Marine life flees the equator to colder waters. A mass extinction could follow

As species flee to colder waters, this will likely have profound implications for marine ecosystems and human livelihoods.

The tropical waters around the equator are 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 decreases as one moves towards the poles.

Ecologists have assumed that this global pattern has remained stable over the past few centuries – until now. Our recent study found that the ocean around the equator has already become too warm for many species to survive, and that global warming is to blame.

In other words, the global pattern is changing rapidly. And as species flee to colder waters towards the poles, this will likely have profound implications for marine ecosystems and human livelihoods. When the same thing happened 252 million years ago, 90% of all marine species died.

The bell curve deforms dangerously

This global pattern – where the number of species starts lower at the poles and peaks at the equator – results in a bell-shaped gradient in species richness. We examined the distribution records of nearly 50,000 marine species collected since 1955 and found an increasing decline over time in this bell shape.

If you look at each line of this graph, you can see a slight drop in total species richness between 1955 and 1974. This worsens considerably over the following decades. (image: Anthony Richardson)

So, as our oceans have warmed, species have followed 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 to the warming at higher latitudes, tropical species have to move farther to stay in their thermal niche compared to species elsewhere.

As ocean warming has accelerated in recent decades due to climate change, the dip around the equator has deepened. We predicted such a change five years ago using a modeling 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 seabed, their richness peaked or declined slightly at latitudes with mean annual sea surface temperatures above 20℃.

Today, species richness is highest in the northern hemisphere at latitudes around 30°N (off southern China and Mexico) and in the south around 20°S (off northern Australia and southern Brazil).

It’s already arrived

We shouldn’t be surprised that global biodiversity has responded so quickly to global warming. This has happened before, and with dramatic consequences.

252 million years ago…

At the end of the Permian geological period about 252 million years ago, global temperatures warmed by 10℃ over 30,000 to 60,000 years due to greenhouse gas emissions from volcanic eruptions in Siberia.

A 2020 study of fossils from that time shows that the pronounced peak in biodiversity at the equator has flattened and spread. During this gigantic rearrangement of global biodiversity, 90% of all marine species have been killed.

125,000 years ago…

A 2012 study showed that more recently, during rapid warming around 125,000 years ago, there was a similar rapid movement of reef corals away from the tropics, as documented in the fossil record. The result was a pattern similar to the one we describe, although there was no associated mass extinction.

The study authors suggested their findings could foreshadow the effects of our current global warming, ominously warning that there could be mass extinctions in the near future as species move to subtropics, where they may find it difficult to compete and adapt.

Today…

During the last ice age, which ended about 15,000 years ago, the richness of forams (a type of hard-shelled single-celled plankton) peaked at the equator and has been declining ever since. This is important because plankton is a key species in the food web.

Our study shows that the decline has accelerated in recent decades due to human-induced climate change.

The Deep Implications

The loss of species in tropical ecosystems means that ecological resilience to environmental change is reduced, potentially compromising ecosystem persistence.

In subtropical ecosystems, species richness increases. This means there will be species invaders, new predator-prey interactions, and new competitive relationships. For example, tropical fish moving through Sydney Harbor compete with temperate species for food and habitat.

The migration of wishes from warmer waters could have serious implications for fishing in tropical countries. (picture: picture)

This could result in ecosystem collapse – as seen at the boundary between the Permian and Triassic periods – in which species disappear and ecosystem services (such as food supply) are altered in ways permed.

The changes we describe will also have profound implications for human livelihoods. For example, many tropical island nations depend on revenue from tuna fishing fleets through the sale of licenses in their territorial waters. Highly mobile tuna species are likely to move rapidly to subtropical regions, potentially beyond the sovereign waters of island nations.

Likewise, many reef species important to artisanal fishers – and highly mobile megafauna like whale sharks, manta rays and sea turtles that support tourism – are also likely to shift to subtropics.

The movement of commercial and artisanal fish and marine megafauna could jeopardize the ability of tropical nations to meet the Sustainable Development Goals for zero hunger and marine life.

Can we do something?

One path is set out in the Paris Climate Accords and involves aggressively reducing our emissions. Other opportunities are also emerging that could help safeguard biodiversity and hopefully minimize the worst impacts of its distance from the equator.

Currently, 2.7% of the ocean is conserved in fully or highly protected reserves. This is well below the target of 10% by 2020 set by the United Nations Convention on Biological Diversity.

But a group of 41 nations is pushing to set a new goal of protecting 30% of the oceans by 2030.

This “30 out of 30” target could ban seabed mining and eliminate 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 refuges where the climate will be stable for the foreseeable future.

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

This article was written by Anthony Richardson, Professor at the University of Queensland; Chhaya Chaudhary, expert at the University of Auckland; David Schoeman, professor of global change ecology at the University of the Sunshine Coast; and Mark John Costello, professor at the University of Auckland. It is republished from The Conversation under a Creative Commons license. Read the original article.

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