Sam Wright L605
19/6/13
To what extent is the conservation of sharks important to marine ecosystems?
It is a common misconception to consider sharks as ruthless killers with no role other than
preying on helpless victims. This misconception is completely wrong. In 2011, only 17
human deaths globally were as a result of shark attacks and so labelling sharks as ruthless
killers appears totally absurd. It is far less common to consider the role all species of sharks
play in marine ecosystems to maintain a balance within the marine food chain. Sharks are
notoriously the most efficient predator in the world’s oceans and are able to survive in all
tropics and ranging environments (Sharks’ Bite Force). Sharks are able to inhabit almost
every marine ecosystem on earth ranging from bottom-dwelling species present in coastal
regions to larger sharks roaming the open ocean (Sharks and Rays pg 104-05). Because of
this, they undertake a vital role of regulating the productivity of fish, crustaceans, coral reefs
and all types of marine organisms in these different environments.
Sharks have dominated the oceanic depths for over 100 million years presenting almost
exactly the same characteristics today as they did 100 million years ago (Marine Biology pg
144-45). This is because sharks have been such successful predators throughout history that
there is no requirement for them to adapt to change their ways. An extremely convincing
argument would highlight the importance of all species of sharks to marine ecosystems and
so would argue for the conservation of vulnerable shark species.
Alongside being notorious for their predation skills, sharks – especially reef sharks, have
regulated marine life in environments such as coral reefs. This is an extremely important
concept in allowing a balance within marine ecosystems since without the presence of
smaller, reef sharks such as the nurse shark; the food chain would become completely
overturned resulting in the destruction of the coral reef. The nurse shark is
one of millions of the fish abundant on coral reefs including many other
sharks. The nurse shark prospers in the shallow waters (up to 50m) of
tropic continents such as Brazil, Mexico and the Caribbean feeding on
crabs, fish and crustaceans (Marine Fish and Sea Creatures pg 156-59). By
doing so, the nurse shark ensures that the reef doesn’t become
overpopulated with ‘browsers’ which feed on and damage the coral reef. Without the
presence of the nurse shark, the prey could be free from predation and would very quickly
overpopulate the reef and cause irreversible damage to it.
This concept was highlighted in 1974 at the Great Barrier Reef in Australia with the
introduction of the ‘crown-of-thorns’ starfish. This species of starfish populated a section of
the reef in which there were no predators to feed on it. This allowed the starfish to freely
roam the reef consuming whatever parts of the reef it wanted. In doing so the starfish was
found to have caused 95% mortalities to the coral reef it inhabited once it was removed
from the site. This occurred due to the fact that there was no predation effecting the
‘crown-of-thorns’ starfish population which allowed it to prosper and overpopulate its
section of reef (Reef Evolution pg 264). This highlights the fact that sharks are essential to
maintaining a healthy and prosperous living environment on reefs as they regulate the
populations of marine ‘browsers’ that are likely to negatively affect the ecosystem of the
reef.
Other examples of the negative effect of removing the sharks as the top predators in the
food chain have been highlighted in South Africa and Tasmania during the 1970s and 80s
(Swimming into the Oblivion). In South Africa, large-meshed nets were erected in order to
catch large sharks. This allowed a huge increase in the abundance of smaller shark species
such as Thresher and Whitetip reef sharks which overfed on small fish species which as a
result suffered huge declined in population numbers. Similarly in Tasmania, a shark fishery
was blamed for the collapse of a neighbouring crayfish farm. The removal of predatory
sharks by the fishery had allowed populations of octopuses to prosper which sought after
the crayfish farms’ crayfish causing the farm to collapse. These examples signify the
importance of sharks’ roles as the top predators in the marine food chain and highlight how
important it is that they are protected and saved through conservation efforts.
In 2005, the Scripps Institution of Oceanography of the University of California observed a
very interesting change in behaviour of prey fish when sharks are removed from the
ecosystem (Swimming into the Oblivion). At the Line Islands in the Pacific Ocean scientists
compared marine ecosystems of two different coral reef communities. Reefs which had the
least amount of human disturbance portrayed the healthiest communities and supported
the greater biomass of fish due to a vast abundance of predatory sharks. In comparison,
reefs which had been subject to human disturbance removing sharks showed a far
unhealthier and unproductive community. Sheila Walsh of the Nature Conservancy based in
Virginia stated that “it doesn’t seem energetically possible” (Swimming into the Oblivion).
Scientists concluded that the absence of predatory sharks caused prey fish to mate and
develop at a far slower rate which resulted in a less productive reef system. Walsh also
suggests that “having sharks around makes for faster, more productive fisheries” which
defies the simple assumption that removing the top predator will allow a greater population
of prey fish. From these findings it is evident that sharks play a crucial role in maintaining a
healthy, functional marine community and without the exploitation of humans, coral reefs
are able to naturally prosper. Therefore the conservation of shark species is clearly an
essential concept in allowing prosperous marine ecosystems.
As Sheila Walsh indicates, there are also benefits for humans when considering shark
conservation projects (Swimming into the Oblivion). The experiment detailed by the Scripps
Institution of Oceanography reveals how the presence of shark species allows fisheries of
both fish and invertebrates such as crayfish to be far more productive. In turn, this allows
local fishermen to access a greater abundance of fish in order to feed local communities.
This not only benefits them personally but would also boost the local economy which
obviously is a huge bonus. Therefore, it is not only in the interest of marine ecosystems that
sharks are present but is also hugely beneficial for human communities. This evidence
clearly supports the implementation of conservation projects to protect threatened sharks
and would appeal to a vast human audience because of the incentives provided by marine
communities populated with sharks.
Furthermore, different sharks show diverse feeding patterns which allow them to prey on
almost all creatures found in the sea. The largest sharks in today’s oceans such
as the whale and basking sharks peculiarly tend to feed on the smallest groups
of organisms found in the ocean – plankton. A “plankton soup” is made up of
very tiny floating plants and animals which are unable to forcibly swim against
the water current (1001 Facts about Sharks pg 80-81). Most of these tiny animals are in fact
larvae (eggs) of smaller fish, crustaceans and other small organisms. This feeding process
also helps to regulate the population growth of smaller organisms by killing them before
they develop into fully grown adults. Basking sharks filter through 1800 tonnes of water
every hour to sought their plankton meal. From this, it is clear to see the importance of not
only smaller, bottom-dwelling sharks but the largest sharks to marine ecosystems because
of their feeding patterns. Therefore, the conservation of all species of sharks is vitally
important in restoring the balance of marine communities.
The most well-known and feared shark is the Great White. It is considered the most
dangerous to humans out of all the species of sharks. However, dangerous, carnivorous
sharks such as the Great White and others alongside it such as bull and tiger sharks also play
key roles in regulating marine ecosystems and without them, the balance of predation and
prey in the oceans would be very different. These larger, carnivorous sharks feed on bigger
mammals and fish of the sea such as seals, dolphins, turtles as well as seabirds and sea lions
(Marine Fish and Sea Creatures pg 156-59). The feeding
patterns of large sharks such as the Great White allow a
constant flow of energy throughout the marine food chain. As
shown in the diagram on the left
(http://teacheratsea.wordpress.com/tag/food-chain/), large
sharks are the top predator in the marine food web. This
allows each consumer lower in the web to feed on their prey
in order to gain energy from them. For example; sunlight
provides energy for the single-celled organisms, these organisms provide energy for
shrimplike creatures once eaten, as small fish consume these creatures they uptake the
energy previously gained. The process continues and results in the large shark gaining the
most energy by consuming the large fish – which it requires in order to survive. Without the
presence of the large shark as the primary consumer/ top predator, the balance of the food
chain is unequal. The number of large fish would increase, resulting in the number of
mackerel and small fish being predated on to a far greater extent. In turn, this means the
energy flow is unbalanced as the larger number of tuna begin to run out of a food source.
This obviously creates a problem and highlights the importance of the primary consumer
and top predator in this food chain.
Following this, it seems feasible to question why there isn’t an overload of large sharks in
the marine food chain since nothing in the sea hunts them. From this it would seem logical
to suggest that sharks don’t need help from humans via conservation projects. However this
would be wrong since humans have overhunted and exploited large sharks
for their fins, teeth, skin as well as for game fishing to such an extent that
large, iconic sharks such as the Great White and Basking shark are listed as
endangered and vulnerable on the IUCN Red List (Marine Fish and Sea
Creatures pg 156-59). Human demand for ‘shark fin soup’ has heavily
increased since 2000 in countries such as China which have profited from a growing
economy. A bowl of high quality ‘shark fin soup’ can cost up to $720.37 (CITES Regulation...)
which demonstrates why sharks have been overexploited for their fins since they are so
valuable. This exploitation of sharks by humans affects not only the largest, dangerous
sharks but also passive unaggressive species such as the Great Hammerhead Shark and
Whitetip Reef sharks which are helpless to human exploits and have been labelled as
‘Threatened’ on the IUCN Red List as a result (Marine Fish and Sea Creatures pg 156-59). In
return, it is essential that humans put forward conservation efforts to try and restore the
balance of sharks in the Earth’s oceans since their decline has largely been caused by human
activity.
Alongside this, sharks are naturally slow to reproduce which limits their ability to adapt to
changing environments quickly. Shark reproductive systems are more similar to that of
mammals rather than that of other fishes. The gestation period inside a female shark may
take up to 2 years after which they lay only a few large eggs to be fertilised by male sperm
(Sharks and Rays pg 106-7). Great White sharks have the lowest reproduction rate of all the
large sharks and have an infant mortality rate as high as 80% which causes serious problems
for this species. Due to Great White sharks’ slow reproductive rates, even slight changes in
their environment can have dramatic consequences for their population (CITES
Regulation...). For example, when just a few members of the adult population are removed,
the whole population of this species is under
serious threat because they are so slow to
reproduce in order to replace fatalities.
Therefore it is essential that humans intervene in
order to help protect vulnerable species such as the Great White. Promising initiatives such
as the IUCN Red List and Appendix II (CITES) have emplaced restrictions on the killing and
trade of white sharks in order to protect the species
(http://www.iucnredlist.org/details/3855/0). The IUCN Red List details all species of all
organisms and labels which of each is endangered/vulnerable/threatened/extinct and so on.
This ‘list’ enables conservation efforts to be put in place to restrict human exploitation of
these species. The Appendix II put forward by CITES (Convention on International Trade in
Endangered Species) represents an effective way to enforce restrictions and protection for
endangered species (http://www.cites.org/eng/app/). As a result of these initiatives the
white shark is fully protected in Australian, U.S, South African, Maltese and Namibian waters
(Biology and Ecology of the White Shark pg 75-76). Even so, this success for the protection
of white sharks needs to be implemented globally for all species of vulnerable sharks that
require human conservation efforts.
However, an alternative interpretation would argue that the shark’s role in the marine
community isn’t important enough for human intervention through conservation projects.
This argument relies on the assumption that other marine predators will replace the shark
as the top, dominant predator throughout the world’s oceans. For example, killer whales
appear as a sensible replacement for large, carnivorous sharks because of their similar diet
patterns. Killer whales are active predators that feed on a wide variety of fishes as well as
marine mammals such as seals, sea lions, baleen whales, penguins and sea otters
(http://www.seaworld.org/animal-info/info-books/killer-whale/diet.htm). In comparison to
large, predatory sharks such as the Great White, killer whales present almost exactly the
same diet. Alongside this, the killer whale is currently ranked as the largest predator of
warm-blooded animals alive today. This would suggest that killer whales could be the
perfect replacement for large sharks as the top predator
in the sea. However, the diagram on the right details the
global distribution of killer whales as highlighted by the
dark yellow print. The diagram depicts a very limited
expanse of sea covered by killer whales
(http://killerwhale.vanaqua.org/page.aspx?pid=1355).
For this reason, it is implausible to suggest that killer
whales could replace large sharks as the dominant predators of the ocean because they
patrol such as limited expanse of sea in comparison to that by large sharks which roam the
open ocean globally. Furthermore, the diversity of top predatory shark species such as Great
White, Bull, Tiger and Lemon sharks heavily outnumber killer whale populations and
distributions. For this reason the interpretation that sharks aren’t important because they
can be replaced is clearly flawed.
It is not only large predatory sharks that would need to be replaced but also bottomdwelling sharks that roam coastal habitats such as coral reefs at shallower waters. It could
be argued that these bottom-dwelling sharks would be easier to replace
since coral reefs are so abundant with fish. For example; barracuda,
octopuses, stingrays and blue marlins are just a handful of predatory fish
that inhabit coastal regions, regulating coral reefs for their prey.
Between octopuses and stingrays they consume prey such as fish,
crustaceans, shrimp, molluscs which are hugely abundant on coral reefs
(http://animals.nationalgeographic.co.uk). In contrast, blue marlin and barracuda tend to
predate on larger reef fish growing up to their own size as well as mackerel and tuna
stretching further outward towards the open sea. Therefore it could be suggested that
coastal sharks such as the Whitetip reef shark, Grey reef shark and nurse shark would be
easily replaced by these other reef predators. Although this may appear plausible at first, if
these predators took dominance there would be nothing to predate on them in order to
limit their population growth. The ecosystem which may appear balanced at first, would
soon become unbalanced with an overpopulation of these predators and an under
population of bottom-dwelling organisms as the predators would over-exploit the
abundance of fish and crustaceans on the reef. In contrast, sharks are incapable of
becoming overpopulated because of their inability to reproduce quickly as the gestation
period in female shark species is usually 2 years at the lowest. Sharks are therefore essential
to the marine community in order to prey on these secondary predators as a way to restrict
their population growth. Without the presence of sharks, there would be no predators large
enough to hunt barracuda, octopuses, stingrays etc efficiently enough which could result in
a terribly unbalanced marine food web.
Some may argue that even though sharks may be important to marine ecosystems, they
have a greater importance in relation to human use. The most essential human benefit from
sharks is for their medicinal use. Sharks are immune to almost all types of disease due to a
chemical called ‘squalene’ present in their liver oil which prevents harmful illnesses such as
cancer from effecting sharks (1001 Facts about Sharks pg 139). Alongside
squalene, shark oils contain chemicals which can control cholesterol levels
which is a major cause of cardiovascular disease in humans. A specific species
of shark named the spiny dogfish produces oils containing 10 times the amount
of vitamin A than that of cod liver oils (1001 Facts about Sharks pg 139). It could
be argued that because of sharks’ production of such valuable and useful oils
which can be put to great use by humans; they become more important for
human use than their role in marine ecosystems.
Furthermore, sharks have great value during medical operations as shark oils
can be used as anticoagulants during heart surgery in order to stop the clotting of blood.
Dried shark brain acts as a general pain killer and can prevent tooth decay (1001 Facts about
Sharks pg 139). On top of this, shark corneas have successfully been used in human
transplant operations along with shark cartilage which can create a synthetic skin to treat
burn victims (1001 Facts about Sharks pg 139). Although it could be argued that sharks’
medicinal use for humans is a reason to allow human exploitation of them, it would be far
more intelligent to conserve and protect shark species in order to create a sustainable
source of these medicinal properties. If sharks are over-exploited now, humans would
benefit from their uses for a short period of time until they become extinct along with the
source of such medical properties. Therefore, it is far more suitable to implement
conservation initiatives in order to create a sustainable supply of shark, medicinal products
from which humans would benefit to a far greater extent over many generations.
An alternative argument also puts forward the idea that herbivores may be more influential
to the productivity of coral reefs in comparison to carnivorous predators such as sharks
(Reef Evolution pg 260-62). Herbivorous organisms are essential to coral reef productivity as
they facilitate the flow of energy to higher trophic levels within
the marine food web. As depicted by the food web on the right,
herbivores, crustaceans and molluscs make up the middle three
sections ranging from trophic level 2 to 4. Without this
foundation, the carnivorous finfish that make up the top trophic
level (level 5) would be without prey and would struggle to
survive. This makes the herbivorous foundation essential to
carnivorous sharks’ survival. It could also be argued to indicate
that herbivorous organisms are able to prosper without the
presence of predatory sharks whereas the opposite is not possible. Therefore it could be
concluded that sharks are not in fact essential to marine communities because the
foundation trophic level populations of herbivorous organisms such as crustaceans and
molluscs are able to continue surviving without their presence.
Moreover, it could be argued that sharks aren’t important to marine ecosystems because
herbivorous organisms promote a high level of productivity on reefs as they regulate algae
and micro-organisms by feeding on the most abundant species of each (Reef Evolution pg
260-62). This enables a greater biodiversity and species evenness within the marine
community as many different species of algae and micro-organisms are allowed to grow
without one species dominating the reef community. Dominant species of micro-organisms
are prevented from thriving which creates greater opportunity for subordinate and
opportunist species to grow instead (Reef Evolution pg 260-62). From this, some may argue
that reefs would be just as productive without the presence of predatory sharks because of
the niche (role) performed by herbivorous organisms. However, it is far more convincing to
realise that both herbivorous and carnivorous marine organisms can survive in unison within
the ecosystem because they undertake such different and diverse niches. Carnivorous and
herbivorous organisms don’t compete with each other for shelter/food/mates and so the
existence of one does not restrict the existence of the other. A reef which inhabits both
carnivorous species such as sharks and herbivorous marine creatures would definitely be far
more productive and healthy than a reef which solely consists of herbivorous species (Reef
Evolution pg 260-62). Therefore, it isn’t a sensible conclusion to believe that sharks are
unimportant to marine communities just as it would be implausible to suggest that
herbivorous creatures are unimportant in the same way.
A final interpretation that may be put forward is to suggest that larger sharks that roam the
open ocean are far less important to marine ecosystems because they inhabit environments
so far from coastal reefs and communities. Large predatory sharks such as the Great White
which feed on marine mammals such as sea lions and seals don’t necessarily contribute to
the marine food chain since these mammals can inhabit landed shores as well as the ocean.
It is clear that reef and coastal sharks provide an essential function to marine communities
by regulating browsing species of fish and crustaceans in order to allow a productive reef.
However, it could be argued that the role of larger predatory sharks isn’t remotely as
essential as that of smaller, reef sharks. Therefore, it may seem acceptable to discourage
conservation projects aimed at protecting these larger sharks which supposedly have very
little impact on the marine ecosystem.
In response to this argument, it is essential to consider the diversity of predatory shark
species. As previously mentioned; different shark species are able to inhabit completely
contrasting marine environments and impact the local ecosystem in their own community.
For example, Bull sharks regularly enter estuaries and fresh water rivers in search for mates
in order to breed (1001 Facts about Sharks pg 94-95). This is why the Bull shark is so
dangerous to humans because it regularly enters fresh waters in which humans are far more
likely to be present. This species is just one example of how diversity is displayed by most
sharks as they can prosper in almost every marine environment available to them. Because
of this, large predatory sharks tend to feed on most large fish and marine mammals they
interact with. Once again, this illustrates how large predatory sharks affect marine
ecosystems as they feed on large mammals towards the top trophic levels in the food web.
Therefore, these sharks are also incredibly important to the marine food chain as a means of
regulating the predators in the trophic level below theirs. This illustrates that there is also a
need to conserve and protect large predatory shark species not only because they are
fascinating creatures but because of their importance in relation to the diverse marine
communities they interact with.
It could be argued that the conservation of sharks is not important to marine ecosystems
firstly because of the potential of other predatory species such as killer whales, barracuda
and blue marlin which may be able to replace sharks as the top predators in the ocean.
Along with this some may argue that marine herbivores would allow coral reefs to prosper
to a similar extent as is possible in the presence of predatory sharks. It could also be
suggested that the human uses of sharks as medicine and in medical operations is far more
important than their relative importance to marine communities. On top of this, it is
possible to argue that large sharks are relatively unimportant to marine ecosystems and so
conservation efforts towards protecting such species are unnecessary.
However, a far more convincing interpretation would realise that all types of species of
sharks are extremely important to marine ecosystems and because of this they do require
conservation efforts in order to protect them. It is evident that coral reefs are far more
productive in the presence of reef sharks which feed on ‘browser’ organisms to prevent the
destruction of the reef. This was highlighted through the examples detailed by the Scripps
Institution of Oceanography at the University of California, the fishery in South Africa as well
as the crayfish farm example in Australia (Tasmania). The argument that sharks should
definitely be conserved is further supported because of the incentives for humans as a
result of shark conservation. As suggested by Sheila Walsh – following the findings from the
Scripps Institution of Oceanography at the University of California investigation, fishermen
benefit hugely from shark presence nearby fisheries because they remove predators that
would restrict the abundance of smaller fish. Furthermore, humans would benefit from a
sustainable supply of shark oils to use for medicinal purposes for future generations which is
obviously an incredibly important incentive for supporting conservation projects aimed at
protecting sharks. The interpretation that sharks are unimportant to marine ecosystems can
be dismissed by highlighting the limitations of this argument. The assumption that marine
predators which have the capabilities of replacing sharks will do so is simply an assumption
and the evidence collected from South Africa, Tasmania and California suggest that the
absence of sharks leads to a major imbalance of the food chain within a marine community.
Therefore it is clear that sharks are an incredibly important asset to marine ecosystems
throughout all environments in today’s oceans and because of this, it is essential that
humans implement conservation projects in order to help protect and save these assets.
References:
Biology and Ecology of the White Shark (book)
http://www.cites.org/eng/app/
http://www.iucnredlist.org/details/3855/0
The Price of Fame: CITES Regulation and Efforts towards International protection of the
Great White Shark (article)
Sharks and Rays (book)
Marine Fish and Sea Creatures pg 156-59 (book)
http://teacheratsea.wordpress.com/tag/food-chain/ (pg 3 pic)
1001 Facts about Sharks (book)
Swimming into the Oblivion (article)
Reef Evolution (book)
Marine Biology (book)
Sharks’ Bite Force (article)
http://www.seaworld.org/animal-info/info-books/killer-whale/diet.htm
http://killerwhale.vanaqua.org/page.aspx?pid=1355 (pg 5 pic)
http://animals.nationalgeographic.co.uk
http://www7.taosnet.com/platinum/data/environment/destruction.html (pg 7 pic)