Today I saw something I’ve never seen before. I was swimming at Table Rock inside the Rickett’s Point Marine Sanctuary when I found an abalone lying upside down. What was doing this, I thought? As I approached, so did a young snapper. But before either of us could inspect further a large but harmless Port Jackson Shark came in and snatched the abalone away. It swallowed it whole! It seems I had found my explanation for the upside-down abalone. I reckon he’d been prising them off the rocks and had returned to finish off a feed. Perhaps the standup paddleboarders had disturbed him earlier. What I was seeing, I realised, was part of how Port Jackson Sharks in Melbourne, abalone and urchins keep our beaches clean.
Sharks are the glue that hold ecosystems together
Port Jackson Sharks are top predators. They are like a cross between a shark and a ray. They have crushing plates evolved to feed on urchins and shellfish and are crucial to the balance and function(Of an ecosystem). A subset of ecosystem processes and structures, where the ecosystem does something that provides an ecosystem service of value to people. More of south Australian coastal reefs.
Once upon a time they would have been far more abundant. Port Jacksons used to be protected but now it’s legal for Melbourne fishers to catch and kill them. Another popular shark that is targeted by fishers all over the bay are Gummy Sharks (see below). These are rarely seen in the shallows. Both species feed on abalone and sea urchins.
It’s easy to assume that predators might have a negative effect on prey but nothing could be further from the truth.
Remove predators from a functioning ecosystem and it collapses like a house of cards. You need sharks that prune abalone, and abalone that prune seaweed, for example. The abalone diversify the seaweed but can’t survive without the urchins – young abalone hide under urchin spines. Remove urchins and your abalone population can collapse as juveniles become more accessible to predators, as was observed in South Africa.
So if you remove any component of an ecosystem you risk all the others. The absence of animals ‘pruning’ the edges of each other’s populations ruins the dynamic that keeps ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More in tip-top condition.
This experience today was a glimpse into the complex relationship that maintains the entire condition of our bay’s coastal reefs. But it’s so rare to see outside our marine parks.
Functioning ecosystems are naturally complex
Imagine what could happen if you begin killing resident urchins in a marine park, which is one of the only places where you’re likely to see natural predator recovery. This is what we are doing in our marine parks in Melbourne.
The fact is, when we interfere with the recovery of ecosystems we create more problems than we solve. We can’t ‘prune’ the ecosystem with enough precision. Even if we did, we might have to prune the predators too … and everything else. That’s a vicious spiral until there is nothing left.
I’ve written a few times about the importance of sea urchins and the dangers of sea urchin eradication in northern Port Phillip Bay. For starters, there’s no evidence that urchins are ‘overabundant’ (in fact, quite the contrary).
But these moves are in direct contradiction to everything we know about how to restore functioning ecosystems. You can’t fix a problem by creating another similar one. You can’t restore a functioning reef ecosystem by removing another layer of animal biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More if the problem started with the removal of the predator biomass in the first place.
The justification for this work, has never considered the relationship between abalone, urchins and sharks. It hasn’t even looked at the impact of urchin removal on coral reef, another relationship that survives only when predator populations are healthy.
Restoring clean beaches means leaving alone
For the most part, Port Phillip Bay has no top predators and few herbivores. Port Jackson Sharks, Gummy Sharks, large snapper, wrasse and grouper have been virtually wiped out. Our marine parks are one of the only places they are left and where natural processes can be allowed to rebuild. We shouldn’t be risking that by interfering during the recovery stage.
As a consequence of the lack of overall diversity and abundance of wildlife, our beaches and seas are dirtier, especially after rainfall and sewage spills, the likes of which have happened numerous times this summer.
Rebuilding resilience by letting nature take its course
Why are Melburnians not more concerned about pollution in our bay? Every day I read posts by well-doers parroting advice from the EPA regarding poor water quality. That advice is … don’t swim! But that’s hardly the point. Surely we should be shouting at those responsible for protecting the bay’s health … these are the same organisations advising we don’t swim (Melbourne Water, the EPA etc).
Meanwhile, the glorious interactions we see snorkelling in the bay, are a reminder of how important our wildlife is. As I discussed recently, even the world’s largest re-insurance company (Swiss Re) are starting to realise that loss of wildlife has had a greater impact than most well-known impacts such as climate and pollution. If you haven’t heard of Swiss Re, they underwrite most of the insurance you have protecting your health, homes and lifestyles.
What they are saying is, allow nature to rebuild itself and the outcomes are better, faster and cheaper.
Which is why we need to tackle pollution while ensuring there are lots of animals working together, to keep the whole system healthy. I’d love to see the return of Port Jackon Sharks and Gummy Sharks to all our coastal waters. Wouldn’t you prefer that, than be told not to swim in the sea?
