When most people hear the word 'shark', what often comes to mind is a magnificent archetypal predator; large, strong and toothy... like something from an action movie. A real "apex predator". Whilst there are certainly some sharks like this, not all species of sharks are at the top of their food chains. In fact, many sharks are quite small and have multiple natural predators themselves. So which sharks are the apex predators? Which are lower on the food chain? And what exactly is an apex predator anyway?
Pass It On
The transfer of energy in an ecosystem is a complex and dynamic process... To simplify it all, the entire web can be divided into different layers known as "trophic levels"; with organisms on lower trophic levels being consumed by animals in higher levels. Energy flows from top to bottom - transferred from one life-form to another when they eat each other. Some energy is lost along the way, which is why you often find the population sizes decrease as you go up the food chain and you only have very small populations of apex predators relative to the animals they eat (Roff et al, 2016).
All energy that animals use for movement and growth comes originally from creatures called "producers" or "autotrophs". Right at the very bottom trophic level, these creatures harness the energy from the sun or from chemicals like sulphur, and convert it to organic matter in their bodies. For example, plants and algae convert solar energy into sugars through a process known as "photosynthesis".
These producers are then eaten by "herbivores". Slightly higher in the food chain, these animals use the energy they gain from plant material for their own growth. These "consumers" are then subsequently eaten by 'predators', which are higher again in the food chain again.
At the Top of the Game
An 'apex predator' is an animal at the very top of their food chain. This means that they are the absolute top predator; consuming other animals at lower trophic levels and having very few (if any) natural predators. For example, the great hammerhead (Sphyrna makkaron) is an apex predator. These guys feed on other large 'carnivores', like bony fishes and even other sharks. Other examples include tiger sharks (Galeocerdo cuvier), bull sharks (Carcharias leucas), and oceanic whitetips (Carcharhinus longimanus) amongst others (Roff et al, 2016).
Apex sharks have gained some notoriety in the media in recent years, as people have become increasingly aware of the major impacts the loss of apex predators can have on an ecosystem. Apex predators exert 'top-down control' over their food web. This means that they influence all the levels of all the animals below them and are important for maintaining the balance of the system. When the apex predators are removed, the ecosystem can shift and lead to drastic "trophic cascades". It has been shown that the removal of apex predatory sharks can lead to population explosions of their prey, which subsequently can lead to smothering and death of entire coral reefs! (Ferretti et al, 2010; Roff et al, 2016).
Top Dogs
The term apex predator can be a little misleading though because it implies that a single species is top dog, but in fact, it is also possible to have more than one apex predator in an ecosystem! In South Africa, great whites (Carcharodon carcharias) are certainly an apex predator - feeding on large seals and other sharks - but they are not alone. There are also broadnose sevengills (Notorynchus cepedianus) and tiger sharks (Galeocerdo cuvier) that are at the top of the food chain, and they all compete with each other for many of the same resources. To learn more you can check out When the great Whites are Away the Sevengills will Play (Hammerschlag et al, 2019).
In recent years South African great whites have even become prey themselves to another apex predator into the area - the orca. To learn more head over to Where Have All the White Sharks Gone? and Clash of the Marine Titans (Hammerschlag et al, 2019).
It is thought that an ecosystem with multiple apex predators is likely to be comparatively robust and able to withstand trophic cascades, because if one species declines, another can come in to take its place (Hammerschlag et al, 2019).
Sharks in the Middle
On the other hand, not all sharks are apex predators! Some sharks are described as 'mesopredators' (meaning middle predator). These animals are eaten by apex predators above them, but feed on animals at even lower trophic levels beneath them. Many mesopredator sharks are even food for other species of sharks at higher trophic levels (Roff et al, 2016).
For example, the pyjama shark (Poroderma africanum) is a voracious predator, renowned for aggressive hunting of octopi and squid [You might recognise pyjama sharks from their appearance in the film My Octopus Teacher, available on Netflix. If you have not seen this film, I would thoroughly recommend it!]. Yet these sharks are not the apex predator in their habitat... The multiple apex predatory sharks in South Africa will all snack on the smaller pyjama sharks if they get the chance (Hammerschlag et al, 2019).
Realistically the vast majority of shark species fall into the category of mesopredators. With over 540 species of sharks currently known to science, I don't think I should list them all here... but a few examples include the catsharks (Family Scyliorhinidae), epaulette and bamboo sharks (Family Hemiscyllidae), smoothhounds (Family Triakidae) and dogfish (Family Squalidae) (Roff et al, 2016).
Size Doesn't Matter!
Whilst it is great that the general public are now increasingly concerned about shark declines and aware of the threat of trophic cascades, it important to note that it is not only apex predatory sharks which are declining worldwide... Overfishing, and habitat degradation and loss has meant that many species of mesopredatory sharks have also been decimated (Dulvy et al, 2014). To learn more, see Will 300+ Species of Sharks Go Extinct?
Tope sharks (Galeorhinus galeus) for instance, are now classified as critically endangered by the IUCN because they have been so severely overexploited by fisheries.
It is not just the loss of apex predator sharks which we need to worry about, because declines of mesopredatory sharks could also seriously alter ecosystem health and significantly change marine habitats (and be a terrible shame!). So, whilst it's great that apex sharks are getting media attention, let's not forget about their smaller, less flashy cousins, who are also just as important!
If you would like to learn more about trophic cascades in shark ecosystems, you can read The Significance of Sharks and Restructuring the Reef.
References
Dulvy NK, Fowler SL, Musick JA, Cavanagh RD, Kyne PM, Harrison LR, Carlson JK, Davidson LNK, Fordham SV, Francis MP, Pollock CM, Simpfendorfer CA, Burgess GTH, Carpenter KE, Compagno LJV, Ebert DA, Gibson C, Heupel MR, Livingstone SR, Sanciangco JC, Stevens JD, Valenti S, White WT (2014). Extinction risk and conservation of the world’s sharks andrays. eLife , 3: e00590. Access online.
Ferretti F, Worm M, Britten G, Heithaus M & Lotze H (2010). Patterns and ecosystem consequences of shark declines in the ocean. Ecology Letters, 13, 1055–1071. Access online.
Hammerschlag N, Williams L, Fallows M & Fallows C (2019). Disappearance of white sharks leads to the novel emergence of an allopatric apex predator, the sevengill shark. Scientific Reports, 1:9, doi: 10.1038/s41598-018-37576-6. Access online.
Roff G, Doropoulos C, Rogers A, Bozec Y-M, Krueck NC, Aurellado E, Priest M, Birrell C & Mumby 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
