Climate change driving global movement of marine species

As predicted by climate warming, the number of species has decreased at the equator and increased in the sub-tropics since the 1950s.

This was the case across all 48,661 species, and when they were split into those living on the seabed (benthic) and in open water (pelagic), fish, molluscs and crustaceans.

The results from the University of Auckland-led research showed that pelagic species had shifted poleward in the northern hemisphere more than benthic. The lack of a similar shift in the southern hemisphere was because ocean warming has been greater in the northern than southern hemisphere.

Previously, the tropics were considered stable and an ideal temperature for life because so many species occur there. Now, we realise that the tropics are not so stable and are increasingly too hot for many species.

The study was the culmination of lead author Chhaya Chaudhary’s PhD at the University of Auckland and was built on a range of studies in a research group that studied the literature and data on particular taxonomic groups in detail, including crustaceans, fish and worms.

The data were obtained from the Ocean Biodiversity Information System (OBIS), a freely accessible online world database whose establishment was led by the University’s Professor Mark Costello as part of the Census of Marine Life, a global marine discovery programme from 2000 to 2010. The records of when and where species were reported were summed into latitudinal bands and a statistical model was used to account for variation in sampling.

Last year, Professor Costello co-authored a study showing that while marine biodiversity peaked at the equator during the last ice age, 20,000 years ago, it had already flattened before industrial global warming. That study used fossil records of marine plankton buried in deep sea sediments to track the change in diversity over thousands of years.

This latest study on a decadal timescale shows this flattening has continued in the past century, and the number of species now dips at the equator. The study, and others in progress, show that the number of marine species declines once the annual mean sea temperature is above 20 to 25 oC (varying with different kinds of species).

As one of the Lead Authors on the current 6th Assessment Report of the International Panel on Climate Change (IPCC), Professor Costello says the findings are significant.

“Our work shows that human-caused climate change has already affected marine biodiversity at a global scale across all kinds of species. Climate change is with us now, and its pace is accelerating.

“We can predict the general shift in species diversity, but because of the complexity of ecological interactions, it is unclear how species’ abundance and fisheries will change with climate change.”