3. Ecosystem health

Predation is the process of animals consuming other animals, and it occurs across varying levels of food webs. The tops of food webs are represented by apex predators, such as great hammerhead and tiger sharks (Section 2.4.8), pelagic fish, birds and marine mammals.⁸²³ Mesopredators (or middle-order predators) include many sharks, emperors, snappers, groupers ⁴⁰⁰ and marine turtles, which eat other animals but are also eaten by apex predators. 

Predation influences the composition and distribution of communities,⁸²⁴ affects behaviour and the evolution of characteristics in prey species ⁸²⁵ and, importantly, transfers energy from the pelagic zone to the reef environment.⁸²⁶

Since 2019, increasing evidence shows that predation may play an important role in controlling crown-of-thorns starfish populations during their planktonic, juvenile and adult life stages. Planktivorous damselfish target crown-of-thorns larvae despite the presence of alternative prey,^827,828 while 26 species of invertebrates have been found to prey upon juvenile crown-of-thorns,³⁴⁹ including the red decorator crab which can eat up to 5 juvenile crown-of-thorns starfish a day. The peppermint shrimp eats juvenile crown-of-thorns starfish (up to 4 months old) and damages older juveniles, through partial predation.⁸²⁹ Adult crown-of-thorns starfish are preyed upon by the giant triton snail⁸³⁰ and various species of fish.⁸²⁸ Crown-of-thorns starfish densities are reduced in no-take marine reserves (where predators are protected) compared to unprotected areas open to fishing, further suggesting that predation may help to reduce crown-of-thorns starfish outbreaks in the Region.⁸²⁸

Several factors affect the process of predation. Disturbance to habitat (for example, following cyclones or coral bleaching) can damage reef structure, potentially benefiting predators by reducing the availability of shelter for prey species.⁸³¹ Projected increases in sea surface temperature may see some species moving to cooler habitats to stay within their optimal thermal range ⁸³² or to maintain access to prey.⁸³³ Tiger sharks are known to move to higher latitudes during La Niña and lower latitudes during warmer years.⁸³³ The increase in temperatures may result in higher metabolic rates that need to be maintained,³⁷⁴ requiring species to eat more to survive. Ocean acidification has been shown to affect sensory systems, affecting how cues are received and processed.⁸³⁴ For example, prey fish may exhibit decreased directionality and responsiveness, while predators commence their strike from a larger distance, suggesting a higher motivation to feed.⁸³⁵

Fishing represents a threat to predation in the Region, through targeted removal of predators.⁸²³ The stock status (in 2020) of predatory fish and shark species fished in the Region is sustainable for some (for example, coral trout and several reef sharks) and depleted for others (for example, Australian snapper, pearl perch and Spanish mackerel) (Section 5.4). Predatory species are more abundant in protected areas.⁸³⁶ The Region’s ecosystems remain impacted by a long history of exploitation of apex predators, and populations of some targeted species remain in poor condition.^49,837

Evidence suggests there is substantial ecological redundancy among the diverse group of sharks and reef fish that act as mesopredators in the Region.^396,394 Although much less is known about predation by smaller fish of mobile invertebrates (other than crown-of-thorns starfish), natural rates of predation across the ecosystem are likely relatively stable due to the complexity and high functional redundancy of coral reef food webs,^394,772 and effective connectivity between coastal and offshore habitats across the Region.⁸³⁸

Depredation, where a predator completely or partially consumes an animal caught by fishing gear, is an emerging interest area.^839,840,841 Shark depredation has been recorded since the 1800s, and recent anecdotal evidence indicates frequency may be increasing in some areas.^407,842,843 The effect of depredation on the Region’s ecosystems is unknown; most studies are fisheries specific.⁸⁴⁴ 

Consideration of the process of predation often focuses on apex predators and mesopredators. Although there is knowledge about predation within other species, such as turtles eating jellyfish ^845,846 and invertebrates with varying diets, there is a gap in information regarding the effect of this other predation on prey populations and the Region as a whole. 

Predation affects a wide range of taxa that are subject to varying pressures. Although natural rates of predation are likely relatively stable, the Region’s ecosystems remain impacted by a long history of exploitation of apex predators and populations of some targeted species remain in poor condition. A large group of predators, the sharks and rays, are in poor condition.