This essay is an example of what could be asked at the end of AQA A - Level Biology paper 3 as part of the synoptic section. This essay achieved 21/25 and the student finished the course with a grade A.
Write an essay on the many different types of relationships and interactions between
organisms (25 marks)
There are numerous ways in which organisms interact with one another, this can take place
on a large scale, such as how humans have an impact on ecosystems at a global scale, and
also a much smaller scale, for example how a virus interacts with a host cell.
One way in which organisms Interact with one another is through courtship behaviours. This
is when organisms of the same species use certain behaviours in order to attract mates of
the same species. A species is a group of organisms that share a common ancestor and are
able to interbreed to produce fertile offspring. An example of courtship behaviours can be
seen in fireflies. There are many species of fireflies, and so when needing to mate, it is
important that an individual attracts an organism of the same species in order for successful
reproduction to occur. Some fireflies use bioluminescence. They will have certain patterns of
flashing when searching for a mate and the pattern that is produced by the signal will only
attract a meteor the same species as they will recognise that pattern and will be attracted by
it, which will then lead to successful mating.
Organisms of different species also interact in various ways. Prey and predator
interrelationships allow both of the populations to survive. They go through natural cycles
where the number of both populations rise and fall based on availability of prey and the
amount of predators. Numbers fluctuate and help the populations to remain at a stable and
sustainable level as when prey numbers increase, predator numbers also increase as there
are more prey to feed on, helping to control prey numbers. Then as the number of prey falls,
the number of predators also fall as food sources become more limited, ensuring that the
prey population is not fully removed from an ecosystem. These healthy cycles allow for both
populations to be able to live in the same ecosystem, allowing higher biodiversity and also
controlled populations. This can also benefit other organisms within the ecosystem. For
example, a species of wolf was reintroduced to Yellowstone national park in the state of
Wyoming to help control elk population. As the elk had no predators and so over time, the
land became overgrazed. When the wolves were introduced, the elk had to stay on the move
and so the land had time to grow and the elk population became more stable. As the land
was able to regrow, other organisms such as birds and bears returned as berries could grow
on bushes, rabbits returned as the grass regrew and river banks sprouted small trees which
changed the course or rivers, helping beaver populations, which in turn created pools that
provided homes for species of fish.
However, humans can negatively affect the relationships between organisms. There are
numerous ways in which human activity has negatively impacted the environment. An
example of this is by constructing dams and reservoirs to store water for irrigation. The
Mekong river, which flows through several Asian countries including Cambodia, China,
Vietnam and Thailand, has been dammed numerous times to store water for irrigation in
farming and to generate hydroelectric power. During the construction process, areas of
forest and other land had to be cleared so that machinery could enter. This led to the
destruction of the surrounding ecosystems and a loss of biodiversity. The machinery is also
run by fossil fuels which leak into the water and contaminate it with pollutants, causing
damage to the surrounding aquatic ecosystems. In the
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