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Restructuring the Reef

Updated: Apr 26

Tropical coral reefs support a remarkable array of marine biodiversity, not limited to just species of fish, but also sea cucumbers, star fish, sea turtles, rays and sharks. 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.


Image source: http://divemagazine.co.uk/

After the emergence of coral reefs approximately 45 million years ago, sharks underwent rapid diversification because prey availability and diversity exploded in these habitats. This allowed the evolution of the wide diversity of sharks we see around reefs today, especially those of the Carchariniformes order. many of these species act as "apex predators" on the reef.

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An apex predator exerts "top-down" effects on an ecosystem because they control the populations of the species below them in the food web. The loss of large apex sharks has caused "trophic cascades" in temperate marine ecosystems. Theoretically, the loss of sharks would result in an increase in mesopredators (known as "mesopredator release", impacting multiple different prey species at lower trophic levels. Therefore, scientists have concerns that removal of sharks (aka "trophic downgrading") could trigger shifts in nutrient cycling, and reduced ecosystem resilience and functioning in coral reef habitats.


Predator presence affects the coral habitat both directly and indirectly. Firstly, sharks prey upon smaller species below them, 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, they enhance coral resilience. Indirectly, the presence of predators affects the foraging and reproductive behaviour of their prey, which also creates population control and balances the ecosystem. These processes are known as "predation pressure" (Roff et al, 2016).


A recent study has confirmed that sharks occupy a diverse range of trophic roles in coral reef ecosystems. The large shark species: tiger sharks (Galeocerdo cuvier) and great hammerheads (Sphyrna mokkaran), are apex predators on the reef, but they are "transient", meaning they move in and out of the reef environment over a large home range. Below these sharks, the sharks of intermediate body size, such as grey reef shark (Carcharhinus amblyrhynchos) and epaulette sharks (Hemiscyllium ocellatum), are mesopredators and tend to be permanent inhabitants of the reef habitat.


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, with multiple species occupying each trophic level. Therefore, even if a shark species was entirely removed, the impact of the trophic cascade could possibly be buffered by other species, which perform a similar role in the ecosystem (Roff et al, 2016).


Comparative reef shark food webs: B. Shark apex predators, C. Shark mesopredators (Roff et al, 2016).

However, the removal of sharks could be detrimental to coral reef habitats in other ways. Firstly, sharks of the reef can be important for controlling the spread of disease within fish populations because they target weak, sick individuals as prey. Secondly, sharks can also aid in ecosystem health by controlling the population of of invasive species, such as the lionfish (Pterois species) (Roff et al, 2016).


Shark feeding on a lionfish (Image credit: YouTube user Patrick Explores)

Reef sharks are also critical for nutrient cycling. Animals often source their food in one location and defecate in a different area, meaning that the nutrients present in their faeces are translocated from their original site. Mesopredatory sharks which live permanently on the reef may have a home range of 50 km and are therefore responsible for the translocation of nutrients between the reef and adjacent habitats. Apex sharks, which undergo large-scale migrations are responsible for relocating nutrients 1000s of kilometers between coastal and "pelagic" ocean regions. Loss of the sharks could therefore have serious impacts on the presence of nutrients in multiple different ocean regions (Roff et al, 2016).


The interactions between organisms in a system do not only go one way; whilst the sharks are critical for the health of the reef ecosystem, the health of the reef is also critical for shark populations. Sharks need the coral and coral needs the sharks...


Corals before and after bleaching on the Great Barrier Reef, Australia (Image source: www.greenandgrowing.org)

In the past few decades, reefs have undergone substantial declines in coral cover, reduced structural complexity and increasing algal dominance. There are rising concerns that with rising water temperatures associated with climate change, coral reefs will become increasingly less resilient and more rare... If we lose the coral reefs the entire community that relies upon them will be decimated! For example, several studies have shown that where coral reefs have become unhealthy recently, sharks have markedly declined in abundance. Therefore, it will be of critical importance to work towards protecting these crucial habitats, whilst simultaneously protecting the endangered species which play a pivotal role in the health of the reef ecosystem (Roff et al, 2016).



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.

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