Module One 11.5 What determines where particular species live and how many of them there are?
The number of organisms present in a habitat is dependant on many things, some of which are listed below.
a) Temperature
The penguin would be the best suited to a cold environment.
b) Light
A plant needs light in order to photosynthesise.
c) Water
A fish lives in water, whereas a cactus will survive quite happily in a very dry environment i.e. desert.
These factors will change depending on the time of day and year.
e.g. 1. night: plants have no light therefore will not be able to photosynthesise and nocturnal animals such as the bat will hunt for food at night, therefore you will find more bats about at night-time.
2. winter: less light is available and the ground is often frozen, plants will be unable to photosynthesise so much and therefore the growth of most plants will slow rapidly.
Organisms will only grow when conditions are suitable which means the number of organisms in a particular place will vary over the year. Most plants will grow best in summer and some birds migrate to hotter climates in winter.
To help organisms survive longer, they have become adapted (or changed).
A good example of this is in some flowers. The flower of some orchids looks like the female of some wasps or bees. This attracts the male insect into the flower and it becomes covered in pollen. When he is later attracted to another flower, some of the pollen will fall into the flower and fertilisation will take place.
Some adaptations act as defence mechanisms. These can include the animal simply blending into its environment, it is camouflaged e.g. a peppered moth will be darker in industrial areas so that it will blend onto the surface of trees covered in pollution whereas in rural areas the moth is lighter.
Other animals have evolved colours and patterns that contrast boldly with their background and make them completely visible to predators. These animals may taste unpleasant and the predator will have learned to recognise them. Their appearance is protective as it warns potential predators that they should stay away.
Animals such as the camel and the polar bear have become adapted to harsh environments:
The Camel- thick long eyelashes, nostrils can be closed
- water can be stored in stomach lining (up to 100 litres)
- kidneys produce very concentrated urine, does not sweat
- tough lips and grinding teeth
- flat wide feet and long legs
- very little fat under skin, fat stored in hump
The Polar Bear
- white fur is good camouflage against the snow when hunting
- the polar bear's skin is black to absorb heat
- its long hairs are hollow to act as an insulator and for buoyancy when swimming
- when walking, it appears bow-legged and pigeon-toed, both adaptations enable it to maintain balance on slippery ice
- it has excellent sense of smell (able to smell a dead whale up to 20 miles away)
- except for the balls of the feet, the soles are covered with hair, making a non-skid tread
- has two layers of fur over a thick layer of fat
- its coat is warm and water proof.
Adaptations of plants to arid (very dry) conditions:
High temperatures increase the rate of transpiration (water loss from the plant).
- In autumn deciduous trees lose their leaves removing the evaporating surfaces and therefore reducing water loss when supply of water in poor (this will also reduce the rate of photosynthesis and the plant cannot make its own food, it becomes dormant).
- Trees like the pine tree (evergreens) survive in cold climates by having small, compact needle-like leaves. These needles have a smaller surface area than normal leaves and therefore not as much water will be lost from the plant.
- Some plants have rolled up leaves, sunken and few stomata (holes in the lower side of the leaf that allow gases to pass in and out) or hairy leaves which all help to keep a layer of air saturated with water near the surface of the leaf.
Cacti are adapted in many ways to the desert conditions in which they live:
- some have no leaves (photosynthesis carried out by the thick green stem), this reduces the surface area for evaporation
- they can store water in their fleshy tissue and use this for photosynthesis
- the stomata stay closed during the day to reduce water loss
- they have a thick waxy cuticle layer, which reduces evaporation.
The body size and surface area also help organisms survive in different environments. A small object has a large surface area in relation to its size. This means it will lose heat quicker than a large object and it will need more energy to maintain its body temperature. A large object has a small surface area relative to its size. This means it will not cool down as quickly as a small object.
All living things compete for resources to stay alive. Competition takes place between organisms of the same species and with other species. When resources are scarce or hard to find not all the individuals in a population will survive.
Animals compete for space, food and water. Plants will compete for space, water and nutrients from the soil.
Predators and preyAnimals that hunt and eat other animals are called predators and the animals which are eaten are called prey. The number of prey present in a habitat will affect the number of predators present and visa versa e.g. if the size of a population of rabbits increases then this means there will be more food for the foxes which eat them. This means more foxes will survive and multiply, eventually there will not be enough rabbits to feed all the foxes. The numbers of foxes will decline and this will mean the number of rabbits will be able to rise again (see below).
Population size What determines how many organisms will be able to survive?
The size of a population will depend on several factors:
a) amount of food
b) competition for food
c) amount of light available
d) disease
e) the number of predators or animals grazing
Adaptations
How Organisms Adapt to Survive in a Particular Habitat - a lesson