By Blaise Jones
Sharks are apex predators; they sit atop of the food chain with no other animals regularly preying upon them. In fact, the only predatory threats that face an adult shark are carnivorous whales like orcas, human fishermen, and bigger sharks.
But how does being an apex predator keep the environment healthy? It turns out that having too much of a good thing can very easily become a bad thing. Sharks and other apex predators keep the populations of their prey animals – nearly the entire ocean’s population – balanced.
Without apex predators ecosystems can undergo a catastrophic series of events known as a “trophic cascade.”
A trophic cascade happens when the top predator in an environment is removed, allowing their prey to experience rapid growth (since they aren’t being eaten), which in turn leads to a massive reduction in their prey, which then alters the food web throughout the entire region.
Most fish species are biologically programmed to consume as many resources and produce as many babies as they can as quickly as possible. This is because these animals have high infant mortality, leaving only a fraction of those born to reach adulthood. However, if you take off one of their major stressors, in this case predation, their odds of reaching adulthood increase significantly. In cases where apex predators like sharks have been completely removed their prey reproduced in such huge numbers that they completely strip their ecosystems barren, killing off all of the other animals living there as well.
This not only makes life more difficult for marine life, it hurts humans as well. A reduction in shark populations all along the Atlantic coast of North America triggered a boom in stingray populations. Large predatory sharks such as bull, tiger, and great whites normally preyed upon these stingrays, keeping their populations in check. Researchers discovered that all 11 predatory sharks in the north Atlantic that preyed up stingrays, most notably the cownose stingray, experienced population declines in the late 20th century and into the 21st century. The reduction in shark populations over the past 30 years allowed the stingrays to overpopulate along the coast. As a result, scallop industries alone the Atlantic coast perished since scallops are one of the favorite foods for cownose stingrays.
The trophic cascade caused by overhunting of predatory sharks allowed stingray populations to explode to the point that scallop industries were destroyed and in 2004 North Carolina officials were forced to shut down scallop harvesting that had thrived in the region for more than a century.
No Fish Hospice
Not only do sharks keep the normal populations of fish in check, they also keep them healthy. While sharks are efficient hunters, they’re practical ones as well, preferring to prey on sick and injured animals. By selectively hunting the old, sick, and weak members of their prey’s populations, sharks are unintentionally ensuring that those populations stay healthy. The sick fish do not spread their diseases to the healthy populations. The old fish are removed, allowing younger fish to take better advantage of resources. This constant cycling out of the old for the new encourages population genetics to remain dynamic, allowing the population to remain flexible in the face of unanticipated disasters.
This is also encouraged by the sharks focusing on the weak fish. The injured fish are easily recognized by the opposite sex as being weak, which dissuades members of the opposite sex from reproducing with them. By removing these fish sharks remove a potential resource competitor from the healthier fish, allowing them to flourish and increasing their odds of reproducing.
So while it may seem like sharks only take from their ecosystems, their particular method of prey selection allows them to take only the perfect amount, leaving their ecosystems in a harmonious balance.
“The Encyclopedia of Sharks” by Steve Parker
“Sharks: The Animal Answer Guide” by George H. Burgess and Gene Helfman
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