Sam Wright L605
19/6/13
To what extent is the conservation of sharks important to marine ecosystems?
Introduction
Sharks are often considered as iconic, ruthless predators roaming the world’s oceans. It is
far less common to consider the role all species of sharks play in marine ecosystems as they
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 (The Science Teacher 2007: 1). 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 (Castro, Huber 2008: 2). This is because sharks have been such
successful predators throughout history that there is no requirement for them to adapt to
change their ways. 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 (Deacon, Last, McCosker, Walker, Tricas 1997: 3). 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.
Possible Outcomes
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.
This conclusion can be made from considering the role of shark species as the top predator
in the marine food chain which is essential in regulating the lower trophic levels of the food
web. Alongside this, it is clear that sharks require human intervention through conservation
projects because they are very slow breeders and struggle to adapt to changing
environments caused by human activity and pollution. It is also a fact that the presence of
sharks in global oceans is actually beneficial to human communities and so it is important
not only for shark species but also humans when that conservation projects are
implemented.
Counter arguments might suggest that other marine predators could replace sharks as the
top predators if sharks were to be removed from their environments. On top of this, it could
be argued that human uses of sharks is more important and so humans should be allowed
to exploit them for their medicinal and economic benefits. Some could be suggest that
herbivorous organisms inhabiting coral reef communities are far more essential than
carnivorous shark species and so the conservation of these shark species isn’t important.
Finally, a counter argument might interpret wider swimming sharks as unimportant because
they prey on marine mammals rather than reef inhabitants and so the conservation of
larger, open ocean sharks is unimportant to marine ecosystems.
Sharks’ role as the top marine predators
The most well-known and feared shark is the Great White (Carcharodon carcharias). 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
(Carcharhinus leacuas) and tiger (Galeocerdo cuvier) sharks also play key roles in regulating
marine ecosystems and without them, the balance of predation and prey in the oceans
would be terribly unbalanced. These larger, carnivorous sharks feed on marine mammals
and fish of the sea such as seals, dolphins, turtles as well as seabirds and sea lions (Beer, Hall
2007: 4). 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 (NOAA 2010: 5), 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
which allows body processes such as respiration to occur. 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 and breed. Without the presence of the large shark as
the primary consumer/ top predator, the balance of the food chain is unequal – resulting in
an increase in the number of large fish. This would create an abundance of larger fish such
as mackerel and tuna predating on small fish to such an extent those populations of these
smaller fish could reduce exponentially. In turn, this means the energy flow is unbalanced as
the larger fish 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 and through game fishing to such an extent that large, iconic sharks such as the
Great White and Basking shark (Cetorhinus maximus) are listed as endangered and
vulnerable on the IUCN Red List (Beer, Hall 2007: 4). Human demand for ‘shark fin soup’ has
heavily increased since the turn of the century 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 (Martin 2007: 6) which demonstrates why sharks have
been overexploited for their fins. This exploitation of sharks by humans affects
not only the largest, dangerous sharks but also passive unaggressive species
such as the Great Hammerhead Shark (Sphyrna mokarran) and Whitetip Reef
sharks (Carcharhinus longimanus) which are helpless to human exploits and have been
labelled as ‘Threatened’ on the IUCN Red List as a result (Beer, Hall 2007: 4). 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
(Deacon, Last, McCosker, Walker, Tricas 1997: 3). 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 (Deacon, Last, McCosker, Walker, Tricas
1997: 3). Due to Great White sharks’ slow reproductive rates, even slight changes in their
environment can have dramatic consequences for their population (Martin 2007: 6).
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 (IUCN Red List 2013: 7). 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 endangered 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 (CITES 2013: 8). As a result of these
initiatives the White shark is fully protected in Australian, U.S, South African, Maltese and
Namibian waters (Bruce 2008: 9). 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.
Other than being notorious for their predation skills, sharks – especially reef sharks,
undertake the role of regulating 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 (Carcharias taurus); 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 (Beer, Hall 2007: 4). By doing so, this species of
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.
Effects of removing sharks as the top predators
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 (Wood 1999: 10). This mortality rate of 95% represents mass destruction of
the reef as almost the whole reef this starfish inhabited was subject to damage. This
mortality rate reached such a high percentage because there was no predation effecting the
‘crown-of-thorns’ starfish population. The starfish population was allowed to prosper and
overpopulate its section of reef and in doing so, destroy their own habitat (Wood 1999: 10).
This highlights the fact that predators such as 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 sharks as the top predators in the food
chain have been highlighted in South Africa and Tasmania during the 1970s and 80s
(Nicholls 2012: 11). 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 (Alopias vulpinus) and Whitetip reef (Carcharhinus longimanus) sharks which
overfed on small fish species causing a huge decline in population numbers. This is terribly
problematic for both marine communities as well as human communities which rely on an
abundant supply of small fish to provide food and jobs which fuels the local economy. This
therefore highlights the importance of the presence of large sharks in marine communities
which are essential for controlling the populations of smaller shark species.
Similarly in Tasmania, a shark fishery was blamed for the collapse of a neighbouring crayfish
farm (Nicholls 2012: 11). The removal of predatory sharks by the fishery had allowed
populations of octopuses to prosper which sought after the crayfish farms’ crayfish which
caused the farm to collapse. Octopus is popular prey for smaller, reef sharks such as the
Nurse shark (Beer, Hall 2007: 4) and so without the presence of species of sharks such as
this, octopus populations were allowed to prosper exponentially. Once again, the
importance of predatory sharks is highlighted by this example of the destruction caused as a
result of removing them from an environment. The example of Tasmania also emphasises
the importance of sharks to human communities.
Scripps Institution of Oceanography Investigation
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 (Nicholls 2012: 11). 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” (Nicholls 2012: 11). 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 (Nicholls 2012:
11). 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. It seems that prey fish are reliant on the presence of predatory shark
species in order to mate and develop at a fast rate. Without the presence of predatory
sharks, the prey fish were described as being “consistently fatter” upon dissection it was
seen that their “intestines were lined with fat” (Nicholls 2012: 11). In comparison, prey fish
at sites undisturbed by humans were described as “slender” which coincided with a more
productive coral reef community. The results of this investigation comprehensively reveal
how essential to all types of marine life the presence of sharks is and illustrates why
conservation of vulnerable sharks is a vital practise. It is obvious from the evidence provided
by the Scripps Institution of Oceanography that sharks are extremely important components
to marine ecosystems.
As Sheila Walsh indicates, there are also benefits for humans when considering shark
conservation projects (Nicholls 2012: 11). 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.
Basking and Whale Sharks
Furthermore, different shark species show diverse feeding patterns which allow them to
prey on almost all creatures found in the sea. The largest sharks in today’s
oceans are the Whale (Rhincodon typus) and Basking sharks (Cetorhinus
maximus) which peculiarly 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 (Pope 2002: 12).
Most of these tiny animals are in fact larvae (eggs) of smaller fish, crustaceans and other
small, marine organisms. Basking sharks filter through 1800 tonnes of water every hour to
collect their plankton meal. This feeding process contributes to the regulation of the
population growth of smaller organisms as they are consumed before they are able to
develop into fully grown adults. This again highlights the role of sharks as regulators of
marine life. Even though large sharks such as the Basking and Whale sharks don’t actively
hunt their prey, they do contribute greatly to controlling the population growth of smaller
marine organisms. From this, it is clear to see the importance of not only smaller, bottomdwelling sharks but the largest sharks to marine ecosystems because of their feeding
patterns. Once again, the conservation of all species of sharks is vitally important in
restoring the balance of marine communities as shown by the diet patterns of even the
largest, less predatory sharks.
Replacing sharks as the top predator
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 they exhibit a similar
diet. Killer whales are active predators that feed on a wide variety of fishes along with
marine mammals such as seals, sea lions, baleen whales, penguins and sea otters (SeaWorld
2013: 13). 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 represents 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 (Vancouver Aquarium
Cetacean Research Program 2011: 14). 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 a limited proportion of the sea in comparison to that by large
predatory 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 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 (National Geographic 2013: 15). 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 (Carcharias
taurus) and Nurse shark would be easily replaced by these other reef predators. Although
this may appear plausible, if these predators took dominance there would be nothing to
predate on them in order to limit their population growth as demonstrated by the
Tasmanian example of octopuses causing the destruction of a neighbouring crayfish farm.
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 are likely to 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 due to the gestation period in female shark species usually lasting at least
2 years. 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 would result in a terribly unbalanced marine food web.
Medicinal uses from sharks
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 affecting their vital organs (Pope 2002: 12). 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
(Squalus acanthias) produces liver oils containing 10 times the amount of vitamin A than
that of cod liver oils (Pope 2002: 12). 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 respective roles in marine ecosystems.
Furthermore, shark oils can have great use during medical operations as they can be used as
anticoagulants during heart surgery in order to stop the clotting of blood. It is able to be
used in this way because it suppresses the enzyme activity of thrombin which would convert
soluble fibrinogen into insoluble fibrin; causing the blood to clot. Moreover, dried shark
brain acts as a general pain killer and can prevent tooth decay (Pope 2002: 12). 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 (Pope 2002: 12).
Therefore, it could be proposed by some that human medicinal uses for sharks are more
important than their roles within their own marine environments. However, it would be far
more intelligent to conserve and protect shark species in order to create a sustainable
source of these medicinal properties for continual human use. 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 population of vulnerable
shark species and in doing so, create a sustainable supply of medicinal products from which
humans would benefit to a far greater extent over many generations.
Role of Herbivores on the reef
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
(Wood 1999: 10). Herbivorous organisms are essential to coral reef
productivity as they facilitate the flow of energy to higher trophic
levels within the marine food web (Wood 1999: 10). As depicted by
the food web on the right, herbivores, crustaceans and molluscs
make up the middle three sections ranging from trophic levels 2 to 4
(World Aquaculture 2000: 16). Without this foundation, the
carnivorous “finfish” placed in the top trophic level (level 5) would
be without prey and would struggle to survive as a result. From this,
it can be concluded that the herbivorous foundation is essential to
carnivorous sharks’ survival. It could also be argued that herbivorous organisms are able to
prosper without the presence of predatory sharks whereas the opposite is not possible.
Therefore some may suggest that sharks are not in fact essential to marine communities but
herbivorous organisms rather are more essential in creating a productive coral reef
community. This is because the foundation trophic level populations of herbivorous
organisms such as crustaceans and molluscs are able to continue surviving even in the
absence of carnivorous “finfish”.
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 (Wood 1999: 10).
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 because of the feeding nature of herbivorous reef organisms which
creates a greater opportunity for subordinate and opportunist species to grow instead
(Wood 1999: 10). 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 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 (Wood 1999: 10). Therefore, it isn’t a
sensible conclusion to believe that sharks aren’t important to marine communities just as it
would be implausible to suggest that herbivorous creatures are unimportant in the same
way.
Unimportance of large sharks
A final interpretation that may be put forward is to suggest that larger sharks that roam the
open ocean are 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 its prey 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 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 (Pope 2002: 12). This is why the Bull shark is so dangerous to humans
because it regularly enters fresh waters in which humans are more likely to be present. This
species represents just one example of the diversity 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 of the large fish and marine mammals they interact
with. Once again, this illustrates how large predatory sharks affect marine ecosystems as
they feed on prey labelled as being in the top trophic levels in the marine 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.
To what extent are sharks important to marine ecosystems?
It could be argued that the conservation of sharks is not important to marine ecosystems
firstly because of their potential replacement by other predatory species such as killer
whales, barracuda and blue marlin. 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 vulnerable sharks
should definitely be conserved is further supported because of the incentives for humans as
a result of this. 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 incredible 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 other
marine predators showing similar diet patterns to sharks have the capabilities to replace
them as the ‘top’ predator 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.
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(book).
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