Losing half of tropical fish species as corals disappear

30 06 2021

When snorkelling in a reef, it’s natural to think of coral colonies as a colourful scenography where fish act in a play. But what would happen to the fish if the stage went suddenly empty, as in Peter Brook’s 1971 Midsummer Night’s Dream? Would the fish still be there acting their roles without a backdrop?

This question is not novel in coral-reef science. Ecologists have often compared reef fish diversity and biomass in selected localities before and after severe events of coral mortality. Even a temporary disappearance of corals might have substantial effects on fish communities, sometimes resulting in a local disappearance of more than half of local fish species.

Considering the multiple, complex ways fish interact with — and depend on — corals, this might appear as an obvious outcome. Still, such complexity of interactions makes it difficult to predict how the loss of corals might affect fish diversity in specific contexts, let alone at the global scale.

Focusing on species-specific fish-coral associations reveals an inconsistent picture with local-scale empirical observations. When looking at the fraction of local fish diversity that strictly depends on corals for food and other more generic habitat requirements (such as shelter and reproduction), the global picture suggests that most fish diversity in reef locality might persist in the absence of corals. 

The mismatch between this result and the empirical evidence of a stronger coral dependence suggests the existence of many hidden ecological paths connecting fish to corals, and that those paths might entrap many fish species for which the association to corals is not apparent.

The identity and relevance of such indirect connections for the functioning and persistence of natural communities is now a hot topic in theoretical ecology. But as much as it is becoming increasingly clear that direct and higher-order interactions are fundamental determinants of community stability, the challenges of unravelling such complexity makes the issue tractable only at a theoretical level or in simple systems characterised by low diversity. Which is not exactly the definition of coral reefs.

Still, predicting the potential effects of coral loss on fish communities globally is a fundamental task, especially considering that reef fish provides protein to millions of people.

To tackle the issue, we have just published a paper in Proceedings of the Royal Society B. We started from the consideration that if coral diversity begets fish diversity, then a relationship between the two was to be expected, which was the case (see figure below).

Current global distribution of coral richness, tropical fish richness, projected fish richness dependent on corals and projected fish richness remaining in a world without corals. (a) Current global distribution of coral genera richness; (b) current global distribution of tropical fish species richness; (c) projected tropical fish species dependent on coral; (d) projected global distribution of tropical fish richness in a coral-less world

However, such a relationship does not tell us much about fish-coral dependency, because part of the correlation might result from shared environmental preferences and biogeographical history.

We therefore devised a model to disentangle the effect of environment, biogeography, and history on fish and coral diversity. We obtained an equation capable of accurately predicting local-scale fish diversity as a response to several environmental variables (such as water temperature, pH and salinity) and coral diversity.

Besides offering a handy model to predict fish diversity under novel environmental conditions, the equations offered a powerful tool to explore how fish diversity will vary in response to changes in coral diversity. That is, it permitted us to answer the hypothetical (but not totally unrealistic) questions of how global patterns of fish diversity will look like in a coral-less world.

The answer is not encouraging and provides robust support to local-scale studies: according to our model, if all corals are lost, fish diversity will drop, on average, by more than 40%. The results for fish diversity in terms of species richness extend to fish phylogenetic and functional diversity. The decline in fish richness phylogenetic diversity would decline by a third, and functional diversity decline by a quarter.

In general, more realistic projections based on different scenarios of carbon emissions until the end of this century do not depict a much brighter picture. Still, they demonstrate that cutting emissions might make a massive difference for the future diversity of corals and fish. 

Giovanni Strona



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