Sharks sit at or near the top of marine food webs, and their influence ripples far beyond the animals they eat. When shark populations decline, the effects cascade through entire ecosystems: mid-level predators balloon in number, prey species get hammered, and habitats like seagrass beds and coral reefs can shift in ways that affect everything from fisheries to carbon storage. That cascading influence is what makes sharks a keystone species, an animal whose ecological impact is disproportionately large relative to its abundance. But the story is richer, and messier, than a simple chain of cause and effect.
How Removing Sharks Reshapes Food Webs
The most dramatic evidence for sharks’ keystone role comes from watching what happens when they disappear. In the coastal northwest Atlantic, populations of 11 large shark species that feed on rays, skates, and smaller sharks declined steeply over about 35 years. As those top predators vanished, 12 of 14 of their prey species increased in abundance. The best-documented consequence involved the cownose ray, which surged in numbers and devoured bay scallops so aggressively that a century-old scallop fishery collapsed entirely.1PubMed. Cascading effects of the loss of apex predatory sharks from a coastal ocean That sequence, top predator lost, mid-level predator surges, prey of mid-level predator wiped out, is a textbook trophic cascade, and it illustrated how removing one functional group of predators can unravel an entire community.
This was not an isolated case study. In Shark Bay, Australia, researchers found that tiger sharks initiated what ecologists call a behaviour-mediated trophic cascade. Large grazers like dugongs and sea turtles changed where and how they fed depending on whether tiger sharks were around, and those behavioral shifts had measurable effects on seagrass ecosystems.2PubMed. Patterns of top-down control in a seagrass ecosystem: could a roving apex predator induce a behaviour-mediated trophic cascade? The sharks did not need to kill every grazer to reshape the habitat. Their mere presence was enough to redirect the behavior of the animals below them.
The Landscape of Fear
This behavioral effect deserves its own discussion, because it may be more ecologically important than direct predation. When sharks patrol an area, the animals they might eat change their feeding patterns, their movements, and even how much of the water column they use. Researchers studying dugongs in Shark Bay found that when tiger sharks were scarce, dugongs foraged in shallow and deep habitats in proportion to how much food was available. When sharks were common, dugongs abandoned the richer but riskier shallow waters and crowded into deeper, safer areas with less food. The shift tracked daily shark abundance almost perfectly.3PubMed. Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)?
The same fear-driven reshuffling shows up in temperate waters. Along a coastal gradient, smooth dogfish in areas with high large-shark exposure were roughly six times less likely to contact bait and more than eight times less likely to actually bite compared to dogfish in low-exposure areas.4PLoS ONE. Effects of exposure to large sharks on the abundance and behavior of mobile prey fishes along a temperate coastal gradient On coral reefs, mid-level predatory fish confronted with a life-sized shark model restricted their foraging to a tight hemisphere around shelter sites, with the vertical axis hit hardest. They would not venture far above or below their hiding spots, effectively shrinking the volume of reef they were willing to exploit.5Oikos. The hemisphere of fear: the presence of sharks influences the three dimensional behaviour of large mesopredators in a coral reef ecosystem
Why does all of this matter for the ecosystem? Because when mid-level predators are too scared to forage aggressively in certain areas, the species they eat get a reprieve. Seagrass keeps growing in zones that grazers avoid. Small reef fish survive in pockets where their predators will not venture. The fear sharks create acts as a kind of invisible zoning regulation across the habitat, distributing grazing and predation pressure more evenly and preventing any single species from stripping an area bare.
Seagrass, Carbon, and the Chain of Consequences
The link between sharks and seagrass meadows has implications that extend well beyond marine ecology. Seagrass beds are among the most effective carbon sinks on the planet, locking away organic carbon in their root systems and sediments. When shark-driven fear keeps grazers like sea turtles from overgrazing these meadows, more seagrass survives, and more carbon stays buried. Experiments on green turtle grazing showed that intermediate levels of grazing actually produced the highest rates of nutrient cycling and carbon storage. But intense grazing, the kind unchecked by predators, caused a collapse of ecosystem functions across the board.6Global Change Biology. Seagrass ecosystem multifunctionality under the rise of a flagship marine megaherbivore In other words, a moderate amount of grazing is healthy; unregulated grazing is destructive. Sharks help maintain that balance.
Sharks also contribute to ocean carbon cycling more directly. When large marine fish die naturally, their carcasses sink to the deep ocean, transporting carbon far below the surface where it can be locked away in sediments for long periods. This is the opposite of what happens on land, where dead organisms generally release their carbon into the atmosphere.7PubMed Central. Let more big fish sink: Fisheries prevent blue carbon sequestration-half in unprofitable areas Carcasses of large species like whale sharks and manta rays can represent a meaningful transfer of carbon from the surface to the deep sea, potentially remaining sequestered in sediments for millions of years.8One Earth. Integral functions of marine vertebrates in the ocean carbon cycle and climate change mitigation Fisheries that remove these animals before they die naturally short-circuit that process.
Sharks as Nutrient Couriers
Beyond predation and fear, sharks play a less obvious role as nutrient transporters. Many species travel vast distances between feeding and resting grounds, and as they move, they carry nutrients in their bodies from one ecosystem to another. A shark that feeds in the deep ocean and rests in shallow coastal waters, for instance, effectively shuttles energy between those two systems. A study of diverse elasmobranch populations found that these animals promoted energetic connectivity between nearshore, open-ocean, and deep-sea ecosystems, contributing to ecological stability and resilience.9PubMed Central. Energetic connectivity of diverse elasmobranch populations – implications for ecological resilience A 2024 review in Science emphasized that sharks can function simultaneously as predators, competitors, facilitators, nutrient transporters, and even as food for other species.10PubMed. Ecological roles and importance of sharks in the Anthropocene Ocean That multifunctionality is part of what makes their loss so destabilizing: you are not just removing a predator, you are removing a nutrient delivery system and a behavioral regulator at the same time.
Where the Evidence Gets Complicated
It would be tidy to say that every reef and coastline with healthy shark populations thrives, and every one without them degrades. But ecology rarely works that way. On the northern Great Barrier Reef, researchers found that a fourfold difference in reef shark density across survey sites had no detectable impact on the density or biomass of mid-level predatory fish or their prey. There was no evidence of trophic cascading at all. Instead, both sharks and many other functional groups responded to environmental drivers like habitat complexity.11Ecology. Revisiting the paradigm of shark-driven trophic cascades in coral reef ecosystems
The same 2024 Science review that catalogued sharks’ ecological roles acknowledged this directly: large ecosystem effects of sharks are not ubiquitous. Context matters enormously. The strength of a shark’s influence depends on the type of ecosystem, the species involved, the abundance of alternative predators, and the complexity of the food web. On some reefs, other large predators like groupers may fill similar functional roles, diluting the shark-specific signal. In open-ocean pelagic systems, the dynamics differ again. Treating “sharks” as a monolithic ecological force oversimplifies what is actually a patchwork of local interactions, some strong, some weak, some undetectable with current methods.
This does not undermine the keystone argument so much as refine it. The strongest evidence for shark-driven cascades comes from systems where a single large shark species dominates the predator guild and where the prey community has few alternative pressures. Shark Bay’s tiger sharks and the northwest Atlantic’s great sharks are compelling examples precisely because those systems had relatively simple predator-prey dynamics. On a complex coral reef with dozens of predatory species, the signal of any one group gets diluted.
Not All Sharks Fill the Same Niche
There are over 500 described species of sharks, and they occupy an enormous range of ecological roles. Some are apex predators of open water. Others are bottom-dwelling invertebrate crunchers. Still others are filter feeders that consume plankton. Lumping them all into one functional category makes about as much sense as lumping hummingbirds and eagles into one category because both are birds.
Research on coastal shark communities in eastern Australia used a combination of ecological and physical traits to map out the functional space sharks occupy: habitat preference (reef, coastal pelagic, oceanic, benthic), feeding group (invertebrate feeders, small-fish feeders, large-fish feeders, megafauna feeders), movement scale, tooth shape, body size, and head dimensions. The range of functional roles was wide, and critically, species that looked similar on the surface sometimes occupied very different niches.12PubMed Central. Long term declines in the functional diversity of sharks in the coastal oceans of eastern Australia When populations decline, it is not just a numbers game. Losing a reef-associated invertebrate feeder removes a different ecological function than losing an oceanic apex predator, even though both are “sharks.” The concept of functional diversity captures this: as shark communities lose species, they lose not just abundance but the breadth of ecological roles those species collectively filled.
Why Shark Populations Recover So Slowly
The ecological importance of sharks collides with a brutal biological reality: most shark species are exceptionally slow to bounce back from population declines. They mature late, reproduce infrequently, and produce few offspring per reproductive cycle. The pelagic thresher shark, for example, reaches sexual maturity slowly, has low fecundity, gestates for long periods, and grows slowly over a long lifespan. If fishing mortality exceeds its already low natural mortality rate, the population can take decades or longer to recover.13Leuser Journal of Environmental Studies. Effect of Life History on Alopias pelagicus Overexploitation Vulnerability: A Literature Review
This vulnerability is not evenly distributed across species. The sharks and rays with the highest economic value in small-scale fisheries tend to be the largest and slowest to reproduce, including bull sharks, silky sharks, makos, wedgefish, and mobulid rays, and they are therefore the most likely to face extinction.14PubMed Central. Linking extinction risk to the economic and nutritional value of sharks in small-scale fisheries Markets preferentially target the species least able to withstand heavy fishing pressure.
The scale of the problem is staggering. In the northwest Atlantic, scalloped hammerhead, white, and thresher sharks each declined by over 75% in just 15 years, based on the largest available dataset for the region.15PubMed. Collapse and conservation of shark populations in the Northwest Atlantic Globally, despite two decades of increased regulatory attention, total shark fishing mortality actually rose from at least 76 million to 80 million sharks per year between 2012 and 2019. Roughly 25 million of those were threatened species.16PubMed. Global shark fishing mortality still rising despite widespread regulatory change More regulation has not yet translated into fewer sharks being killed.
What Conservation Efforts Are Actually Achieving
Marine protected areas remain one of the most direct tools for shark conservation, and the evidence on their effectiveness is cautiously encouraging. At Palmyra Atoll within the U.S. Pacific Remote Islands Marine National Monument, satellite-tagged grey reef sharks overwhelmingly stayed inside the protected boundaries. Two thirds of tagged sharks were detected exclusively within the refuge for up to about 1.3 years after release.17Biological Conservation. Assessing the effectiveness of a large marine protected area for reef shark conservation Even a relatively small no-take zone of about 583 square kilometers showed signs of rapid shark recovery, including among apex species. As larger predators returned, the proportion of lower-level predatory species declined, suggesting that the trophic structure was reassembling itself.18Biological Conservation. Evidence for rapid recovery of shark populations within a coral reef marine protected area
But protected areas have limits, especially for highly migratory species that cross jurisdictional boundaries. And international trade regulation, the other major policy lever, has proven disappointingly porous. In 2013, five threatened shark species were listed on Appendix II of CITES, meaning that any uncertified export became illegal. Yet between 2015 and 2021, fins from four of those species were commonly found in Hong Kong, the world’s largest shark fin trading hub, indicating substantial ongoing illegal trade.19PubMed Central. International trade regulations take a limited bite out of the shark fin trade An analysis of that same Hong Kong market found that two thirds of species in the trade are threatened with extinction, and many coastal shark species remain largely unprotected by either fisheries management or trade regulations.20Conservation Letters. Two thirds of species in a global shark fin trade hub are threatened with extinction: Conservation potential of international trade regulations for coastal sharks
The economic case for keeping sharks alive is compelling where it can be measured. In Palau, a group of about 100 sharks that regularly interacted with dive tourists would have been worth at most around $10,800 if harvested by fishers, a trivial sum. As a living tourist attraction, those same animals generated far more revenue, distributed across multiple sectors of the economy, while keeping the population ecologically functional.21Biological Conservation. Socio-economic value and community benefits from shark-diving tourism in Palau: A sustainable use of reef shark populations Shark-diving tourism will not save every species in every place, but it demonstrates that the financial incentives can align with conservation when communities have alternatives to extraction.
Climate Change Is Redrawing the Map
Even where shark conservation succeeds locally, climate change is reshuffling the deck. Tiger sharks tracked off the North American coast have shifted their migrations poleward during periods of unusually warm sea-surface temperatures, arriving at northern latitudes earlier in the year. An analysis spanning nearly four decades confirmed that areas of highest tiger shark catch density have progressively crept poleward, and catches occur earlier in the season. During warm anomalies, tracked sharks moved beyond the spatial management zones that had been protecting them from commercial fishing and bycatch.22Global Change Biology. Ocean warming alters the distributional range, migratory timing, and spatial protections of an apex predator, the tiger shark (Galeocerdo cuvier)
This matters for every ecological function described earlier. If tiger sharks shift their range, the behavioral cascades they trigger in Shark Bay or other tropical systems may weaken as their presence becomes less predictable. Meanwhile, temperate ecosystems that historically had little shark pressure may suddenly acquire a new apex predator, with unpredictable consequences for local food webs. Static protected areas designed around historical distributions become less effective as the animals they were meant to protect move outside their borders. Conservation strategies will need to become as dynamic as the ocean itself.
The Perception Problem
One of the more frustrating obstacles to shark conservation is not ecological or economic but psychological. Media coverage of sharks heavily emphasizes negative interactions with people. Research on news portrayals has found that species commonly involved in bites, like white and tiger sharks, receive disproportionate coverage, and that reporting tends to lean on fear-laden language. Politicians have invoked pop-culture imagery, including the movie Jaws, to justify lethal shark-control measures.23Marine Policy. The impact of news media portrayals of sharks on public perception of risk and support for shark conservation Meanwhile, the commercial shark-diving industry goes in the opposite direction, often minimizing any risk to attract paying customers.
Neither extreme serves the animals or the public well. Framing sharks as mindless killers undermines support for protections. Framing them as harmless cuddly creatures undermines the credibility of conservation advocates. The ecological reality, that these animals regulate ocean ecosystems through a combination of predation, fear, nutrient transport, and carbon cycling, does not fit neatly into either narrative. Getting that more nuanced story into the public conversation is one of the harder but more important tasks facing shark conservation. The science is increasingly clear on what sharks do for the ocean. The challenge is making that matter to the people who decide whether they survive.