Apex Predators- their impact and importance.
Every ecosystem, be it on land, in the air or in the sea, has its Apex predator (or Top predator or Super predator.) An Apex predator is an animal who, adult, has no natural predator within its ecosystem.
The marine environment is vast, varied and made up of many different zones, each one an individual ecosystem: Coastal zones, Arctic or Antarctic zones, Coral reefs, Benthic (sea floor) zones, Demersal zones just above the sea floor and Pelagic zones which are all waters not close to coastlines or the sea floor and which can be divided into different sub-zones according to the depth (epi-pelagic- sunlit, meso-pelagic- twilight, bathypelagic- midnight, abyssopelagic- lower midnight and hadopelagic.) This variety of marine ecosystems provides us with a diversity of life-forms and their predators. The majority of these are meso-predators, intermediate predators which feed on smaller organisms yet are prey items for Apex predators. In the pelagic environment, tuna or billfish are mesopredators who feed on baitfish but are preyed upon by some shark species. In coastal zones, seals and sea-lions prey on fish but are themselves taken by sharks.
Sharks are an often-cited example of an Apex predator, but this is not entirely true. Smaller, sluggish shark species fall prey to larger, more rapid shark species. These in turn can be taken by an even larger individual of one of the bigger, more aggressive species. While the Great White Shark is an Apex predator in practically every coastal zone where it occurs, off the Californian coast and Farallone Islands, it is prey to the Orca, who roll it over using co-ordinated teamwork to exploit the sharks’ natural ‘tonic immobility.’ They then devour its huge, oily, highly nutritious liver. Orcas are Apex predators in every region where they occur.
Apex predators play a major role keeping their ecosystems in check. This is known as ‘trophic dynamics.’ Studies have shown that on land, or in the ocean, their removal can have a dramatic impact on that ecosystem and results in even larger problems. A reduction of Apex predators results in an increase in the number of meso-predators which causes a decline in prey populations- this is the ‘Top Down trophic cascade.’ Apex predators keep meso-predators at a reasonable number and ensure abundant prey populations.
Off the East Coast of the United States, the population of Hammerhead sharks was greatly reduced by overfishing. Cownosed Rays, usually predated upon by the Hammerheads, grew in numbers and consumed increasingly larger quantities of clams and other shellfish, resulting in a collapse of those fisheries on the East coast.
Trophic dynamics are much easier to study on land and the most commonly cited example of how Apex predators affect their ecosystem is Yellowstone National Park. After decades of absence, the Grey Wolf was reintroduced to the park in 1995. Dramatic changes have occurred throughout the whole ecosystem since then. Elk, their favourite prey, became less abundant. They changed their behaviour and moved from exposed riparian zones. Freed from constant grazing, these zones saw a renewed growth of Cottonwood, Willow and Aspen which created habitat for beaver, moose and dozens of other species. The wolf’s presence also had a positive effect on the park’s other Apex predator, the Grizzly. Emerging from hibernation, they would scavange off wolf kill, replacing lost body fat more quickly. Wolf kill also ensured they were better prepared for hibernation. Cubs are born in winter and better prepared mothers produce more nutritious milk so cub survival rates increased. Many other scavanging species not only survived, but thrived from wolf kill- eagles, raven, coyote, black bears and others.
Now if we imagine a similar scenario occurring in a marine environment, we can see just how important marine predators are for trophic dynamics and species abundance and survival.
It is for precisely these reasons that we must re-assess fishing methods, the species targetted and quantities caught and impose strict quotas, respect threatened species and change our fishing methods. We must also re-evaluate our impact on the marine environment in the pursuit of recreational, industrial and transportation activities and make a concerted effort to minimise our impact on these ecosystems and their fragile populations, so easily disrupted and disturbed.