Sid Wales/Supplied
Caulerpa from Okupu Beach on Great Barrier Island. (File photo)
Dr Ian Davidson is team leader, invasion ecology and management at Nelson’s Cawthron Institute.
OPINION: For some invasion biologists and biosecurity managers “caulerpa” is a word synonymous with a worst-case scenario in marine invasions. Books and science papers describe this “killer algae” and how it quickly overgrows and fills every contour of seabed with uniform green mats of leafy seaweed.
When it invades, it can cover ground prodigiously by itself, and it can jump across a seascape with the help of boats and anchors. As a marine plant, it is quite attractive in small doses and aquarium enthusiasts value is bright green foliage – like branches of a Christmas tree – to add colour and texture to their tanks.
This allure is one of its weapons of invasion, however, because caulerpa escape from fish tanks has aided and abetted its invasion history around the world.
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New Zealanders are now grappling with invasive caulerpa at multiple sites on our northeast coastline, most likely delivered by boat anchors rather than as an aquarium escape. We have nine native caulerpa species on New Zealand’s coastlines, part of our rich biodiversity and mostly in warmer waters of the North Island.
The new invader appears to have a competitive advantage over native seabed life, at least for now. There is a heightened sense of concern and urgency to respond effectively to prevent worst-case impacts that could play out in future.
The first obvious impact is its mere presence in places that were not previously dominated by a carpet of intertwined creeping stolons and rhizoids (little root-like structures), matted over by wafting branches and branchlets of green weed. It is jarring to see a marine space converted to something so characteristically different to what it looked like before.
This, in and of itself, looks like a takeover or exclusive property development for one species alone and diminishes the mauri of marine places for mana whenua and coastal communities.
Beyond the monopolised coverage of space, the impacts of caulerpa get a little murkier and can be slippery to get a handle on. Certainly, sustained presence of a dominating single species “meadow” can crowd out native species and valued kai moana.
Its dominance prompts unwanted changes to habitats and for native species. The ability of the native community of seafloor species and fish to interact with caulerpa (compete for space, eat it, use it as new habitat) is uncertain, but problems could persist.
The worst predictions don’t always play out either and projections of seagrass exclusion from invaded sites have not always occurred in long-invaded sites in the Mediterranean, for example.
The trajectory of caulerpa during every invasion may wax and wane with the seasons, and operating in uncertainty is the only sure thing ahead.
The challenge now is three-part: (1) develop strategies and tools that can reduce the dominance of this invader at a local scale, and ideally eradicate it; (2) dramatically reduce its opportunity to jump to new sites from the current infestations of Great Barrier Island, Great Mercury Island, and the Bay of Islands; and (3) develop an understanding of its impact on the ecology of our unique marine spaces.
There is a fraught history of eradication efforts in the sea – it’s a tricky environment to operate in and there aren’t off-the-shelf tools available like there are for pest control in homes, gardens, farms, and the landscape. But New Zealanders are leaders in biosecurity – developing these tools and strategies is another opportunity to show that leadership
SUPPLIED
Dr Ian Davidson is team leader, invasion ecology and management at Nelsonâs Cawthron Institute.
Our world-leading systems to reduce the risk of new invasions are not a failure because of this new incursion. Unwanted species often reach the doorstep and some inevitably make it in. This new caulerpa invasion presents another challenge, but through New Zealander’s ambition and practicality we can work to stop the spread and even reverse it.
This will require innovation and resources–we need more, and better, pest control tools developed specifically for marine systems – but it can serve to tackle the current problem and set us on a much stronger path to tackle the next invader that arrives at our shores.