Why do we need sharks?PhD
All Shapes and Sizes, Forms and Functions
Sharks have been on our planet for 450 million years. They have evolved into many different forms, from streamlined sharks that cruise at top speeds to flattened rays that sail the sea floor. Sharks can be the swift predators we imagine from epic documentaries, but they can also be small and camouflaged like shysharks; gigantic planktivores like whale and basking sharks; or deep-water scavengers like sixgill sharks.
Worldwide, there are more than 500 different types (species) of sharks. That number rises to more than 1,300 if you include the skates, rays and chimaeras that make up the group of cartilaginous fish we call chondrichthyans (fish with cartilage skeletons). This diversity means that there are many sharks playing different roles and functioning in various ways across all habitats and depths of the ocean.
Healthy Sharks, Healthy Oceans, Healthy Humans?
We often hear that we need healthy oceans if we are to survive and thrive. We also hear the well-worn message that sharks help to keep our oceans healthy by maintaining a delicate balance of life. But what exactly does that mean? How do sharks keep our oceans – and therefore - ourselves healthy?
Answering this question seems intuitive, but to be truly scientific about it we have to look at some of the evidence of the ecological roles that sharks play in different ocean habitats. The trouble is, good marine science is challenging. Answering complex questions to give measurable results (what scientists call empirical evidence) takes many years, significant funding and no small amount of effort and innovation. With so many shark species in the oceans, and the sheer scale of those oceans, we’ve got lifetimes of work ahead of us to understand all the different roles that sharks play.
We’ve pieced together some stories of fascinating research that is digging into the specifics of how some sharks function in different habitats and what that means for the oceans – and for us. It may feel like the tip of a very large iceberg for now, but that’s the exciting thing about research and our incredible natural world: there is so much to discover!
How do sharks keep our oceans healthy?
A landscape of fear
Top predators – those animals at the apex of the food web – can shape how ecosystems look. Not necessarily only by controlling populations numbers by eating prey, but also by influencing how other animals in that ecosystem behave.
You might know of the example of the wolves that were reintroduced to Yellowstone National Park. Initially, scientists dubbed the effect of their return the 'landscape of fear’. Elk, which are grazers that influence the parks' vegetation, started avoiding certain areas at times of the day or night when the threat of these returned predators was greatest. The result? The elk couldn’t graze whole areas all day and their grazing pressure on certain areas was reduced.
Evidence from Shark Bay in Australia now shows that tiger sharks may play a similar role in the seagrass meadows where green turtles graze. Scientist Dr Michael Heithaus and a team of international researchers investigated the idea that as key predators of green turtles, tiger sharks may help regulate the populations of their prey – but more importantly, they influence the behaviour of their prey. In areas where tiger shark populations are healthy, green turtles avoid high-risk grazing areas, allowing sea-grass meadows to flourish. Where tiger shark numbers have dwindled, populations of green turtles boom and they feel bold enough to roam the sea-grass meadows at will. The result? The scientists’ trials in places like Bermuda and Indonesia showed that where turtles overgraze seagrass meadows, the whole ecosystem can change.
But why would the return of tiger sharks matter to us? Well, seagrass meadows happen to be incredible carbon sinks that capture and store carbon from the atmosphere, and are therefore a critical asset in our bid to mitigate climate change. Healthy tiger shark populations could mean healthier seagrass meadows, helping us slow the worst effects of our changing climate.
Shovelling and shaping
Elephants, beavers and termites. What do these animals all have in common?
They’re what scientists call ecosystem engineers: creatures whose habits and behaviours can shape how a habitat looks and functions. Elephants do this by pulling down trees, beavers by creating dams and termites by tunnelling their way through a subterranean world, determining the soil profile as they go.
Proving that an animal is an ecosystem engineer is tricky, but there have been studies looking at the shovelling, snuffling and sucking that flat rays (the group scientists call batoids) do on sand flats, which may qualify as 'engineering'. Rays rooting around in the sand in their search for delectable prawns, snails and other invertebrates create pits and indentations in the seafloor and kick up flurries of sand that can change the water clarity and flick little creatures secreted in the sand (the infauna) into the water column. The effect is called bioturbation and nutrient cycling. Scientist Dr Jeremy Vaudo also showed that the behaviour of rays shuffling around the seafloor in shallow coastal seas may be influenced by the same ‘landscape of fear’, which means that the sandflats may only be shaped when these rays feel bold enough to access them, but are left to die when the risk of predation is high.
When considering why sharks are needed, we often overlook their role as couriers that transport nutrients from one habitat to another. Whether diving deep or migrating thousands of kilometres, they often feed in one location and deposit their waste in another, helping to cycle nutrients and connect disparate marine ecosystems.
For example, scientists in Seychelles were scratching their heads at a conundrum: warm tropical waters are often nutrient-poor relative to the temperate (cooler) parts of our oceans. However, some isolated tropical coral reef systems have been shown to be hotspots of productivity, their waters containing enough nutrients to host abundant life.
What – or who – was responsible for enriching these stand-out ecosystems?
One hypothesis was that highly mobile animals moving great distances, like sharks, turtles and whales, might contribute by bringing nutrients from further afield. Dr Lauren Peel and the team from the Save Our Seas Foundation D’Arros Research Centre (SOSF-DRC) set about testing what reef manta rays were eating. They found that 45% of the rays' diet consisted of pelagic zooplankton, tiny animals near the ocean’s surface. Another 38% of their diet was made up of emergent zooplankton that rise from the seabed at night, while the remaining 17% comprised mesopelagic plankton, found in deeper waters further offshore.
Lauren’s study showed that reef mantas were munching pelagic zooplankton in the St Joseph Channel during the day, and then socialising at a coral reef cleaning station to the north of D’Arros Island. The researchers believe that while these mantas fraternise and are being deep-cleaned, they deposit their faeces that may be packed with nutrients picked up from feeding at night, in deeper waters, and in different areas. Thus reef manta faeces could be a fertiliser of sorts, helping corals to flourish. While reef manta rays are not the only factor contributing to the health of these coral reef systems, studies like this help us tease apart a complex web of life that may be disrupted if we push its resilience too far.
But what do reef manta rays do for us? Coral reefs are incredible ecosystems worth billions to our economies, feeding people with the fish that live there and providing ecotourism income. Healthy coral reefs can protect coastlines from huge storms and they are important to many people for recreational, cultural and spiritual reasons. If reef manta rays are contributing in any way to the better functioning of these beleaguered systems, then we can be grateful for the indirect effects of these ocean gardeners.
A treasure trove of knowledge
As we continue to gather incredible stories of how sharks, rays and their kin function in the oceans, there are other non-ecological reasons why sharks are important players in our marine ecosystems.
The long period of time that sharks have ruled the oceans has given them an evolutionary advantage over the relative ‘blink of an eye’ that human-beings have dominated the earth. What has enabled these animals to persist so successfully and for so long? We still have much to learn from them. Biomimicry experts (those innovators who look to nature to find sustainable solutions to modern-day problems) have used shark skin as inspiration for streamlining in aeronautical engineering and Olympic swimwear. Prof. Mahmood Shivji, the director of the Save Our Seas Foundation Shark Research Center in Florida, has shown that understanding the genetics of sharks might advance our own medical research and help us gain new insights into human disease, wound healing and how our own genetics function. We stand to lose a lot more than we know when we lose shark species from our oceans: we lose not only their current function, but their potential too.
A link to our livelihood
Many philosophers and environmentalists would argue that we need sharks in our oceans simply because they have what is called an intrinsic value. They argue that nature has a value, irrespective of whether it benefits human-beings or not. The persistence of sharks in our oceans should have nothing to do with whether we need them or not: they have their own, intrinsic right to exist.
However, we can also show that sharks have a numeric value. We rely on sharks for income as fishers or tour operators, for recreation (many divers love to seek out the diversity of sharks while snorkelling or on scuba-diving) and for cultural or spiritual ties. Researcher Dr Ruth Leeney has detailed the deep ties that many communities still have with sharks and rays, celebrating their existence in art and stories and revering them in their belief systems. These are links to sharks and rays that are sometimes more difficult to make sense of than hard numbers and empirical evidence, but these animals' ability to help us make sense of our place in the natural world and keep us connected to it may be equally important.
Finding space for sharks and ourselves
Perhaps there’s something to be said for those who support the intrinsic value of nature. What might our world look like without the eccentricity of a wobbegong shark sailing, tassled and camouflaged, along the seafloor? What else do we stand to lose when the last whale shark earns a dive operator a final paycheque, or a reef manta no longer brings its well-travelled fertiliser to a reef? Could our planet, the only known one in our solar system with such vibrant life and bewildering diversity, simply just feel poorer for losing one more of the 500 species of sharks that have found how to make their way in the oceans over millennia?
Would we even notice?
A lot more funding and effort are needed to support research into exactly what role sharks play in the myriad of habitats and depths of the ocean. The trouble is, sound science and conservation action don’t always act on the same timescale. At times, the speed at which we’re damaging ecosystems outstrips our ability to investigate thoroughly how they function.
No one wants to jump to conclusions and most good scientists hold it is a matter of pride to conduct sound scientific studies. In law and philosophy, we are familiar with the idea of the precautionary principle. The idea is to pause, reflect and accumulate more information before making decisions that may have consequences that we cannot have foreseen.
Sharks matter. We might not yet know exactly how they all matter, or what role they play in every one of the diverse ocean ecosystems on earth, but the question to ask yourself after reading these snippets is whether you now too have an intuitive sense that we need sharks in our seas? The reasons may be as varied as the sharks themselves, and they might feel as personal as your own connection to the natural world. While our understanding of sharks grows and evolves, it appears that we’re going to need them if we’re to make sense of our changing world, and our place in it.
Lauren R. Peel, et al., 2019, Stable isotope analyses reveal unique trophic role of reef manta rays (Mobula alfredi) at a remote coral reef, Royal Publishing Society.
Michael R. Heithaus, et al., 2014, Seagrasses in the age of sea turtle conservation and shark overfishing, Frontiers in Marine Science.
Melanie Fyda, 2019, Foraging ecology and behavior of batoids and their influence on coastal sandflats .