Several species of sharks are described as apex predators in their communities, including, but not limited to the great white (Carcharodon carcharias), the broadnose sevengill shark (Notorynchus cepedianus) and the tiger shark (Galeocerdo cuvier). These guys are the biggest, baddest, beasts in their habitats; all the smaller guys run from them and no-one is able to eat them. Right? Well, after several great whites were found up washed on beaches dead in South Africa, scientists have started to ask questions. We are now learning that the place of an apex predator may not be as black and white after all...
Top Dog(s)
You may be familiar with the term “apex predator”, which scientists use to describe a species which is at the top of its respective food chain, above all other predators. Apex predators are critical for ecosystems health and maintenance of biodiversity, as they exert “top-down” control of the system; balancing mesopredator, herbivore and producer populations below them in the chain (Roff et al, 2016).
But ecosystems are anything but simple and in reality food webs can have more than one apex predator co-existing within one habitat. This can occur because each top-dog occupies a slightly different “niche” (the position of a species and its ecological role within an ecosystem), so apex predator species are able to persist side-by-side (Roff et al, 2016).
For axample, in False Bay, South Africa both great whites and sevengills can be found. Both predate upon pinnipeds (such as cape fur seals (Arctocephalus pusillus pusillus)) and cetaceans (such as common dolphins (Delphinus delphis)) and other, smaller sharks (such as bronze whaler sharks (Carcharhinus brachyurus)), so both are apex predators. Yet it could be argued the great white ultimately comes out on top because they will also hunt the sevengills. To avoid this, the seven gills take refuge in kelp forests that white sharks are reluctant to enter (Engelbrecht et al, 2019).
All Change
Scientists studying the marine ecosystems in the area have noted that how sharks use the space in False Bay has been shifting. Where the sevengills normally lived in the kelp forests of Miller's Point and the great whites roamed around within the bay, since the end of 2015, there has been multiple incidents when sevengills have very suddenly disappeared from their kelp forest homes overnight. Sadly, these have been followed by the discovery of sevengill carcasses in the area (Engelbrecht et al, 2019).
Similarly, the numbers of great whites in the area has been steadily declining, to an 18-year-low in 2018. There have also been several incidents of white shark carcasses being discovered, which were followed by periods of several days, or even weeks, when white sharks were entirely absent in False Bay (Hammerschlag et al, 2019).
What was particularly baffling is that the carcasses of these sharks were mostly completely intact with only the pectoral girdle broken open and the liver removed through the tear in the abdominal wall (Engelbrecht et al, 2019). So what on Earth is going on!?
The New Boss
Since 2009 there has been a marked increase in the abundance of orca (Orcinus orca) in False Bay. In the past, these orca preyed upon dolphins and humpback whales (Megaptera novaeangliae), so they occupied a different niche to that of the sharks (Engelbrecht et al, 2019).
However, in 2015 when local ecotourism charters started reported sightings of a completely new pod of orca in the region around the time of sevengill and great white deaths, experts started to wonder if they had found their suspects (cue the CSI theme music!). Later reports of orca hunting large sharks cinched it...
We now know that orca predate large sharks in South Africa. This means that the previous balance of apex predatory white sharks and sevengills has been completely upended by the orca's new behaviour (Engelbrecht et al, 2019).
Why Now?
The question that remained was, why have orca suddenly started hunting large sharks in South Africa? Pods of orca have been transient residents in South Africa in the past without such effects being reported in the shark community. So what changed?
Orca pods have highly organised social structures based upon a matriarchy and each pod tailors its own unique diet and behaviour. They can even have noticable differences in markings between different groups. It has even been suggested that vocalisations are so specialised within a group, that they are something akin to a dialect. We refer to these differences as “ecotypes”, meaning that, despite being classified as a single species, orca are capable of varying enormously between pods, similar to cultural differences in human beings. For instance, some orca pods are specialised to target marine mammal prey predominantly, whereas other groups take a more generalist approach and have a broader diet. It might be surprising, but it's true that pods of orca are certainly capable of hunting large sharks. But not all groups of orca bother (de Bruyn et al, 2013; Foote et al, 2016; Engelbrecht et al, 2019).
It seems that, there is now a new pod of orca that have started hunting for food in South Africa. These orca, named Port and Starboard after their opposing bent dorsal fins, are a different ecotype to other pods in the area, as they are partial to shark liver snacks. Scientists supect that the appearance of these new apex predators is likely to have played a role in the decline of white sharks at their aggregation sites throughout South Africa (Engelbrecht et al, 2019; Towner et al, 2022). For more on this topic, you can check out Where Have All the White Sharks Gone?
We cannot predict whether these shark killing sprees will become a regular occurrence in the future, or whether other groups of orca will follow Port and Starboard’s example, but what we do know is that apex predatory sharks are critical to the health of the marine ecosystem in South Africa. If shark populations decline due to increasing predation or because they flee the area, this could have a catastrophic effect on the rest of the ecosystem known as a “trophic cascade”. Future research will be critical to understand why these orca have shifted their ecotype to target sharks in Soutgh Africa because if anthropogenic activity (such as pollution, overfishing or climate change) has caused the shift in their behaviour, it is possible that management strategies will need to be implemented in order to vulnerabl protect South African sharks (Engelbrecht et al, 2019; Towner et al, 2022).
Acknowledgements
Great thanks go to Tami Kaschke for the use of her photographs in this article.
References
de Bruyn PJN, Tosh CA & Terauds A (2013). Killer whale ecotypes: is there a global model? Biological Reviews, 88:1, 62-80. Access online.
Engelbrecht TM, Kock AA & O’Riain MJ (2019). Running scared: When predators become prey. Ecosphere, 10:1, e02531. Access online.
Foote AD, Vijay N, Ávila-Arcos MC, Baird RW, Fumagalli M, Gibbs RA, Hanson MB, Korneliussen TS, Martin MD, Robertson KM, Sousa VC, Durban JW, Veira FG, Vinař T, Wade P, Worley KC, Excoffier L, Morin PA, Gilbert MTP & Wolf JBW (2016). Genome-culture coevolution promotes rapid divergence of killer whale ecotypes. Nature Communications , 7:11693. 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, 9:1908. 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. Access online.
Towner AV, Watson RGA, Kock AA, Papastamatiou Y, Sturup M, Gennari E, Baker K, Booth, T, Dicken M, Chivell W & Elwen S (2022). Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site. African Journal of Marine Science, 44:2, 139-152. Access online.
By Sophie A. Maycock for SharkSpeak
