Seals are marine mammals belonging to the group called pinnipeds. As carnivores, they spend most of their lives navigating the oceans to find food, establishing them firmly as active consumers within the global marine ecosystem. An ecological consumer is an organism that obtains energy and nutrients by feeding on other organisms. This places the seal within a food web, determining its role in energy transfer.
Defining Seals as Secondary and Tertiary Consumers
Seals are classified as both secondary and tertiary consumers, depending on the specific diet and the food chain being examined. Secondary consumers are carnivores that prey on herbivores, a role many seals fill by consuming small, forage fish that feed on zooplankton. Seals also operate as tertiary consumers, preying on other carnivores or larger fish. For example, Leopard seals routinely consume penguins and smaller seals, positioning them higher up the food chain. Elephant seals also function as tertiary consumers by preying on large squid and deep-dwelling fish.
Specialized Hunting and Feeding Strategies
Seals are successful underwater pursuit predators due to physiological adaptations. Their bodies are streamlined for efficient swimming, and many species, like the Northern Elephant seal, are exceptional deep divers, reaching depths exceeding 5,000 feet. These mammals rely on highly sensitive facial whiskers, known as vibrissae, which detect subtle changes in water movement created by moving prey.
Feeding strategies vary widely across the approximately 33 species of seals. Crabeater seals have specialized, sieve-like cheek teeth that allow them to filter-feed almost exclusively on Antarctic krill. Conversely, Harbor seals employ biting and suction feeding to capture demersal fish like cod, herring, and flatfish. Larger species, like the Leopard seal, ambush prey at the ice edge and use a biting technique to handle larger victims such as penguins and seal pups.
The Impact of Seals on Prey Populations
The consumption habits of seals significantly influence the structure of marine communities through a process known as top-down control. When seal populations are healthy, they regulate the numbers of their prey species, which prevents any single fish or invertebrate population from dominating an area.
This consumption role sometimes creates direct competition with human economic interests, particularly commercial fisheries. While seals consume commercially fished species, their impact on fish stocks is often less significant than factors like environmental changes and the intensity of human fishing. For example, recovering Harbour seal populations have been estimated to cause substantial mortality on certain demersal fish in localized areas, which fuels conflict with fishermen. However, removing seals does not automatically lead to a recovery of fish stocks, as ecosystem dynamics are complex.
Major Predators of Seals
While seals are consumers, they also serve as a food source for larger marine and terrestrial carnivores, completing their role in the food web. In the open ocean, the primary predators of seals are the Orca, or Killer Whale, and large sharks, such as the Great White Shark. These aquatic hunters target seals due to their high caloric blubber content. In Arctic regions, the Polar Bear is the dominant land-based predator, relying on various seal species as its main food source.
Seals employ several defensive behaviors to mitigate predation risks. Their exceptional speed and agility in the water are their best defense against aquatic threats, and they frequently haul out onto land or ice to avoid marine hunters. Pups and smaller seals are at the highest risk, though larger adult seals, such as the five-ton male Elephant seal, can occasionally deter attackers.