Ecology is a complex science and so often, our simplest assumptions about how things work, don’t hold up in practice. Take sharks for example. How do you reduce shark attacks? We imagine more sharks means more attacks but in order to have fewer shark attacks, we probably need more sharks. We know other large marine predators like billfish (marlin, sailfish) are commonest near our coasts during El Niño. This is when warm water from the Eastern Tropical Pacific dominates the ocean’s top layer and a strengthened East Australian currentA warm water current that runs south down the east coast of Australia, starting in the Coral Sea before turning east and heading towards New Zealand. The south extent can sometimes reach south of Tasmania. The sea level can be almost a metre higher in the north (because the warm surface expands), causing it to flow 'downhill' at rates of More pushes nutrient-rich warm-water deeper. The depth of this mixing layerThermoclines are boundaries between gradients of temperature and salinity. The Mixing Layer Depth is usually defined as the depth where it is 0.5°C cooler than the surface temperature, that accounts for changes in salinity [1]. When the mixed layer is shallower, is has more prey abundance, because the boundary layer nutrients are easier to reach. Mixing layer depth varies from More determines how productively animals can feed. It takes more energy to reach deeper food.
How do mobile marine animals find food?
During these times, mobile animals like sharks and billfish, swim up against coastlines or islands, where the mixing layer is lifted closer to the surface, after it collides with the land (above, left). El Niño and La Niña are two opposites of a climate cycle that swings back and forth like a pendulum.
In La Niña, the surface is less warm, so the mixing layer is shallower (above, right). This doesn’t directly help the predators either. In these conditions, there is more surface nutrientEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More, so preferred food becomes thinly spread-out across the ocean, also making it a struggle to find enough to eat. Predators travel longer distances offshore in search of ‘hotspots’ and can’t gather together and breed.
This creates a contradictory situation.
Seabirds for example, have bumper breeding years during El Niño, when overall, there is least available (deeper) surface nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More. This is because extremely high-energy prey, such as filter-feeding anchovy are also forced nearer to islands where the mixing layer is shallow and where the birds breed. It becomes a lot easier for seabirds to raise chicks, when there is an abundance of prey in adjacent waters. Sharks benefit the same way, as they are feeding in the same food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More.
Predators and prey cycle together
Already you can see how an assumption about quantity of food versus quantity of animals can be the opposite of what we might expect. It’s not as simple as saying, more food equals more animals or more predators equals fewer prey.
Marine predators benefit from periods when there is less surface productivity … but only as long as there are plentiful periods in between. This boom-bust cycle, where animals expand and then contract geographically, is necessary to allow recovery of fish populations. Fluctuations in the numbers of predator and prey can then track each other closely over the long-term (see the example, below). The patterns are predictable.
Now imagine the peaks and troughs in these cycles as the amount of energy the system is absorbing. That’s it’s biodiversityWhat is the definition of biodiversity? When we ask, what is the definition of biodiversity? It depends on what we want to do with it. The term is widely and commonly misused, leading to significant misinterpretation of the importance of how animals function on Earth and why they matter a great deal, to human survival. Here I will try to More value and when the peaks are highest, there are more fish, more stable ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More and a robust and predictable ocean and atmospheric climate.
Removal of predators destabilises ecosystems
When we kill sharks through overfishing, bycatch, or in nets and drumlines, in an effort to reduce shark attacks, we make the situation worse, not better. Our efforts are counter-effective as we suppress the productivity of the ecosystem and the natural cyclical patterns break down.
Remember, these animals are selectively manipulating ecosystem processes over massive areas and through sheer mobility and abundance, doing things at a scale we cannot possibly comprehend or replace. The impact of animals like sharks is more awesome than anything science is currently able to describe–mostly because the sharks are gone, so there is nothing left to measure!
More shark attacks during La Niña definitely doesn’t mean there are more sharks. Fewer sharks should mean fewer attacks but that isn’t happening. The only other explanation, and the one that’s most likely from an ecosystem perspective, is that overall shark distribution has changed, forcing a higher proportion of remaining sharks into some coastal areas. There are also countless examples of where animals move closer to areas of human occupation, as these tend to be the last vestiges of habitat, for all animals, as we deplete other areas.
Proving exactly how this works would be virtually impossible but what we do know, is that the extinction of predators leads to a more chaotic system. We move from the rhythm of a steady stable-state(of an ecosystem) where free surplus energy is minimised, where there is maximum entropy production and minimum waste. In such a system, there is expected to be relatively small fluctuations in atmospheric and other chemistry and where disruption or disturbance occurs, the resulting changes can be absorbed quickly by a succession of new plants and animals that enter to fill More ecosystem to one that doesn’t conform to expectations, where neither other animals, or human fisheries, can depend on where and when to find food.
Chaos and disorder is not good for sharks or people
It’s this move towards disorder(Of energy and ecosystems). Ecosystems are thermodynamically driven. Disorder occurs when energy dissipates and becomes more chaotic. For example, the release of hot air into the atmosphere results in that energy is freer to disperse (maximum entropy). The opposite is true when energy is locked into biological processes, when it is stored inside molecules (minimum entropy). Stability in ecosystems occurs More that explains how removing top predators reduces ecosystem richness and predictability. We are looking in the wrong place for an explanation of what is happening to our planet. We need to learn to think in terms of order <-> chaos, rather than in terms of more <-> fewer animals.
The impact of animals like sharks is more awesome than anything science is currently able to describe–mostly because the sharks are gone, so there is nothing left to measure!
If you happen to live somewhere with an inherently higher-risk of shark attack, the risk may increase, when shark populations are destroyed elsewhere and overall ecosystem productionThe power of an ecosystem to process energy. The most productive ecosystems have reached a steady stable-state with maximum entropy production. That’s to say, the number of species has reached an optimum and the functions they fulfil, have translated free surplus energy into nutrients that is either stored inside plants and animals, or is entrained within the biological cycles that More has declined. You are observing an intermediate effect, where starving sharks move into coastal areas, desperate to find food, just like they would have during El Niño.
When you remove top predators from the ecosystem, it drives energy into lower levels of the food pyramid, to the advantage of animals like squid and jellyfish–this is not an alternative state, it’s a state of disorder that could take thousands of years to reset, once the predators are gone. This is already happening off West Africa and the Eastern Tropical Pacific and it’s collapsing whole fishing economies, threatening the livelihoods and food security of millions of people.
Sharks attacks is a sign of ecosystem stability collapse
This isn’t a situation confined to the poorest countries either. There are a litany of examples of fisheries collapse from the Grand Banks in Canada to the world’s entire stocks of the most commercially-valuable tuna. If you don’t think wealthy western societies depend on fish stocks, you’re very wrong. Even the final days of negotiating a Brexit deal for the UK in December 2019 were stalled by disagreements about fisheries allocation.
Squid and jellyfish not only compete against sharks, their smaller size and abundance, creates a lot more random waste and chaos – free surplus energyThe energy of a system that is emitted as waste and is not part of ecosystem processes. There is always some free surplus energy as this creates the basis for evolution where new species exploit gaps in the ecosystem where free energy becomes available. Surplus energy can occur as a result of disruption or disturbance. When free surplus energy reaches More – that sharks would have stabilised. As we continue to deplete the ocean, we flatten the peaks and troughs in the natural predator-prey cycles (above) and this leads to animal populations redistributing, breaking the ocean-climate-linked cycles that would shield us from global warming and further warming the ocean.
Sharks, like any large animal, shield us against climate instability by regulating the excess energy in a system that is constantly bombarded by solar radiation.
The latest report in the journal Nature [2] finds that we have lost 71% of the world’s sharks and rays since the 1970s.
Why won’t killing all the sharks make things better?
Ultimately, if you kill all the sharks, you’ll have no shark attacks. That’s for sure. In the short term though, you’re going to increase shark attacks.
There can be no doubt that this catastrophic loss of top predators is already having a huge impact on our fisheries and food security. If you think the only good shark is a dead shark but you like eating seafood or going fishing, then you’re going to end up disappointed.
Sharks are essential for the stability of food chains. Without them, we tip the balance in favour of a system that competes with sharks–we push ecosystem energy down the food chain to levels below that which we humans can survive in. When you consider sharks eat many of the same fish that we like to eat, we’re cutting our nose off to spite our face … we’re removing the sharks that make it possible for us to have healthy fisheries.
Meanwhile, if you want to live somewhere with fewer shark attacks, you aren’t going to make things any better by killing them. At best, you’re going to increase the uncertainty of shark attack and at worst, you might increase the number of sharks in certain places.
As a society we need to start understanding the ecological significance of our actions and make decisions to rebuild wildlife populations before it’s too late.
The question we need to ask ourselves is, can we afford to live in a world without sharks? How do you reduce shark attacks? I think the answer is obvious. Rebuild a better environment for people and sharks.
Spotlight
Another new conservationWhy is animal conservation important? Animal conservation is important, because animals are the only mechanism to create biodiversity, which is the mechanism that creates a habitable planet for humans. Without animals, the energy from today’s plants (algae, trees, flowers etc) will eventually reach the atmosphere and ocean, much of it as carbon. The quantity of this plant-based waste is so More frontier opens up as recreational fishermen call for shark culls due to ‘higher numbers’ of sharks. WA has suffered repeated marine heatwaves and collapse of ocean food chains are driving remaining sharks close to coasts. Killing more sharks isn’t the answer. Read more.
- MacLulich D A (1937) Fluctuations in the Number of the Varying Hare (Lepus americanus) (Univ. of Toronto Press, Toronto)
- Pacoureau, N., Rigby, C.L., Kyne, P.M. et al. Half a century of global decline in oceanic sharks and rays. Nature 589, 567–571 (2021). https://doi.org/10.1038/s41586-020-03173-9 https://www.nature.com/articles/s41586-020-03173-9
