Invasive sea urchin endangers giant kelp forests

Feral Herald |

For the first time, a marine ecological community has been listed under federal environment laws – the sinuously beautiful, marvellously diverse Giant Kelp Marine Forests of South East Australia. They are endangered by climate change and invasive species.

You could almost watch a giant kelp forest grow. The giant kelp species Macrocystis pyrifera can shoot up more than 50 mm a day, up to 2 metres a year. It is the world’s largest benthic (sea bottom) organism.

Macrocystis pyrifera
Giant kelp (Macrocystis pyrifera). Photo: Richard Ling (Flickr, Creative Commons licence)

Giant kelp forests inhabit rocky reefs along the east and south coastlines of Tasmania in relatively nutrient-rich, cool waters mostly deeper than 8 metres. Some patches probably also occur in western and northern Tasmania, south eastern South Australia and Victoria.

These forests provide shelter and habitat for a great array of animals – fish, sea snails, lace corals, worms, crustaceans, sea urchins, seastars and sponges – and protect the shore from waves.

Lost from more than 60 per cent of their former Tasmanian distribution, the giant kelp forests have been disappearing since the mid-1900s, with declines accelerating over the past 20 to 30 years. Their losses are largely due to climate change facilitating an invasive species.

Over the past 60 years, wind systems strengthened by ozone depletion and climate change have pushed the East Australian Current about 350 km further south, increasing sea  surface temperatures and salinity along the eastern Tasmania coast. The temperature increase is trending at 2.3°C a century, the highest in the Southern Hemisphere and three times the average rate of warming in the world’s oceans.

The East Australian Current brings warm, nutrient-poor water from the Coral Sea and kelp-munching black sea urchins (Centrostephanus rodgersii) from NSW, where they are native. The warming is detrimental for giant sea kelp and helps  sea urchins establish, by increasing temperatures above the 12°C threshold they need for reproduction.

Centrostephanus rodgersii
Black sea urchin (Centrostephanus rodgersii). Photo: Saspotato (Flickr, Creative Commons licence)

The sea urchin has spread rapidly. It arrived in Bass Strait about 40 years ago, was found off the Tasmanian mainland in 1978, and now occurs round to southwest Tasmania.

In many areas, the sea urchins have grazed giant kelp out of existence, leaving ‘urchin barrens’. Past over-fishing may have facilitated this by depleting rock lobsters, the principal predators of sea urchins.

Another invasive species, the Japanese seaweed Undaria pinnatifida, is also spreading on Tasmania’s east coast. It can form dense stands and is likely to threaten giant kelp forests in future.

A recovery plan will be developed. Recommendations of the Threatened Species Scientific Committee relevant to invasive species include:

  • Support research into effective control methods for invasive species such as Centrostephanus rodgersii.
  • Manage sites to prevent introduction or further spread of new invasive exotic species, and support targeted control of existing key species which threaten the ecological community.
  • Manage shipping and aquaculture practices to minimise potential invasion of exotic species.

References

Commonwealth listing advice and conservation advice for Giant Kelp Marine Forests of South East Australia

Ridgeway K, Hill K. 2009. The East Australian Current. Marine Climate Change in Australia: Impacts and Adaptation Measures 2009 Report Card. In: Poloczanska ES, Hobday AJ, Richardson AJ (eds). Marine Climate Change in Australia. National Climate Change Adaptation Research Facility.

Check out BBC footage of sea urchins eating giant kelp off the Californian coast

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