The Importance of Sharks in an Ecosystem

Sharks are often the “apex” or top predators in their ecosystems because they have
few natural predators.

Sharks feed on the animals below them in the food web, helping to regulate (control)
and maintain the balance of marine ecosystems. Apex (top) predators directly limit
the populations of their prey, which in turn affects the prey species of those animals,
and so on.

This allows top predators to switch prey species when certain populations are low,
thereby allowing all prey species to remain present in the ocean ecosystem.

Apex predators also

control the spatial
distribution (where
they are found) of
potential prey through
intimidation (being
scared). Fear of shark
predation (attack)
causes some species
to alter their habitat
use and activity level.

This means that more

predators lead to
greater diversity (a
greater number of
different species within the ecosystem)

Comparisons of areas with and without apex predators show that apex predators
provide greater biodiversity and higher densities of individuals, while areas without
apex predators experience species absences.

Without apex predators there is the potential for unchecked predation by other lower
predatory species. This means smaller fish and other species may overeat vegetation
that ultimately affects the species richness (number of different species) and
abundance (amount there is) within the system.

In addition to regulating species abundance (amount), distribution (where they are

found) and diversity (variety of species), top predators provide essential food sources
for scavengers and remove the sick and weak individuals from prey populations,
keeping those populations healthy.
 Loss of Sharks in North Atlantic
Surveys show that the number of the 11 great sharks (sharks more than two meters in
length) along the eastern coast of the United States has declined (reduced) to levels
that essentially mean they have been eliminated (no longer are there). This means
that the sharks are now unable to perform their ecological role (job within the
ecosystem) as top predators and help to regulate (control) the ecosystem.

All of the shark species in this area, except for the mako, have declined by more than
50 percent in the past eight to 15 years. Scalloped hammerhead, white and thresher
shark numbers are estimated to have declined by more than 75 percent in the past 15
years.

During this same period, their prey — 12 species of rays, skates and smaller sharks —
have increased in numbers by as much as ten times. The species that increased most
was the cownose ray, which migrates up and down the eastern coast eating scallops,
clams and oysters.

The effect of this has altered the entire ecosystem, resulting in the collapse of a
century old scallop fishery. This was because by 2004 cownose rays had eaten most of
the bay scallops, ending the North Carolina fishery.

Without bay scallops to eat, the cownose ray, along with other rays, skates and small
sharks, is expected to expand its foraging (looking for food sources) to clams and
oysters meaning these species will also soon disappear in this area.

This has already been seen with the loss of another species, a hard clam known as
the Quahog which is a key ingredient in clam
chowder. Since it is now hard to find, many
restaurants have had to remove this American
classic from their menus, affecting the economy and
culture of the coast.

Loss of species can also be caused by changes to

the habitats. Hungry rays roaming the waters and
hunting for food have the potential of uprooting
seagrass at higher rates, leading to poorer quality
nursery grounds for fish which reduces the numbers
found in the ocean.

Clams and scallops also act as filters for the oceans. When their numbers decline it
means an increase in the amount of algae that can grow which further damages the
ocean ecosystem.
 Caribbean Coral Reefs
Healthy coral reefs provide a complex, 3D habitat that encourages great
species diversity (a large number of different types of plants and animals) and
lots of life. The loss of sharks in a reef ecosystem, like that of the Caribbean
Sea, can trigger a chain reaction that is felt throughout the entire food web,
ultimately leading to the degradation (lowering the quality of the environment)
of coral reefs on a local or even regional scale.

Since algae (simple plants that grow quickly and can cover hge areas) compete
with coral for settlement on reefs, coral depend on fish to eat the algae and
provide space for coral to settle and grow. A reduction in herbivorous fish (fish
that eat plants) prevents coral from thriving (being successful).

The loss of sharks as top predators in the ecosystem allows the number of
grouper, which eat other fish species, to increase. The groupers in turn reduce
the number of herbivores, such as parrotfish, blennies and gobies, in the
system. Without these herbivores to eat algae off the coral, algae will take over
a reef system.

This overgrowth of algae makes the habitat all the same so fewer species can
find the habitat they need. This reduces the number of different species found
and can also mean that coral reefs themselves also disappear.

In summary, the removal of sharks from

the coral reef ecosystem can affect the
resilience of coral reefs to disturbance
(how able they are to withstand
changes), leading to a habitat with
reduced species diversity and numbers of
fish.

The coral reefs of Jamaica show this

change. Over the past 30 to 40 years,
the species composition in Jamaica
has changed drastically. Sharks,
snappers, jacks, triggerfish and
groupers are now replaced by small
herbivorous fish. The amount and
range of corals found has also declined
because even though the remaining
fish are herbivores, they are too small to
clear all the algae.

Because coral cannot compete , algae

now cover more than 90 percent of the
reefs and this has happened as a direct
result of the loss of sharks from the
ecosystem.
 Tiger Sharks in Shark Bay
Tiger shark intimidation
(being scared of them)
alters the spatial
distribution (where the
animals they hunt are
found) of their prey and
affects how seagrass
grows and supports a
range of other animals.

In Shark Bay, Australia, sea

cows and green sea turtles,
which are common tiger
shark prey, alter their
habitat selection based on
the presence of tiger
sharks. They choose to live in areas where there are fewer tiger sharks. This creates a
trade-off between safety and food quality for the species.

Sea cows and green turtles prefer the nutritious seagrass found in the middle of large
grassy patches, but it is very difficult to escape from a tiger shark in these locations.
When tiger sharks are found in high numbers sea cows and green turtles choose to
feed on the lower quality seagrass located near a patch’s edge, thereby reducing
their risk of being eaten as they can escape.

When grazing, sea cows and green turtles remove the entire seagrass plant so their
presence of tiger sharks makes sure some seagrass remains as the species that eat it
will not live in areas where the sharks are.

Tiger sharks not only influence the distribution

of their prey (the sea cows and green turtles),
but also intimidate species that rarely appear
in their diet. Tiger sharks have been shown to
directly and indirectly affect the location of
bottlenose dolphin and pied cormorant (a sea
bird) foraging (looking for food).

Although shallow waters are the most

productive habitat, bottlenose dolphins and
pied cormorants avoid shallow waters when
sharks are present. Once sharks leave the area,
dolphins and pied cormorants are able to
occupy all habitats and freely hunt their food
which reduces the numbers of these species
found, further affecting the balance of the
environment.
 Sleeper Sharks in Alaska
Harbor seals change their behaviour according to the presence of
sleeper sharks in the oceans. They eat differently and live in different
areas when the sleeper sharks are absent.

Usually sleeper sharks intimidate (scare) the seals into keeping their
distance which allows other fish species to live freely without seals eating
them.

Pollock and herring are the 2 top foods for the seals. Herring live near the
surface whereas pollock live in deeper waters where the sleeper sharks
swim.

When sleeper sharks are present, shark intimidation reduces seal

foraging
(looking for
food) in the
deeper
waters and
therefore
directly
increases the
seals eating
of herring
(fish found
near the
surface)
while
reducing
their hunting of pollock (fish found in the deeper water).

The removal of sharks changes this relationship by stopping the seals

from being scared of being eaten by a shark. This allows them to increase
their use of deep waters to consume pollock and decrease their foraging
of herring on the surface. This changes the numbers of both species of
fish and affects the ecosystem food web (how all the species link together
within the environment) as a whole.

The presence of sleeper sharks directly alters the behavior of their prey -
the pollock and herring are able to eat more and more as they no longer
fear being eaten. By removing the sleeper sharks the animals and
organisms that the pollock and herring feed on may also experience
huge losses and possibly extinction as they will be eaten even more.