Echinoderms from Rottnest Island photographed during research
Echinoderms from Rottnest Island photographed during research

The marine ecosystems off the coast of Western Australia, specifically the rocky reefs of Rottnest Island, are experiencing a significant decline in populations of shellfish and sea urchins. This alarming trend can be primarily attributed to the ongoing increase in ocean temperatures caused by climate change.

A recent study conducted by scientists from Curtin University in Australia has revealed that the increasing temperatures of the Indian Ocean are responsible for the significant decline in populations of invertebrates, including molluscs and sea urchins, on Rottnest Island. Located just 20 km off the coast of Perth in Western Australia, Rottnest Island’s western end has experienced a “catastrophic decline” in biodiversity, as noted by the study’s lead author, Assistant Professor Fred Wells from Curtin’s School of Molecular and Life Sciences.

Professor Wells, who has been leading biodiversity monitoring efforts on Rottnest Island, Cottesloe, Trigg Point, and Waterman since 1982, revealed concerning findings. Despite being sanctuary areas with the highest level of protection against human activities, Radar Reef and Cape Vlamingh on Rottnest Island have experienced a “catastrophic decline” in biodiversity between 2007 and 2021. This decline is believed to be linked to the warming effects of the Leeuwin Current, with rock reefs at the western end of Rottnest Island showing a depletion of 90% or more in biodiversity and mollusc density.

In contrast, the coastal areas of Australia that are not impacted by the Leeuwin Current have managed to maintain their biodiversity intact.

The question of ocean warming

Rising ocean temperatures are a global phenomenon, but their impact is not uniform across regions, with certain areas experiencing more rapid warming compared to others.

Ocean temperatures in the Southern Hemisphere have shown a significant rise of approximately +0.5°C since the early 2000s, when compared to the average temperatures recorded between 1900 and 2000.

According to Professor Wells, the west coast of Australia has experienced multiple marine heatwaves in recent years, resulting in unusually high ocean temperatures. This, in turn, has had detrimental effects on the plant and marine animal populations in the region.

The most severe marine heatwave was documented during the summer of 2011, with ocean temperatures surpassing the average by a staggering 3°C for three consecutive months, and reaching anomalies of +5°C along the coast for shorter durations. These findings highlight that even protected areas are not immune to the impacts of global climate change, underscoring the urgency of addressing this pressing issue.

Contrary to expectations, the warming waters did not result in an increase in tropical species west of Rottnest, as all three groups, including molluscs and echinoderms, experienced significant declines, as stated by Professor Wells. However, there is hope for recovery, as populations in other areas off the metropolitan coast are in good condition and could potentially serve as a source of larvae to repopulate Radar Reef and Cape Vlamingh, as concluded by Assistant Professor Fred Wells.