Tropicalisation of Australian temperate reefs
This article was written by Dr. Daniela Ceccarelli, independent marine ecology consultant and part of our advisory board. See her bio below.
In September I took part in a survey trip along the rocky reefs of Australia’s subtropical east coast, from the Sunshine Coast to South West Rocks, a few hours north of Sydney. The team was studying the spread of tropical species into colder waters as the ocean warms due to our greenhouse gas emissions. My job was to identify and count the fish, which we do systematically along predetermined blocks of reef, so that we can come up with an estimate of what kinds of species live on these reefs, how many there are, and how big they tend to be. I was apprehensive because so far, all my work has been on tropical coral reefs, and I knew I was going to be cold, but I also wondered how I would perform with a whole new group of species that I had never seen before. My friend and colleague Dr. Maria Beger, who I was filling in for, sent me a list of species she had compiled from her previous trips, and I had been studying names and pictures.
We began at Flinders Reef, a little north of Brisbane, and my first impression was that I recognised almost all the fish – so many tropical species! I was told that this was traditionally the most tropical site. As we moved south, there was always a component of tropical species, with more and more temperate (cold-water) species, creating the unique mix that makes subtropical zones so interesting. I’m reminded of reports from many years ago of tropical fish making it all the way into Sydney Harbour in the summer, on the warm East Australian Current (EAC). In the past, these fish would all die during the winter; now more and more are surviving, making it to the next summer, growing and reproducing (https://www.abc.net.au/news/2019-09-13/sydney-growing-own-coral-reef-with-help-from-tropical-fish/11466192).
Research on the “tropicalisation” of temperate marine ecosystems is growing, and there are two lines of questions. The first is: what will happen to temperate plants and animals when tropical species move in? The second question is: can temperate regions like Sydney and Japan become refuges for coral reefs as the ocean near the equator becomes too hot for them to survive?
Tropicalisation is not so concerning when we’re talking about a few species that turn up every summer and then disappear over winter, but, as Dave Booth and Will Figueira from the University of Technology in Sydney are finding, more species are surviving the warmer winters. A few pretty tropical fish in Sydney Harbour are not necessarily a problem, but it’s a different story when important foundation species are threatened. Adriana Vergés and her collaborators have been studying this phenomenon both in the Mediterranean and in Australia. In the Mediterranean, they found that tropical rabbitfish can severely reduce the biomass and biodiversity of temperate reefs at a scale of hundreds of kilometres. The reefs were previously dominated by diverse seaweeds that provided food and shelter for many animals; the rabbitfish ate the seaweed, and the resulting seascape was too barren for most species. In Australia and other temperate coastal regions, kelp is a huge seaweed that grows in cold water, and where it flourishes, a whole ecosystem grows around it. Not only plants and animals, but people in these regions have become dependent on intact kelp communities for tourism and fisheries. But when tropical species move in and eat the kelp, the whole system collapses, and is replaced by something else – it could be other seaweeds, low-lying turf, or even corals.
Why is it a problem, as long as there’s still life in the sea? Actually, the problem is all about us: we have come to rely on the presence of certain species in certain locations for our fisheries and tourism, and for our ideas about conservation, which rely on species being where we’re used to protecting them.
The second question, especially for coral reef scientists, is: can these range shifts mean that corals and coral reefs might find a refuge further north and south as low-latitude (equatorial) waters become too hot for them to survive? Maria Beger and her students, who are organising this field trip, are working to answer this question, together with Dr. Brigitte Sommer from the University of Sydney. The answer is still unclear, but for now it seems that coral growth rates and reproduction are still too low for them to form persistent reefs. This may change as the ocean warms, but it’s too early to tell. In Australia, the bigger problem is that, south of the Great Barrier Reef, the continental shelf becomes too narrow and mostly too deep for corals to settle and survive. We might see patches of reef growing around islands, shoals and rocky outcrops, but there’s simply not enough suitable ground for the Great Barrier Reef to migrate south.
What can be done? Adriana Vergés suggests “marine protected areas to increase resilience and connectivity, the development of new fisheries that target range-expanding invaders, and assisted evolution and migration strategies to facilitate the dominance of large habitat formers like corals or seaweeds”. We can protect what we have, but we also need to embrace change and be adaptable in our protection and management of the seas around us.
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Dr. Daniela Ceccarelli is an independent marine ecology consultant with extensive training and experience in tropical marine ecosystems. She completed a PhD in coral reef ecology at James Cook University in 2004, and since then, her fieldwork has taken her to the Great Barrier Reef, Papua New Guinea, Tuvalu, Tonga and the Marshall Islands, and to remote reefs of northwest WA and the Coral Sea. For the last decade she has worked as a consultant for government, non-governmental organisations, industry, education and research institutions on diverse projects requiring field surveys, monitoring programs, data analysis, reporting, teaching, literature reviews and management recommendations. Her research and review projects have included studies on coral reef fish and invertebrates, seagrass beds and mangroves, and have required a good understanding of a wide range of topics. More recently, she has been contributing to the spatial planning efforts in the Pacific through the IUCN’s MACBIO program. She also continues to collaborate with colleagues in academic institutions to further her research interests on coral reefs.