, 90% of all marine species died.
This global pattern, where the number of species begins lower at the poles and peaks at the equator, results in a bell-shaped gradient of richness types. The researchers looked at distribution records for almost 50,000 marine species collected since 1955 and found a progressing dip in this bell shape. Meaning, as our oceans get hotter, species will track their preferred temperatures and move in the direction of the poles.
The dip at the equator has deepened as ocean warming has accelerated over recent decades due to climate change. The researchers predicted this kind of change five years ago using computer modeling, and now they have observational evidence.
Although the warming around the equator of 0.6°C in the last 50 years is relatively modest compared with heating at higher latitudes, tropical species need to move further to remain in their thermal niche compared to species elsewhere.
For each of the ten significant teams of species they studied (including reef fish, pelagic fish, and mollusks) that reside on the seafloor or in the water, their richness either declined slightly or plateaued at latitudes with mean annual sea-surface temperatures above 20°C.
Today, species richness is most significant in the following hemispheres: northern latitudes around 30°N (off southern Asia and Mexico) and within the south around 20°S (off north Australia and south Brazil).
We mustn’t be amazed worldwide biodiversity has responded so quickly to global heating. It has happened before.
At the end of the geological Permian period 252 million years ago, Earth’s temperatures warmed by 10°C over 30,000-60,000 years due to greenhouse gas emissions from volcano eruptions in Siberia. A 2020 study associated with the period’s fossils shows the pronounced biodiversity at the equator flattened and distributed. During this colossal rearranging of biodiversity, 90% of all marine species died.
A 2012 study revealed that more recently, during rapid warming 125,000 years ago, there was a similar quick movement of reef corals away from the tropics, per fossil record documentation. The result showed a pattern similar to the one previously described, although no mass extinction event was connected.
The study authors suggested their results might foreshadow the consequences of our current global heating. They ominously caution there could be mass extinctions shortly as species move into the subtropics, where they may struggle to compete and adjust.
During the last ice, which ended approximately 15,000 years ago, the richness of forams (a type of hard-shelled, single-celled plankton) peaked around the equator and has now been dropping there ever since. This is undoubtedly substantial as plankton is a crucial species in the food web.
The new study found that decline has accelerated in current decades because of human-driven climate change. Losing species in tropical ecosystems means reducing ecological resilience to environmental modifications, potentially compromising ecosystem perseverance.
In subtropical ecosystems, species richness is increasing. This implies there’ll be species invaders, novel predator-prey interactions, and new competitive relationships. For example, tropical fish migrating into Sydney Harbor would compete with temperate types for food and habitat. This might lead to ecosystem collapse — as had been seen at the boundary involving the Permian and Triassic periods — during which species went extinct and ecosystems were permanently altered.
The modifications the researchers describe will also have profound implications on human livelihoods. For example, many tropical areas depend on the income from tuna fishing fleets through the selling of licenses in their territorial waters. Highly mobile tuna will probably rush toward the subtropics, potentially beyond sovereign waters of island countries.
What can we do about this? One pathway is presented in the Paris Climate Accords and involves aggressively reducing our emissions. Other opportunities will also be emerging to help protect biodiversity and minimize the worst impacts. For example, a band of 41 nations is pushing to set a new goal of getting 30% of the oceans reserved by 2030 – dubbed the “30 by 30” target. Through it, they hope to ban seafloor mining and fishing in reserves that will destroy habitats, which releases just as much CO2 as international aviation. At the moment, only 2.7% of the oceans are protected.
