The Stats Say

ISSUE NO. 01
DECEMBER 2021

The Stats Say

Written by: Keagan Mattison

The tropical water at the equator is renowned for having the richest diversity of marine life on Earth, with vibrant coral reefs and large aggregations of tuna, sea turtles, manta rays and whale sharks. The number of marine species naturally tapers off as you head towards the poles.

Ecologists have assumed this global pattern has remained stable over recent centuries - until now.

A recent study found the ocean around the equator has already become too hot for many species to survive, and that global warming is responsible. In other words, the global pattern is rapidly changing. And as species flee to cooler water towards the poles, it’s likely to have profound implications for marine ecosystems and human livelihoods.

When the same thing happened 252 million years ago, 90% of all marine species died.

During the last ice age, which ended around 15 000 years ago, the richness of Forams (a type of hard-shelled, single-celled plankton) peaked at the equator and has been dropping there ever since. This is significant as plankton is a keystone species in the food web and that decline has accelerated in recent decades due to human-driven climate change.

So, as our oceans warm, species have tracked their preferred temperatures by moving towards the poles. Although the warming at the equator of 0.6℃ over the past 50 years is relatively modest compared with warming at higher latitudes, tropical species have to move further to remain in their thermal niche compared with species elsewhere.

Photo: Gabriel Kuettel

As ocean warming has accelerated over recent decades due to climate change, the dip around at the equator has deepened. We predicted such a change five years ago using a modelling approach, and now we have observational evidence. For each of the 10 major groups of species we studied (including pelagic fish, reef fish and molluscs) that live in the water or on the seafloor, their richness either plateaued or declined slightly at latitudes with annual sea-surface temperatures above 20℃.

Today, species richness is greatest in the northern hemisphere in latitudes around 30°N (off southern China and Mexico) and in the south around 20°S (off northern Australia and southern Brazil). Is there anything we can do?

One pathway is laid out in the Paris Climate Accords and involves aggressively reducing our emissions. Other opportunities are also emerging that could help safeguard biodiversity and hopefully minimise the worst impacts of it shifting away from the equator.

The United Nations Paris Agreement

Currently 2.7% of the ocean is conserved in fully or highly protected reserves. This is well short of the 10% target by 2022 under the UN Convention on Biological Diversity. But a group of 41 nations is pushing to set a new target of protecting 30% of the ocean by 2030. This “30 by 30” target could ban seafloor mining and remove fishing in reserves that can destroy habitats and release as much carbon dioxide as global aviation. These measures would remove pressures on biodiversity and promote ecological resilience.

Designing climate-smart reserves could further protect biodiversity from future changes. For example, reserves for marine life could be placed in refugia where the climate will be stable over the foreseeable future.

We now have evidence that climate change is impacting the best-known and strongest global pattern in ecology. We should not delay actions to try to mitigate this.

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