From the Top to the Bottom

One of the most compelling arguments in support of shark conservation measures is the concern that the loss of sharks may have significant impacts on wider ecosystems; causing the collapse of fish stocks or the decline of coral reefs. It's great PR to encourage the general public to understand that sharks are pivotal players in marine ecosystems, however, in reality, it is hard to predict how the loss of sharks might affect an ecosystem. So, what is evidence that losing sharks will trigger trophic cascades? And what happens in these cases? Or is there actually evidence that shark-driven trophic cascades don't exist?

Theoretically, the loss of sharks could lead to trophic cascades so severe that even the plant life in their ecosystems would be affected (Image Credit: Lindsey Lu / Shutterstock)

On the Web

Trophic cascades are when changing the abundance of a certain organism in an ecosystem causing cascading shifts throughout other levels of the food web. These can include direct effects, such as an increase in predator populations causing populations of their prey to decline. The impacts can also be indirect, where the effects travel further through the food chain; effecting organisms one or more trophic levels away. For example, the increase in predators that caused their prey to decline could release pressure on plant life that these prey animals graze on, which could in turn affect the physical structure of the habitat they live in (Desbiens et al, 2021; Hammerschlag et al, 2025).

Scientists are concerned that the loss of sharks could lead to trophic cascades that affect all organisms at every level of their food chains (Roff et al,

Trophic cascades can also have an effect on organisms in different ways. The most obvious is that the removal of a predator will cause prey populations to rise, as fewer individuals are eaten by the carnivores. However, the behaviour of prey animals can also be significantly altered because of "predator release" - they will be less inhibited by the constant threat of predators, so can be free to feed and mate with less caution. This can subsequently have an impact on their habitat or organisms beneath them in the food web. So the removal of a top predator can theoretically affect multiple layers of the food web (Desbiens et al, 2021; Hammerschlag et al, 2025).To learn more, check out Restructuring the Reef.

Wolfing Down

The most famous example of a significant multi-level trophic cascade happening in a natural system is that of the loss and subsequent reintroduction of wolves into Yellowstone National Park. Wolves were eradicated from this ecosystem in the 1920s, which released the elk from predation pressure. However, the reintroduction of a pack of wolves in the mid-1990s not only caused the elk populations to decline back to their previous levels, but also had an effect on the physical habitat. Under the threat of predation, the elk changed their grazing habits; avoiding the open areas they had confidently grazed before, causing shrubs and trees to recover back to their previous coverage. Amazingly, this has even changed the flow of the river through the system, as the altered vegetation affected the soil and water runoff in the area (Ripple & Beschta, 2012).

No Fear

The presence or absence of sharks has been shown to directly cause trophic cascades in their ecosystems. For example, scientists have discovered a textbook trophic cascade after the loss of great white sharks (Carcharodon carcharias) from their stomping grounds in False Bay, South Africa (Hammerschlag et al, 2025).

White sharks exert top-down control over their food webs (Image Credit: Sergey Uryadnikov / Shutterstock)

To learn more, see Where Have All the White Sharks Gone? 

Whilst they were once found there year-round, great white sharks started to decline from 2015 onwards and are now completely absent from False Bay. As a result, their prey species have experienced significant behavioural and physiological changes. Cape fur seals (Arctocephalus pusillus pusillus) experiencing less stress from the threat of predation shifted their movement patterns and their population boomed with a massive 520% increase in numbers. This in turn caused a reduction in the abundance of the seals' prey and cape horse mackerel (Trachurus capensis) populations declined by as much as 61% in the region. This is evidence of a three-level cascade (Hammerschlag et al, 2025).

The abundance of pyjama sharks has been affected by the absence of white sharks in South Africa (Image Credit: Henryp982 / Shutterstock)

Additionally, in the great whites' absence, broadnose sevengill sharks (Notorynchus cepedianus) began to forage in new areas, where they would previously have been vulnerable to predation by the larger sharks. Where they were never seen prior, these sharks began to be spotted every hour during boat surveys of the region. For more info, head over to When the Great Whites are Away the Sevengills Will Play. As a result, the abundance of their prey items: smoothhounds (family Triakidae) and pyjama sharks (Poroderma africanum), began to drop - by as much as 50% (Hammerschlag et al, 2025).

Scientists suspect that the release from predation pressure is allowing the lower level predators, like sevengills and cape fur seals, to forage more effectively and with less caution. Therefore, in this case there is clear evidence that great whites exert "top-down" control over their ecosystem by controlling population numbers and altering behaviour through fear (Hammerschlag et al, 2025).

There is evidence that the loss of white sharks has altered the habitat use of broadnose sevengills in False Bay, South Africa (Image Credit: Tomas Kotouc / Shutterstock)

Fear Factor

Sharks are also known to be able to bring about trophic cascades through fear alone. For example, the absence of sharks can affect algae or seaweed coverage across coral reefs. A study conducted in Fiji, showed that the presence of sharks - including blacktip reef sharks, (Carcharhinus melanopterus), whitetip reef sharks (Triaenodon obesus) and tawny nurse sharks (Nebrius ferrugineus) - in an area created 'hot-spots of fear' that altered how herbivorous prey species behaved. As a result, reduced grazing in these areas meant that seaweed coverage was significantly higher than in parts of the reef where sharks were absent. This could mean that already-threatened reef systems may be subject to additional stress, potentially contributing to the decline of coral reef coverage (Rizzari et al, 2014; Rasher et al, 2017; Desbiens et al, 2021).

Scientists have proven that shark presence alters the algal coverage on reefs, so they are critical to the health of coral ecosystems (Image Credit: Elmer Laahne PHOTOGRAPHY/ Shutterstock)

Bottoms Up

On the other hand, evidence for cascading ecosystem effects over multiple trophic levels after the loss of sharks is quite sparse and it is unclear whether the effects that have been confirmed are likely to actually have a serious impact on the ecosystem as a whole. The effects tend to be strongest in the closer levels and dissipate as you move further away through trophic levels, and no study has yet to prove that the loss of sharks causes cascades that affect every layer in the system (Grubbs et al, 2016; Desbiens et al, 2021; Hammerschlag et al, 2025).

By changing the abundances of their prey species, sharks can actually alter the physical habitat and producer abundance. Therefore, overfishing sharks can lead to changes in coral and algae coverage (Casey et al, 2017)

In fact, there are several studies that have proved that no trophic cascade has occurred in the absence of sharks. Research conducted on the Great Barrier Reef in Australia, for example, found that the amount of sharks in an area did not have an impact on the density of mesopredators or prey. In this case, the absence of sharks did not lead to higher levels of middle predators and the subsequent suppression of algae-eating herbivores. Thus, the concern that the loss of sharks will lead to algae-dominated reefs might not always hold true (Casey et al, 2017; Desbiens et al, 2021).

The evidence for or against shark-driven trophic cascades is conflicting, so scientists are unsure how serious they could be (Image Credit: Tomas Kotouc / Shutterstock)

"[The] effects [of] predator removal from oceanic food webs remain unpredictable"

Casey et al, 2017

Similarly, a study conducted around the Caribbean reef ecosystems found that the loss of apex predator sharks did not result in any change in the density of their prey species, suggesting there was little to no top-down control of these populations by the sharks (Desbiens et al, 2021).

Scientists suspect that in these cases where the loss of sharks has little to no trophic cascading may be due to "functional redundancy" in the system. This means that the food webs are so complex, with some many organisms involved, that if one is absent, another creature fills their niche. In these large ecosystems, sharks may not be such strong regulators of community structure as we thought (Grubbs et al, 2016; Desbiens et al, 2021).

Scientists have proven that the removal of tiger sharks alters herbivores activity around seagrass beds in Western Australia (Imagr Credit: Kris Mikael Krister / WikimediaCommons)

Mad Scientists

Alarmingly, there have been examples of scientific studies that have been conducted so poorly that their claims of finding trophic cascades were completely misleading. The classic example is that of a series of reports which claimed that the loss of large predatory sharks in the northwest Atlantic had caused the collapse of a centuries-old scallop fishery in North Carolina. The researchers claimed that the declines of several large shark species had released cownose rays (Rhinoptera bonasus) from predation pressure, so their booming populations had chomped their way through the scallop population (Grubbs et al, 2011; Myers et al, 2007; Casey et al, 2017).

Poorly-conducted science meant cownose rays were blamed for the declines of scallops in the northwest Atlantic ocean (Image Credit: Cliff from Arlington / WikimediaCommons)

However, it has since been determined that these papers were all based bogus science; with poor methodologies and massive jumps to extreme conclusions that were not supported by the data. In reality there was no evidence at all for a correlation between shark / ray / scallop numbers (Grubbs et al, 2011; Grubbs et al, 2016).

The dissemination of this inaccurate information actually resulted in the decline of cownose rays in the region, as local campaigns to 'Save the Bay' by eating a ray caused fishers to actively target them (Grubbs et al, 2011; Grubbs et al, 2016).

"Hypothesized trophic cascades should be closely scrutinized as spurious conclusions may negatively influence conservation and management decisions".

Grubbs et al, 2016

As marine food webs are so complex, with several organisms occupying similar niches, scientists suspect that in some cases the loss of predatory sharks may not cause such significant trophic cascades as we previously thought (Desbiens et al, 2021)

Better Safe Than Sorry

Large, apex predator sharks are amongst the most threatened animals in the world today. Massive overfishing, habitat loss and degredation, and the effects of climate change are driving many species close to the brink of extinction in the wild (Hammerschlag et al, 2025)

Therefore, there is a very real need to understand how the loss of sharks may impact upon wider marine ecosystems. As there are some cases where trophic cascades have been triggered we have serious cause for concern. So potentially, until we can be absolutely certain that the evidence is definitive, we should probably assume the worst case scenario applies to every case and anticipate that we always need to reserve our sharks in order to protect our marine ecosystems. It's better to be safe than sorry!

References

Casey JM, Baird AH, Brandl SJ, Hoogenboom MO, Rizzari JR, Frisch AJ, Mirbach CE & Connolly SR (2017). A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs. Oecologia, 183. Access online.

Desbiens AA, Roff G, Robbins WD, Taylor BM, Castro‐Sanguino C, Dempsey A & Mumby PJ (2021). Revisiting the paradigm of shark‐driven trophic cascades in coral reef ecosystems. Ecology, 102:4, e03303. Access online.

Grubbs RD, Carlson JK, Romine JG, Curtis T & McElroy D (2011). Save the bay, eat a ray: A purported trophic cascade mediated by declines in large shark populations and the consequences of applying simplistic models to complex ecosystems. Conference abstract. In 141st American Fisheries Society meeting. American Fisheries Society, CITY, Maryland. Access online.

Grubbs RD, Carlson JK, Romine JG, Curtis TH, McElroy WD, McCandless CT, Cotton CF & Musick JA (2016). Critical assessment and ramifications of a purported marine trophic cascade. Scientific reports, 6, 20970. Access online.

Hammerschlag N, Herskowitz Y, Fallows C & Couto T (2025). Evidence of cascading ecosystem effects following the loss of white sharks from False Bay, South Africa. Frontiers in Marine Science, 12, 1530362. Access online.

Myers RA, Baum JK, Shepherd TD, Powers SP & Peterson CH (2007). Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science, 315:5820. Access online.

Rasher DB, Hoey AS & Hay ME (2017). Cascading predator effects in a Fijian coral reef ecosystem. Scientific Reports, 7, 15684. Access online.

Ripple WJ & Beschta RL (2012). Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction. Biological Conservation, 145. Access online.

Rizzari JR, Frisch AJ, Hoey AS & McCormick M (2014). Not worth the risk: apex predators suppress herbivory on coral reefs. Oikos, 123. Access online.

Roff G, Doropoulos C, Rogers A, Bozec YM, Krueck NC, Aurellado E, Priest M, Birrell C & Mumby1 PJ (2016). Reassessing shark-driven trophic cascades on coral reefs: A reply to Ruppert et al. Trends in Ecology & Evolution, 31:8. Access online.

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