Restructuring the Reef

Tropical coral reefs are some of the richest and most beautiful habitats on our planet - supporting a remarkable array of biodiversity, not limited to just fish, but also sea cucumbers, star fish, sea turtles, rays and sharks. All these creatures live together as a part of a delicate balance, that we call an "ecosystem". As "apex predators" on the reef, sharks are critical to supporting the health of the reef ecosystem and, counterintuitively, to ensuring the survival of the reef species that they prey upon. So what would happen if we lost sharks? Can reefs survive without sharks?

Sharks are critical to the health of coral reef ecosystems (Image Credit: frantisekhojdysz / Shutterstock)

Reefs Support Shark Biodiversity

Coral reefs first appeared about 45 million years ago. Sharks had already been around hundreds of millions of years before this, but the emergence of reef habitats caused an explosion in shark evolution. Sharks underwent rapid diversification, as coral reefs supported so many different prey species. Today we see wide diversity of sharks we see around reefs, especially those of the Carchariniformes order (Roff et al, 2016).

But the interactions between organisms in an ecosystem do not only go one way! Whilst the reef is critical for shark populations, sharks are also vital for the health of the reef ecosystem!

Reefs support high biodiversity, including multiple species of sharks, like these lemon and Caribbean reef sharks (Image Credit: Albert Kok / Wikimedia Commons)

Reef Ecosystems Rely on Shark Populations to Remain Healthy

Sharks are predators. This means they eat other animals and are towards the top of the food chain (this is known as occupying high "trophic levels"). As they target weak, sick individuals as their prey, sharks are important for controlling the spread of disease in fishes that live around coral reefs. Sharks also aid in ecosystem health by controlling the population of invasive species like lion fish (Pterois species) (Roff et al, 2016).

In fact, many species of sharks act as "apex predators" on the reef, meaning they are the very top predator in the system. An apex predator exerts "top-down" control of an ecosystem because they eat other animals and therefore control the population sizes of the species lower in the food chain (Roff et al, 2016).

Food chain with a shark at the highest trophic level (Roff et al, 2016).

The presence of sharks creates "predation pressure, which affects the coral habitat both directly and indirectly:

Firstly, sharks prey upon smaller species, which directly limits population growth and controls the levels of herbivory. In reef habitats, herbivores consume fleshy macroalgae that outcompete the corals for space and thus, sharks help to enhance coral resilience (Roff et al, 2016).

Indirectly, the presence of predatory sharks affects the foraging and reproductive behaviour of their prey. Smaller animals will be more cautious when a shark is around. This also creates population control and balances the ecosystem (Roff et al, 2016).

Coral Reef Ecosystems are Very Complex

But not all sharks are apex predators! In fact, sharks occupy a diverse range of trophic levels in coral reef ecosystems. The large shark species, like tiger sharks (Galeocerdo cuvier) and great hammerheads (Sphyrna mokkaran), are most certainly apex predators on the reef. However, at lower trophic levels are the sharks of intermediate body size, such as grey reef shark (Carcharhinus amblyrhynchos) and epaulette sharks (Hemiscyllium ocellatum). These species are "mesopredators" (Roff et al, 2016).

Epaulette sharks are mesopredators, with small home ranges around coral reefs (Image Credit: Anne Hoggett, Lizard Island Research Station / Wikimedia Commons)

Sharks Are Critical for Nutrient Cycling

Sharks are not only important for the reef habitat, but they are also vital for transporting nutrients from the productive reef ecosystem into other areas. Animals often find their food in one location and defecate in a different area (basically they don't s#*t where they eat!). This means that the nutrients from their faeces are translocated from their original site to another region (Roff et al, 2016).

As predators, sharks occupy high trophic levels in the food web. Not all sharks occur the same levels though - smaller sharks are mesopredators and larger sharks are apex predators (Roff et al, 2016)

Mesopredator sharks often live permanently around coral reefs and have small home ranges, but larger, apex predator sharks are more transient and undergo long-range migrations (Roff et al, 2016).

The mesopredatory sharks are important for the movement of nutrients between the reef and adjacent habitats, where the apex sharks are responsible for relocating nutrients 1000s of kilometers between coastal and "pelagic" ocean regions (Roff et al, 2016).

The loss of either groups could have a serious impact on the presence of nutrients in many different marine habitats (Roff et al, 2016).

Some sharks are apex predators in reef ecosystems, where others are mesopredators (Roff et al, 2016).

The Loss of Sharks can Lead to Trophic Cascades

If sharks are removed from a habitat, it can have a huge impact on the balance within that ecosystem. This is because the loss of sharks results in "mesopredator release". When the top predator is no longer eating the mid-level predators, their populations can explode. This has a knock-on effect on all the different prey species at lower trophic levels and the whole systems shifts out of balance. This is known as a "trophic cascade" (Roff et al, 2016).

The loss of large apex sharks has already caused trophic cascades in some temperate marine ecosystems (Roff et al, 2016).

Each shark species has a different, yet equally vital, ecological role in a coral reef ecosystem (Roff et al, 2016)

It is important to note though, that categorisation of sharks as apex predators or mesopredators can be misleading because, in reality, the system is much more complex. The food web is dynamic and multifaceted, with multiple species occupying each trophic level. If a shark species was entirely removed, it may not necessarily lead to a trophic cascade, as another species (that performs a similar role in the ecosystem) might move in to replace them. The problem is that we cannot be certain what impact the loss of sharks could have (Roff et al, 2016).

Apex predator sharks, like the great hammerhead, are important for the long-range relocation of nutrients (Image Credit: Albert Kok / Wikimedia Commons)

We Cannot Predict How the Loss of Sharks Might Impact Fragile Reef Ecosystems

In recent years, reefs have undergone substantial declines in coral cover, reduced structural complexity and increasing algal dominance. There are serious concerns that with rising water temperatures and acidity associated with climate change, coral reefs will become increasingly less resilient and more rare (Roff et al, 2016).

Several studies have shown that sharks become noticeably less abundant when coral reefs are unhealthy. So the loss of reefs could mean we see a decline in reef shark populations. And it would not only sharks that will be affected! If we lose the coral reefs, the entire community that relies upon them would be decimated (Roff et al, 2016).

Sharks populations have been seriously depleted on a global scale and many species are now at risk of extinction. The loss of sharks will adversely affect the health of coral reefs and at the same time, the loss of coral reefs will contribute to a decline insharks. What a vicious circle! It is of vital importance that we make it a priority to conserve, not only endangered species themselves, but also the critical habitats that they call home (Roff et al, 2016).

A shark in a pristine reef habitat (Image Credit: Picasa / Wikimedia Commons)

References

Roff G, Doropoulos C, Rogers A, Bozec Y-M, Krueck NC, Aurellado E, Priest M, Birrell C & Mumby1 PJ (2016). The ecological role of sharks on coral reefs. Trends in Ecology and Evolution, 31:5, 395-589. Access online.

By Sophie A. Maycock for SharkSpeak.