What eats sharks?Scientific writer, PhD
Fierce, aggressive, efficient and strong. Is your first image of a shark that of an agile hunter finning through the oceans? Or have you considered that the hunter is sometimes the hunted; that the ocean’s largest sharks are planktivores (eating microscopic plants and animals floating in the water column), scavengers or even the prey of other sharks and marine animals? Our perception of sharks might mean that we’ve tended to see them from only one point of view, but ecology is always complex enough to throw us a surprising curveball. Sharks are some of the ocean’s most amazing predators and this is part of what makes them important to the healthy functioning of the oceans. But sharks also play an important role in ocean ecology by being the prey of other animals that you might never have expected. Read on to learn about how sharks might rule the ocean in one ecosystem, but are food for anything from orcas and sperm whales to snails and baboons in others. We’re here to interrogate the traditional idea of the apex predator and refresh our old preconceptions of sharks at the top of the ocean food chain.
What is an apex predator?
Apex predators are carnivores (meat-eating animals) at the top of the trophic ladder (or food chain) that hunt other animals as prey. They seldom have natural predators of their own in their ecosystem. Shark species that are commonly considered apex predators include white sharks, tiger sharks and bull sharks. We know that ecosystems are a little more complex than a linear ‘chain’ of producers, consumers and predators, so ecologists usually talk about a food web. Apex predators are important because they influence the food web (and therefore the health and character of the greater ecosystem) by regulating populations of their prey. For this reason, certain apex predators can also be keystone species; if they are removed from the ecosystem, their loss leads to cascading effects that echo across the food web in what are called trophic cascades. In this way, apex predators also influence the population sizes of smaller and medium-sized carnivores (called mesopredators, or ‘middle’ predators) that sit lower down in the food web. The loss of apex predators can cause a ‘mesopredator release’. For example, a study by scientist Ransom Myers and colleagues shows that declines in populations of coastal shark species, including hammerhead, bull, tiger, dusky, sandbar and blacktip sharks, was associated with increases in the number of rays and smaller sharks. Another study by Jonathan Ruppert and fellow researchers suggests that the abundance of mesopredatory fish like snappers and breams (Lutjanidae and Lethrinidae) increased on reefs where sharks were heavily fished. These studies use the term ‘top-order’ predators to describe the ecological role of sharks in these instances.
Why are apex predators so rare?
Apex predators often share certain life-history characteristics, especially when they are large carnivores. They tend to grow slowly, live longer, reproduce later in life and have fewer offspring. These characteristics mean that their populations are smaller overall than those of species that breed earlier and often and produce higher numbers of offspring.
Their life-history characteristics also make them vulnerable to population declines when they are overexploited (which reduces their numbers) or lose habitat (space for their population to grow without the overcrowding that leads to competition and disease). Since apex predators typically have no natural predators in their ecosystem, their population size may be controlled by the abundance of prey available to them. Ecologists call this ‘bottom-up’ control. However, their population size can also be controlled by their exposure to disease, natural disasters and the threat of human activities (‘top-down’ control).
Apex predators are rare because they have smaller populations overall, which may be combined with life-history characteristics that make it difficult for their populations to recover if they are overexploited, are exposed to disease or disaster or they lose their habitat.
Why are sharks called apex predators?
Whether an animal is called an apex predator or not depends on a few definitions. The first is what one considers a predator. Many (but not all!) shark species are large carnivores, but only a very few species are considered to be at the top of the food web (and even then, only in a particular ecosystem or setting). They may eat a wide range of other animals, including fish, squid, marine mammals, crustaceans (crabs and shrimps), gastropods (snails) and other sharks. These characteristics define some shark species as predators if they hunt their prey rather than scavenge animals that are already dead.
A few sharks also have no natural predators in their specific habitat or ecosystem (that is, only until another predator moves into the area – but more about that later). For this reason, they could be considered apex predators, but only really in that context. Sharks that fit this definition include species of large, coastal sharks like white shark, bull shark and tiger shark. In the open ocean environment, species like the oceanic whitetip could be considered apex predators.
Large predatory sharks also tend to be what ecologists call K-strategists: they grow slowly, breed later, have fewer offspring and live longer. Many shark species invest significant energy in producing their young and so their population is smaller overall. This contrasts with other marine fish, which tend to be ‘live fast, die young’ R-strategists that broadcast spawn, producing millions of eggs that are fertilised externally. Although the K-strategy does not define sharks as apex predators, the life-history characteristics that make them particularly vulnerable are often shared with many other apex predator species. Overall, many predatory shark species are highly endangered and are threatened by overfishing, illegal trade, habitat loss, pollution and climate change – a predicament we can associate with many predators, but not really use to define sharks necessarily as apex predators
So large predatory sharks are sometimes considered top-order predators, but as you’ll come to see, the term ‘apex’ is problematic when it becomes fixed.
Are there predators on sharks?
Yes! Most sharks are not apex predators – and many are not even what we would typically think of as predators at all. In fact, sharks fill many different ecological roles . Having evolved in the ocean over more than 400 million years, sharks have become everything from the predators that we expect – hunting seals, other sharks and fish – to those we don’t, like planktivores (basking sharks and whale sharks devour microscopic plants and animals call plankton) and even scavengers (sixgill sharks patrol waters as deep as 2,500 metres, or 8,200 feet, and converge on whale carcasses that have sunk from the surface).
There are mesopredators that are eaten by larger sharks, fish, marine mammals and reptiles and there are even species that we’ve been taught are ‘apex’ predators that become mesopredators in the presence of a different species.
Many predatory sharks are eaten by other, larger sharks and marine mammals. In False Bay, South Africa, the entire order of a local food web was disrupted with the arrival of a new apex predator in the ecosystem: the orca . False Bay is an aggregation site for both white sharks and sevengill sharks. These two predators are thought to co-exist, eating largely the same diet, because sevengill sharks keep to the region’s kelp forests and hunt at night to avoid detection by white sharks, which prey on sevengills. However, the two male orcas, dubbed Port and Starboard, that were first seen in False Bay in 2009 gave ecologists a real-time view of how a large predator can be toppled from its position at the top of the local food web when they hunted white sharks and sevengill sharks, targeting their rich livers. These orcas were thought to belong to a shark-specialist ecotype that was new to the bay. In waters further offshore, this ecotype of orca has been documented eating blue and mako sharks caught on pelagic fishing. The arrival of Port and Starboard coincided with the local disappearance of white sharks and sevengill sharks; it is thought that, as the hunters became the hunted, these sharks moved out of the ecosystem to avoid predation, or changed their habits to avoid detection.
On the rocky Cape coastline of South Africa, the puffadder shyshark is a mesopredator that hunts small fish, crustaceans (crabs and shrimps), cephalopods (squids and octopuses) and polychaete worms. This shark is preyed on by other larger sharks (and even medium-sized sharks in the same ecosystem, like the pyjama shark). Astonishingly, this species was documented being preyed on by the Cape clawless otter, an aquatic land mammal that uses both marine and freshwater habitats. The Cape clawless otter has a flexible and wide diet that includes fish, crabs, frogs and insects – and clearly, when the opportunity presents itself, small sharks!
Even the largest predators must go through life stages when they are helpless, starting out in nurseries as young pups that are vulnerable to predators that they may otherwise not encounter. In the iSimangaliso Wetland Park and World Heritage Site in South Africa, Nile crocodiles have been recorded preying on bull shark pups. St Lucia, located at the southern end of the park, is Africa’s largest estuarine lake. Bull sharks are one of the very few shark and ray species that can move between fresh and salt water and the St Lucia estuary was a vital nursery area for bull shark pups. When the St Lucia estuary mouth closed between 2002 and 2021, bull shark pups disappeared from that system. However, the estuary mouth opened again on 6 January 2021 and bull shark pups very quickly began using the system after a 13-year hiatus. And it was here that a marine predator became prey for a freshwater predator – the Nile crocodile.
Some sharks lay eggs and the embryo developing inside the egg is at risk from a wide range of predators. Although shark egg cases are often tough and leathery to protect their contents, an egg without its adult parents to defend it makes for vulnerable prey. In Australia, large fish like the eastern blue groper, a wrasse that likes rocky habitats, have been recorded eating the eggs of Port Jackson sharks , which are buried in rocky crevices after the sharks amass on shallow, rocky coastal reefs to mate in the Australian winter season. The black stingray) also preys on the eggs of this endemic shark (found nowhere else in the world), but by far its main predator is another shark, the crested horn shark, which has been observed chewing the eggs and in one case, swallowing the egg whole!
The northern elephant seal and northern (Steller) sea lion have diets that are very varied. Both species are known for eating shark eggs, and the northern elephant seal is known to prey on adult sharks and rays too. Pinnipeds (seals and sea lions) aren’t the only marine mammals that have snacked on a shark. The sperm whale is a marine mammal that is considered an apex predator in certain habitats and it has been recorded as having eaten shark eggs.
Chacma baboons struggling to find food on land during the South African summer have learnt to forage for shark eggs at low tide. The rocky shores of Cape Point make for prime pickings for these adaptable primates. Using dextrous hands, they pull the eggs of endemic shyshark and catshark species from where they are entwined in the kelp and seaweed left exposed by the tide.
Many shark eggs are preyed on by marine snails called gastropods. One of the prime culprits in this group of animals is the whelk, a predatory snail that bores holes into its prey to slurp out the nutritious innards. A study on the coast of South Africa found that certain shark species may have responded to the high rates of predation by whelks by evolving to produce a larger number of eggs and having shorter incubation periods. The quicker the baby sharks hatch, the less time they spend vulnerable to egg predators. Scientists have even recorded the green urchin (a member of the group of animals called echinoderms, which includes sea stars and brittlestars) preying on the eggs of the little skate in the Gulf of Maine.
What is the apex predator in the ocean?
Orcas have no natural predators and although their diet depends on their ecotype or geographic location, they are all carnivorous predators. Orcas are unlike many sharks, whose tenuous (and contested!) title of ‘apex’ predator changes depending on the presence of other species in their ecosystem. However, if the definition of predator is broadened to include animals that hunt and use their prey, rather than only directly consume their prey, we can include human beings as apex predators that have developed technologies to overexploit all other apex predator populations. Even orca population numbers are controlled by our other activities: noise pollution from seismic surveys and shipping lanes, ship strikes, oil spills and the loss of their habitat and prey through overfishing.
Sharks are undoubtedly important for keeping our oceans healthy and functioning. However, their importance isn’t limited to the narrow ideas we’ve long held about their ferocity and position at the top of the food web. Even the biggest sharks risk predation by other animals and all are threatened by human activities. It’s all about ‘eat or be eaten’, and most hunting sharks become the hunted when only a few things in their ecosystem change. This still makes sharks essential in the ocean environment: they can influence populations of prey, but they can also maintain biodiversity (the richness of life on our planet) by becoming prey.
What eats sharks?
- Cape clawless otters
- Nile crocodiles
- Seals and sea lions
- Large fish
- Other sharks
- Chacma baboons
- Sperm whales
- Marine snails
- Sea urchins
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The Ecological Function of Apex Predators (ibiology.org) – The Ecological Function of Apex Predators
The Ecological Function of Apex Predators (ibiology.org) – Apex Predators: Sea Otters and Kelp Forests
The Ecological Function of Apex Predators (ibiology.org) – Apex Predators Around the World