When coral dies, tiny invertebrates boom. This could dramatically change the food web on the Great Barrier Reef

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Kate Fraser, University of TasmaniaThis week, international ambassadors will take a snorkelling trip to the Great Barrier Reef as part of the Australian government’s efforts to stop the reef getting on the world heritage “in danger” list.

The World Heritage Centre of UNESCO is set to make its final decision on whether to officially brand the reef as “in danger” later this month.

To many coral reef researchers like myself, who have witnessed firsthand the increasing coral bleaching and cyclone-driven destruction of this global icon, an in-danger listing comes as no surprise.

But the implications of mass coral death are complex — just because coral is dying doesn’t mean marine life there will end. Instead, it will change.

In recent research, my colleagues and I discovered dead coral hosted 100 times more microscopic invertebrates than healthy coral. This means up to 100 times more fish food is available on reefs dominated by dead coral compared with live, healthy coral.

This is a near-invisible consequence of coral death, with dramatic implications for reef food webs.

When coral dies

Tiny, mobile invertebrates — between 0.125 and 4 millimetres in size — are ubiquitous inhabitants of the surfaces of all reef structures and are the main food source for approximately 70% of fish species on the Great Barrier Reef.

These invertebrates, most visible only under a microscope, are commonly known as “epifauna” and include species of crustaceans, molluscs, and polychaete worms.

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When corals die, their skeletons are quickly overgrown by fine, thread-like “turfing algae”. Turf-covered coral skeletons then break down into beds of rubble.

We wanted to find out how the tiny epifaunal invertebrates — upon which many fish depend – might respond to the widespread replacement of live healthy coral with dead, turf-covered coral.

A sample of epifauna under the microscope.
Kate Fraser

I took my SCUBA gear and a box of lab equipment, and dived into a series of reefs across eastern Australia, from the Solitary Islands in New South Wales to Lizard Island on the northern Great Barrier Reef.

Underwater, I carefully gathered into sandwich bags the tiny invertebrates living on various species of live coral and those living on dead, turf-covered coral.

But things really got interesting back in the laboratory under the microscope. I sorted each sandwich bag sample of epifauna into sizes, identified them as best I could (many, if not most, species remain unknown to science), and counted them.

I quickly noticed samples taken from live coral took just minutes to count, whereas samples from dead coral could take hours. There were exponentially more animals in the dead coral samples.

The Great Barrier Reef may soon be listed as ‘in danger’
Rick Stuart-Smith

Why do they prefer dead coral?

Counting individual invertebrates is only so useful when considering their contribution to the food web. So we instead used the much more useful metric of “productivity”, which looks at how much weight (biomass) of organisms is produced daily for a given area of reef.

We found epifaunal productivity was far greater on dead, turf-covered coral. The main contributors were the tiniest epifauna — thousands of harpacticoid copepods (a type of crustacean) an eighth of a millimetre in size.

In contrast, coral crabs and glass shrimp contributed the most productivity to epifaunal communities on live coral. At one millimetre and larger, these animals are relative giants in the epifaunal world, with fewer than ten individuals in most live coral samples.

Dead coral rubble overgrown with turfing algae.
Rick Stuart-Smith

These striking differences may be explained by two things.

First: shelter. Live coral may look complex to the naked eye, but if you zoom in you’ll find turfing algae has more structural complexity that tiny epifauna can hide in, protecting them from predators.

A coral head is actually a community of individual coral polyps, each with a tiny mouth and fine tentacles to trap prey. To smaller epifauna, such as harpacticoid copepods, the surface of live coral is a wall of mouths and a very undesirable habitat.

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Second: food. Many epifauna, regardless of size, are herbivores (plant-eaters) or detritivores (organic waste-eaters). Turfing algae is a brilliant trap for fine detritus and an excellent substrate for growing films of even smaller microscopic algae.

This means dead coral overgrown by turfing algae represents a smorgasbord of food options for the tiniest epifauna through to the largest.

Meanwhile, many larger epifauna like coral crabs have evolved to live exclusively on live coral, eating the mucus that covers the polyps or particles trapped by the polyps themselves.

Harpacticoid copepod are just an eighth of a millimetre in size.
Naukhan/Wikimedia, CC BY

What this means for life on the reef?

As corals reefs continue to decline, we can expect increased productivity at the base level of reef food webs, with a shift from larger crabs and shrimp to small harpacticoid copepods.

This will affect the flow of food and energy throughout reef food webs, markedly changing the structure of fish and other animal communities. The abundance of animals that eat invertebrates will likely boom with increased coral death.

We might expect higher numbers of fish such as wrasses, cardinalfish, triggerfish, and dragonets, with species preferring the smallest epifauna most likely to flourish.

The dragonet species, mandarinfish, feeds on the smallest harpacticoid copepod prey.
Rick Stuart-Smith

Invertebrate-eating animals are food for a diversity of carnivores on a coral reef, and most fish Australians want to eat are carnivores, such as coral trout, snapper, and Spanish mackerel.

While we didn’t investigate exactly which species are likely to increase following widespread coral death, it’s safe to say populations of fish targeted by recreational and commercial fisheries on Australia’s coral reefs are likely to change as live coral is lost, some for better and some for worse.

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The Great Barrier Reef is undoubtedly in danger, and it’s important that we make every effort to protect and conserve the remaining live, healthy coral. However, if corals continue to die, there will remain an abundance of life in their absence, albeit very different life from that to which we are accustomed.

As long as there is hard structure for algae to grow on, there will be epifauna. And where there is epifauna, there is food for fish, although perhaps not for all the fish we want to eat.

Kate Fraser, Marine Ecologist, University of Tasmania

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Repeating mistakes: why the plan to protect the world’s wildlife falls short

The forty-spotted pardalote is one of Australia’s rarest birds.
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Michelle Lim, Macquarie UniversityIt’s no secret the world’s wildlife is in dire straits. New data shows a heatwave in the Pacific Northwest killed more than 1 billion sea creatures in June, while Australia’s devastating bushfires of 2019-2020 killed or displaced 3 billion animals. Indeed, 1 million species face extinction worldwide.

These numbers are overwhelming, but a serious global commitment can help reverse current tragic rates of biodiversity loss.

This week the UN’s Convention on Biological Diversity released a draft of its newest ten-year global plan. Often considered to be the Paris Agreement of biodiversity, the new plan aims to galvanise planetary scale action to achieve a world “living in harmony with nature” by 2050.

But if the plan goes ahead in its current form, it will fall short in safeguarding the wonder of our natural world. This is primarily because it doesn’t legally bind nations to it, risking the same mistakes made by the last ten-year plan, which didn’t stop biodiversity decline.

A lack of binding obligations

The Convention on Biological Diversity is a significant global agreement and almost all countries are parties to it. This includes Australia, which holds the unwanted record for the greatest number of mammal extinctions since European colonisation.

However, the convention is plagued by the lack of binding obligations. Self-reporting to the convention secretariat is the only thing the convention makes countries do under international law.

All other, otherwise sensible, provisions of the convention are limited by a series of get-out-of-jail clauses. Countries are only required to implement provisions “subject to national legislation” or “as far as possible and as appropriate”.

The convention has used non-binding targets since 2000 in its attempt to address global biodiversity loss. But this has not worked.

More than 3 billion animals were killed or displaced as a result of the 2019-2020 bushfires.
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The ten-year term of the previous targets, the Aichi Targets, came to an end in 2020, and included halving habitat loss and preventing extinction. But these, alongside most other Aichi targets, were not met.

In the new draft targets, extinction is no longer specifically named — perhaps relegated to the too hard basket. Pollution appears again in the new targets, and now includes a specific mention of eliminating plastic pollution.

Is this really a Paris-style agreement?

I wish. Calling the plan a Paris-style agreement suggests it has legal weight, when it doesn’t.

The fundamental difference between the biodiversity plan and the Paris Agreement is that binding commitments are a key component of the Paris Agreement. This is because the Paris Agreement is the successor of the legally binding Kyoto Protocol.

The final Paris Agreement legally compels countries to state how much they will reduce their emissions by. Nations are then expected to commit to increasingly ambitious reductions every five years.

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If they don’t fulfil these commitments, countries could be in breach of international law. This risks damage to countries’ reputation and international standing.

The door remains open for some form of binding commitment to emerge from the biodiversity convention. But negotiations to date have included almost no mention of this being a potential outcome.

Ecosystems humans rely on are in peril, such as the Great Barrier Reef which was recently recommended to be placed on the world heritage ‘in danger’ list.
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So what else needs to change?

Alongside binding agreements, there are many other aspects of the convention’s plan that must change. Here are three:

First, we need truly transformative measures to tackle the underlying economic and social causes of biodiversity loss.

The plan’s first eight targets are directed at minimising the threats to biodiversity, such as the harvesting and trade of wild species, area-based conservation, climate change and pollution.

While this is important, the plan also needs to call out and tackle dominant worldviews which equate continuous economic growth with human well-being. The first eight targets cannot realistically be met unless we address the economic causes driving these threats: materialism, unsustainable production and over-consumption.

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‘Revolutionary change’ needed to stop unprecedented global extinction crisis

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Second, the plan needs to put Indigenous peoples’ knowledge, science, governance, rights and voices front and centre.

An abundance of evidence shows lands managed by Indigenous and local communities have significantly better biodiversity outcomes. But biodiversity on Indigenous lands is decreasing and with it the knowledge for continued sustainable management of these ecosystems.

Indigenous peoples and local communities have “observer status” within the convention’s discussions, but references to Indigenous “knowledges” and “participation” in the draft plan don’t go much further than in the Aichi Targets.

Actions in one part of the globe can have significant impacts to biodiversity in other parts.
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Third, there must be cross-scale collaborations as global economic, social and environmental systems are connected like never before.

The unprecedented movement of people and goods and the exchange of money, information and resources means actions in one part of the globe can have significant biodiversity impacts in faraway lands. The draft framework does not sufficiently appreciate this.

For example, global demand for palm oil contributes to deforestation of orangutan habitat in Borneo. At the same time, consumer awareness and social media campaigns in countries far from palm plantations enable distant people to help make a positive difference.

The road to Kunming

The next round of preliminary negotiations of the draft framework will take place virtually from August 23 to September 3 2021. And it’s likely final in-person negotiations in Kunming, China will be postponed until 2022.

It’s not all bad news, there is still much to commend in the convention’s current draft plan.

For example, the plan facilitates connections with other global processes, such as the UN’s Sustainable Development Goals. It recognises the contributions of biodiversity to, for instance, nutrition and food security, echoing Sustainable Development Goal 2 of “zero hunger”.

The plan also embraces more inclusive language, such as a shift from saying “ecosystem services” to “Nature’s Contribution to People” when discussing nature’s multiple values.

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But if non-binding targets didn’t work in the past, then why does the convention think this time will be any different?

A further set of unmet biodiversity goals and targets in 2030 is an unacceptable scenario. At the same time, there’s no point aiming at targets that merely maintain the status quo.

We can change the current path of mass extinction. This requires urgent, concerted and transformative action towards a thriving planet for people and nature.

Michelle Lim, Senior Lecturer, Macquarie Law School, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.