A new global study reveals that the biodiversity of marine species around the equator has dropped, as warming seas force tropical species south into already faltering ecosystems.
The research team, led by the University of Auckland in collaboration with the University of the Sunshine Coast (USC), the University of Queensland and CSIRO, examined the distribution data of 48,661 marine species since 1955.
The alarming results, published in the journal PNAS, confirm that climate change is impacting species diversity across latitudes, with the number of species levelling off or declining at latitudes with average sea surface temperatures exceeding 20°C.
According to Professor David Schoeman, co-author of the study from USC, species attached to the seafloor like corals, oysters and seaweeds have not declined but free-swimming species like fish have dropped significantly.
“The decrease in numbers of species at the equator doesn’t mean that sea life is becoming extinct from the planet,” he says. “Instead, it means extirpation, or local loss of those species.”
The findings suggest that rising sea temperatures are already making tropical ecosystems too hot to some species to survive – so they move south, where historically cooler subtropical waters are also warming.
This movement of tropical marine species southwards is known as tropicalisation. This process is visible along Australia’s east coast as far south as Sydney, where tropical fish are increasingly common in existing reef systems. The complicated interplay between invasive northern species and declining southern species threatens diverse marine ecologies around Australia, including in our cooler, southern waters.
An example of this type of ecosystem disruption already in play is the massive decline in Tasmanian kelp forests. Cornerstone species that furnished the island’s ocean ecosystems – and Indigenous cultural life – for millennia, kelp are now swiftly vanishing due to warming waters allowing an influx of predatory sea urchins.
Schoeman warns that current warming is merely a fraction of the ocean temperature increase expected by 2050, which will cause more widespread disruption and destruction to marine ecosystems. For example, a recent report released by Australia’s top climate scientists says that at 2°C global warming, 99% of coral reefs are projected to disappear, and at 3°C they will entirely vanish.
“Declining numbers of species at the tropics also puts the livelihoods of our tropical-island neighbours at risk, both in terms of seafood resources and tourism attractions,” Schoeman says, calling for the threat of climate change to be taken seriously.
Amalyah Hart is a science journalist based in Melbourne.
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