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Why are predators so important for reef biodiversity?

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A pickhandle barracuda (Sphyraena jello) swims in the open ocean where it can single out its prey and engage in a high-speed chase.

With streamlined bodies, and fins that are well adapted for quick movements, barracudas are built for their role as an apex (top of the food web) predator in reef ecosystems. Apex predators play an important role in controlling populations of mesopredators (organisms found in the middle of a food web) and are essential for maintaining biodiversity across the entire reef.

Mesopredators feed on a range of smaller organisms, many of which are algae grazers. Having grazers on the reef is essential in order to prevent fast growing algae from outcompeting and overgrowing live corals and other encrusting organisms. If a community of mesopredators were to become significantly high, this would likely reduce the number of grazers in that environment as a higher proportion of them would be predated on. This may have several indirect effects on the survival and growth of corals and other reef building organisms that attach to substrates as algal communities would no longer be controlled. Predation of mesopredators from apex predators controls the size of mesopredator communities, allowing a high number of algae grazers to be sustained on the reef.

Certain species living together on a reef may share similar ecological niches to one another (roles played within a community). Species that do may compete with each other when trying to gain access to a shared or limited resource. If several different organisms compete for the same resource, it is possible that a superior species may outcompete all other inferior species and dominate that area. This would likely inhibit certain species from living in that area, resulting in a significant reduction in biodiversity.

The diet of most apex predators is typically varied. These predators will commonly feed on several different prey species living in a particular area. For individuals with a varied diet, predation often favours the prey species that exists in relatively high abundance due to members of this species being encountered more often than other species. As predation rates increase with population size, this prevents any species from reaching a size where they are likely to outcompete other species, therefore allowing several species to coexist with one another at any given time. This process, known as density dependant predation is common amongst apex predators and plays an extremely important role in maintaining biodiversity of organisms that occupy a similar ecological niche on the reef.

Apex predators are often pelagic (living slightly off the reef, in open ocean), and are an easy target for fishing. Furthermore, the value of barracudas and other similar pelagic fish is significantly high, and so fishing efforts are often focussed on these apex predators. In many areas of the world, fishing practices are largely unregulated and are therefore conducted in an unsustainable manner. This may cause populations of pelagic fish species to decline, often resulting in the removal of apex predators from an ecosystem and having significant consequences to nearby reefs. Before populations that are heavily fished can be managed, it is essential to have a good estimated size of an adult stock. This allows figures to be generated for total allowable catches (TACs) in order to prevent overfishing of a certain species.

Monitoring programs running at the Roctopus ecoTrust conduct ongoing surveys of pelagic fish populations and include several species that play important roles as apex predators on nearby reefs. Many species included in these surveys are commonly fished, yet lack sufficient data on their population trends. Ongoing monitoring of these pelagic fish communities aim to identify populations trends that exist for key predators of the reef, identify potential areas which are being affected by overfishing, and monitor the effects that heavy fishing may have on other reef organisms.

 

 -- Click here to see more of Piers' underwater photos of marine organisms. -- 

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