A grey reef shark, one of the species studied in the paper. Via Wikipedia
Reef systems worldwide are in peril, with pollution and warming waters leading to coral bleaching, and overfishing of tropical fish decimating biodiversity. Here's another finding to add to the pile: the decline of shark populations caused by overfishing is having negative effects all the way down the food chain.
The paper, published in PLOS One, looks at the systemic effects caused by the loss of carnivores. Every ecosystem is the result of a balance of all of its constituent life forms, and changing that balance can have effects that cascade throughout the entire food chain.
The study focused on reef systems in the eastern Indian Ocean. As the authors write, "Because long-term monitoring of fish and coral communities has been conducted on these reefs for over a decade, they provide an ideal ecosystem-scale (hundreds of kilometres), natural experiment to investigate this subject."
One of the main goals was to figure out if shark communities were actually being altered by fishing, and if those changes cascaded downward through the trophic structures of other fish communities. The other was to see if benthic changes to the environment caused by algaes replacing dead coral could cascade upward to effect species higher in the food chain.
As to the first question, the team found that fishing has indeed depressed shark levels, with shark levels at a protected reef around three times higher than those at fished areas. This was to be expected. As the authors note, "Indonesian fishermen provide a chronic disturbance on the reefs by targeting 'banquet' species of high economic value," which includes sharks. Around 100 million sharks are killed worldwide each year, and many local populations of sharks have been measured to be in decline.
On the coral side, the team looked at two types of regions in each fished and unfished zone: Areas where coral was intact at historical levels, and areas where environmental changes has caused coral cover to decline. In most cases, that decline means a corresponding rise in algaes and other fast-growing marine plants.
As the above figure shows, both factors affect reef systems. Red bars are from fished reefs, and green are from protected areas; the darker bars are from regions that had lower coral cover due to environmental disturbances, and the lighter bars are from non-disturbed regions.
Starting from the top, fished areas—areas with lower sharks—had higher levels of mid-sized carnivores. Sharks prey both on those carnivores and their food, so with sharks out of the picture, their populations were able to increase.
On healthy reef systems, whether sharks are fished for or not, herbivore levels are low enough (due to a lack of algae) that they don't appear to have been significantly affected by increased levels of mid-size predators. But in disturbed reef systems, where herbivore populations are much higher, higher levels of mid-size predators does appear to correspond with lower levels of herbivores.
Shark levels don't seem to affect the bottom levels of the food chain—coral and plankton eaters—but the health of reef systems definitely do. Both groups had significantly lower populations when reefs weren't healthy, regardless of how many sharks were around.
What does it all mean? Reef systems are being hurt by anthropogenic effects from two directions: by environmental changes causing reef collapse at the bottom, and by overfishing of sharks at the top. While this study doesn't show that declining shark populations has effects that cascade all the way down to reefs, or vice versa, the combined declines are changing the makeup of the entire system.
This can lead to unpredictable changes, as highlighted by the detritivore population graph above. Under most conditions—healthy reefs with high or low sharks, unhealthy reef with high sharks—detritivore populations remain stable. But when coral and sharks go away, the population suddenly doubled, perhaps because more algae and fewer sharks meant more food and less death for those fish.
These types of cascade effects are well documented ecological phenomena, especially when it comes to top predator populations disappearing. For example, the reintroduction of wolf populations to Yellowstone led to beaver populations rebounding. Why? Wolves eat elk, and elk browse on willow saplings that beaver rely on. Keeping elk in check (and more vigilant about gnoshing on trees) let beavers have their trees back.
Now it appears that a similar situation is happening with reefs. It's well known that coral collapse has huge ramifications for marine ecosystems, but it's important to note that overfishing sharks can have top down effects as well. And with changes rippling through from top and bottom, the health of fragile coral reefs are caught perilously in the middle.