The Great South­ern Reef

The Monthly (Australia) - - NEWS - by James Bradley

Any­body who dives or snorkels along the south­ern Aus­tralian coast will be fa­mil­iar with the kelp beds that line that part of the con­ti­nent. Win­kled into the to­pog­ra­phy of the sea floor, they are of­ten things of beauty, fish dart­ing in and out as the dark brown and honey gold fronds shift in the cur­rent. But even when the wa­ter is more tur­bu­lent, and the mass­ing weight of the kelp sucks and pulls, they are ma­jes­tic, the strength of their grip upon the rock im­pres­sive. These kelp beds pro­vide the eco­log­i­cal foun­da­tion of a string of tem­per­ate rocky reefs that fol­low the coast from Kal­barri in the west, across the Great Aus­tralian Bight to Vic­to­ria and Tas­ma­nia, and north again to By­ron Bay. Dense with di­verse and en­demic life (in some places, as many as 80% of species are found nowhere else), these en­vi­ron­ments span 71,000 square kilo­me­tres and are eco­nom­i­cally sig­nif­i­cant, gen­er­at­ing as much as $10 bil­lion an­nu­ally, pri­mar­ily through fish­ing and tourism. Un­til re­cently, these reef ecosys­tems have been stud­ied and man­aged mostly in iso­la­tion from one an­other. This is partly an ac­ci­dent of his­tory – a re­flec­tion of our state-based sys­tem of fish­ery reg­u­la­tion – but it’s also a

With the reef’s bi­o­log­i­cal en­gine gone, the ecosys­tem quickly col­lapses, the kelp re­placed by al­gae, en­demic species by the in­vaders.

re­flec­tion of our lim­ited un­der­stand­ing of these habi­tats, es­pe­cially those fring­ing the sparsely pop­u­lated coasts of South Aus­tralia and Western Aus­tralia. In a pa­per pub­lished in 2016, a team of sci­en­tists led by marine bi­ol­o­gists Scott Ben­nett and Thomas Wern­berg ar­gued that this was a mis­take, and that these reefs are in fact one vast sys­tem stretch­ing from one side of the con­ti­nent to the other. A Great South­ern Reef to match the Great Bar­rier Reef to the north. Al­though it has yet to find broad recog­ni­tion, it is a pow­er­ful and rev­e­la­tory idea – recog­nis­ing not just the in­ter­con­nect­ed­ness and in­ter­de­pen­dence of these ecosys­tems but also kelp’s role as the reef’s foun­da­tion species. The mo­ti­va­tion be­hind Ben­nett and Wern­berg’s pro­posal isn’t purely sci­en­tific. In­stead, it re­flects their frus­tra­tion with the lack of pub­lic un­der­stand­ing of the chal­lenges fac­ing many of Aus­tralia’s marine ecosys­tems. While the Great South­ern Reef has yet to ex­pe­ri­ence any­thing as cat­a­strophic as the re­cent bleach­ing of the Great Bar­rier Reef, Wern­berg ar­gues its sit­u­a­tion is no less crit­i­cal, point­ing to events such as the 2011 “marine heat­wave” off Western Aus­tralia, an oc­cur­rence of un­prece­dented warm sea sur­face tem­per­a­tures that wiped out kelp beds along more than 100 kilo­me­tres of coast, caus­ing mass die-offs of many fish, shell­fish and crus­tacean species. The kelp beds lost in the 2011 heat­wave seem un­likely to re­cover, and Wern­berg ar­gues that this is only the most vis­i­ble in­stance of a much larger prob­lem. We have also lost “sea­weed in Syd­ney, kelp around Tas­ma­nia and kelp forests in the South Aus­tralian gulfs”.

Nor is the prob­lem con­fined to the Great South­ern Reef. “In the past two years alone we’ve seen more than 1000 kilo­me­tres of man­groves die in the North­ern Ter­ri­tory, we’ve had many, many square kilo­me­tres of sea grasses lost in Shark Bay in Western Aus­tralia, yet none of it re­ceives the same at­ten­tion as the Great Bar­rier Reef. The only dif­fer­ence is per­cep­tual.” Un­sur­pris­ingly, the most sig­nif­i­cant pres­sure upon the Great South­ern Reef is cli­mate change. Yet, while warm­ing wa­ter can ad­versely af­fect kelp, the re­la­tion­ship is not as di­rect as it is with corals, which sim­ply die when wa­ter tem­per­a­tures rise be­yond a cer­tain level. Warm­ing wa­ter al­lows her­biv­o­rous trop­i­cal species to move south, where, un­con­strained by nat­u­ral preda­tors, they quickly de­vour the kelp. With the reef’s bi­o­log­i­cal en­gine gone, the ecosys­tem quickly col­lapses, the kelp re­placed by al­gae, en­demic species by the in­vaders.

Largely as a re­sult of urchin graz­ing, 95% of the kelp forests around Tas­ma­nia are now gone.

Adri­ana Vergés, a marine ecol­o­gist at the Univer­sity of New South Wales, has been study­ing this process of “trop­i­cal­i­sa­tion”. Like Wern­berg, she ar­gues that the pres­sures on kelp forests are sig­nif­i­cant and grow­ing, point­ing out that over the past decade the range and health of kelp forests in north­ern New South Wales have sig­nif­i­cantly di­min­ished. In some ar­eas, such as the Soli­tary Is­lands, off Coffs Har­bour, kelp beds have dis­ap­peared en­tirely. “The south-east of Aus­tralia is warm­ing more than two times faster than the global av­er­age,” Vergés says. “That’s re­sult­ing in the rapid south­ward move­ment of many trop­i­cal and warm-wa­ter species.”

This process has had its most dev­as­tat­ing out­comes in Tas­ma­nia, where the gi­ant kelp forests that en­cir­cled the is­land and filled its bays were once so dense they fea­tured on ship­ping maps. In re­cent years, the East Aus­tralian Cur­rent has ex­tended its reach, sweep­ing warm wa­ter down from the trop­ics. This has raised wa­ter tem­per­a­tures around Tas­ma­nia by as much as 2.5 de­grees Cel­sius, and al­lowed a host of species pre­vi­ously con­fined to main­land wa­ters to mi­grate south. The most sig­nif­i­cant of these is the long-spined sea urchin. A fa­mil­iar sight along the east­ern Aus­tralian coast, this marine an­i­mal is ru­inously vo­ra­cious, strip­ping ar­eas of sea­weed and other marine plants. Largely as a re­sult of urchin graz­ing, 95% of the kelp forests around Tas­ma­nia are now gone. To­gether with over­fish­ing, this has had a dev­as­tat­ing ef­fect on pop­u­la­tions of both abalone and lob­ster. Vergés ar­gues that these de­vel­op­ments pose a fun­da­men­tal chal­lenge to our ap­proach to marine con­ser­va­tion. Where a tra­di­tional ap­proach would aim to con­serve and re­tain his­tor­i­cal con­di­tions, trop­i­cal­i­sa­tion re­quires us to ac­knowl­edge both the in­evitabil­ity of species mov­ing be­yond their tra­di­tional ranges and the emer­gence of new ecosys­tems as ex­ist­ing habi­tats are trans­formed. An em­pha­sis on pre­serv­ing the con­nec­tiv­ity of the Great South­ern Reef’s com­po­nent parts would give species greater ca­pac­ity to re­spond to warm­ing wa­ters and to re-es­tab­lish them­selves after pe­ri­ods of loss. Vergés points to re­search that shows that kelp forests in marine re­serves have proved more re­silient than those else­where, per­haps be­cause preda­tor species ca­pa­ble of con­trol­ling in­vad­ing her­bi­vores are more com­mon in ar­eas where fish­ing is re­stricted. In this re­gard, Vergés is par­tic­u­larly alarmed by the fed­eral govern­ment’s re­cent pro­posal to al­low for ex­panded fish­ing and seafloor trawl­ing in marine pro­tected ar­eas.

Wern­berg ac­cepts that ris­ing wa­ter tem­per­a­tures is a “trend that only seems to run one way”. Yet he ar­gues that be­cause so many of the prob­lems faced by the Great South­ern Reef de­pend upon the in­ter­ac­tion be­tween warm­ing wa­ter and lo­cal fac­tors, in many in­stances its ca­pac­ity to with­stand cli­mate change can be strength­ened by lim­it­ing other stres­sors such as run-off, pol­lu­tion and over­fish­ing. These are small sliv­ers of hope. But while there is ev­i­dence some sea­weed beds can be re-es­tab­lished, many of the losses ex­pe­ri­enced to date are likely to be per­ma­nent. And, more omi­nously, Aus­tralia’s geog­ra­phy means there is only so far that the Great South­ern Reef can con­tract south­wards be­fore we start see­ing lo­cal, and pos­si­bly global, ex­tinc­tion events. “One of the rea­sons we coined the term Great South­ern Reef,” Wern­berg says, “[is] to get peo­ple to fo­cus on the fact we have a truly unique ecosys­tem here. We tend to for­get that.”

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