Population Interactions
The niche
In ecology a niche is an organisms place within an ecosystem. All species have certain limitations that
affect their distribution, be it the temperature, light intensity, competition with other species etc. The
fundamental niche is the potential mode of existence given the adaptations of the species. The
realized niche is its actual mode of existence resulting from its adaptations and competition with
other species. Red squirrels are adapted to living in both coniferous and broadleaved forests, but they
need trees for food, shelter and to escape from predators, so they cannot live in other habitats. This is
their fundamental niche. However, when the grey Squirrel was introduced to Britain from America it
was very successful and spread quickly. It was more competitive than the Red Squirrel in the
broadleaved forests but less successful in the coniferous forests. In Britain the realized niche of the
Red Squirrel is the coniferous forests (though on some islands they can still live in broadleaved
forests because the Grey Squirrel never got there).
From: http://rsst.org.uk/about-us/faqs/
Competition
When two species live in similar niches there will be competition for resources. When two species
compete for resources it is known as inter-specific competition. There may not be enough food for
them both, or nesting sites, etc. This can affect both populations and keep the numbers down, or one
species might be excluded completely. In the example above, we saw how competition between Red
and Grey Squirrels in Britain resulted in the Red Squirrels dying out in broadleaved forests. They
could not compete for resources as well as the Grey Squirrel and there was no room for both species
in that niche. This is an example of competitive exclusion, because one species (the Red Squirrel)
is excluded by competition from a habitat that it could otherwise have occupied.
In other situations there is less direct competition for resources because the niches are slightly
different, so both species can live together. Brown Bears and Bald eagles both feed on Salmon in the
rivers of North America. If there were fewer bears there would be more food for the eagles and vice
versa, so there is competition. However, both species have different niches, so the competition is not
so high that one dies out.
Individuals of the same species can also compete for resources, known as intra-specific
competition. As a population grows the competition for resources will become greater, which may
limit further population growth.
Herbivory
Herbivores feed on plants, so can affect the plant populations. Overgrazing in Mongolia is an example
of this, with the vegetation on the steppe being much shorter than what it would be naturally.
Overgrazing also affects the type of species growing, as some species are more able to survive such
grazing pressure than others. Equally the availability of plants determines the herbivore population
feeding on it. In East Africa the famous migration of the wildebeests is due to the different growth of
plant species in different areas at different times.
Predation
As with the relationship above between herbivores and plants, there is a relationship between
predators and their prey. Worldwide, there has been a general decline in the populations of top
carnivores (such as tigers, lions wolves etc), resulting in changes in the populations of many prey
species and the increase in the populations of many smaller carnivores. In tropical Africa Lions and
Leopards have declined, so other predators (such as baboons) have filled the gap and become more
common.
Where there is a close link between one predator species and one prey species (such as the Lynx
and the Snowshoe Hare in Canada) we often have a classic predator prey cycle (see diagram below).
If the hare population goes up there is more food for the lynx, so within a few years the lynx
population is higher. This results in more hares being eaten, so the hare population drops very
quickly. The fall in the hare population causes a massive reduction in the lynx population. For the next
few years (while the lynx population recovers) the hares do extremely well and the population
increases rapidly, resulting in the lynx population rising, etc.
From: https://missbakersbiologyclasswiki
Parasitism
A parasite does not kill and consume its prey, like a predator, but it gets nourishment from its host and
usually causes it some harm. See examples below.
Tapeworms live in the intestines of humans and many
other species. They have suckers and hooks on the
head to attach to intestine wall and they absorb nutrients
from the food eaten by the host. Humans and pigs are
both parasitized by the same type of tapeworm. Humans
mostly get tapeworms from eating pork that has not been
cooked properly.
From: http://2.bp.blogspot.com
Elephantiasis is caused by a number of
different parasitic worms that are
transmitted by mosquitoes. The worms
live in the lymphatic system where they
block vessels, causing a build-up of
lymph. It causes a thickening of the skin
and underlying tissue resulting in gross
malformations of the body.
From: http://www.buzzoff.org
Mutualism
This is where two species gain benefit from associating together (see examples below).
Hermit crabs do not have their own shells, but
live in the old shells of other species. They
often have a sea anemone attached to the
shell. The anemone has stinging tentacles that
help to defend the crab from attack by
predators. The anemone gains as well
because when the crab feeds it produces lots
of bits of food for the anemone to take from the
water. When the crab grows it needs to find a
new shell. When it moves to a new shell it
takes its anemone with it.
from: http://media.photobucket.com
Cleaner fish get food by removing parasites from
bigger fish and they will even enter the mouth of
predators (such as the eel above) to get food
stuck in the teeth. The cleaner fish benefits by
getting free food. The larger fish benefits by
getting cleaned and will not eat the cleaner fish,
even when it goes into its mouth.
From: http://www.islandive.com