Factors That Control Populations

Red Fox
Copyright Mike Brown

The Red Fox (above) is a typical predator. 

Factors that influence populations include competition, predation, parasitism and symbiosis.

1. Competition

Competition is the struggle between organisms for the same resource e.g. grass, dandelion, buttercup and daisy compete for space, light, water, minerals; fox, thrush and hedgehog compete for earthworms.

Competition reduces population numbers.Intra-specific competition occurs between members of the same species. Inter-specific competition involves competition between different species e.g. blackbirds and thrushes competing for snails and insects.

Two types of competition are:

  1. Contest competition is an active physical confrontation between two organisms which enables one to win a resource e.g. competing stags (red deer) interlock anthers until one withdraws when competing for a mate.  Robins – birdsong in spring is male robins warning off others from their territory.
  2. Scramble competition each organism tries to acquire as much of the resource as possible e.g. chicks in nest depending on parent for food.

Effects of competition

  • Enables evolution of a better-adapted species and elimination of the less well-adapted species.

Adaptations to survive competition

  • A grass plant produces large quantities of pollen increasing its chance of reproducing.
  • Blackbird ‘song’ to warn competitors to stay away.
  • Yellow petals of buttercups to attract insect pollinators.
  • Bacteria in soil secrete chemicals to inhibit their competitors.
  • The caterpillar of the cabbage white butterfly chews on cabbage leaves, while the adult butterfly drinks nectar from flowers.

Some effects of biotic factors on organisms

Factor Effect Example
Food As more food becomes available a greater number of organisms will survive Number of berries impacts the number of blackbirds. Quantity of plankton affects the number of mussels
Parasitism (organism taking food from living host) A parasite weakens the host and may reduce their numbers Fleas infect foxes and rabbits. Sea lice can infect fish
Pollination and seed dispersal Many plants need animals to carry pollen and seeds Insects are effective at pollinating plants. Birds disperse seeds when they consume fruits and egest the seeds
Humans Humans can have an impact (positive/negative) on other organism Pollution damages the environment. New parks for new environments for life.

2. Predation

Predation refers to the catching, killing and eating of prey by its predator.

Prey Predator
Insects Spiders
Fish Bears
Aphids Ladybirds
  • Predator adaptations

A fox has reddish fur to camouflage and avoid detection by rabbits. It also has developed long canine teeth to kill prey and tear flesh, and can travel at speed to outrun prey to capture it.

  • Factors that improve the efficiency of predators:
    • Good eyesight (e.g. hawk and other birds of prey), hearing, sense of smell and dentition.
    • Catching whatever is easy, reduces energy and effort.
    • Adapting diet as numbers of prey change e.g. foxes and spiders.
    • Living in packs can help locate food and make prey easier to catch.
    • Ability to migrate to areas where the prey is more plentiful
    • Camouflage
    • Catch large, rather than many small prey.
    • Ladybirds have strong mouthparts to enable them to chew aphids.
  • Prey adaptations

A rabbit digs narrow underground burrows which prevents large predators from entering. It also has long ears that provide good hearing to detect a predator. It also has a white tail to act as a warning signal to other rabbits.

Plants can develop thorns, spines and stings e.g. holly and cacti to deter predation. Some plants also have a bad taste to deter predators e.g. giant hogweed.

Animals can often swim, fly and run faster than predators. They can also mimick animals the predator would normally avoid e.g. hoverfly is a harmless insect but protects itself by mimicking the colouration of wasps. Animals often stay in large groups to avoid predation e.g. in flocks and herds like deer. They can camouflage to enable them to blend with surroundings e.g. greenfly, stick insects, frogs. Some animals are poisonous to predators e.g. caterpillar of large white butterfly, ladybirds contain large amounts of formic acid, which is unpalatable to predators. Mice also flee and hide to avoid being eaten.

  • .Effects of predation in ecosystem:

Predation maintains the prey species at a sustainable level and is a major factor in the evolution of prey species.

  • Predation initially increases the numbers of predators and decreases the number of prey. 
  • Predators and prey often show repeated cycles of increasing and decreasing numbers.
  • The predator-prey relationship is used in biological control of pests e.g. ladybirds are used to control greenfly and certain bacteria are used to control the larvae of butterflies and prevent them from destroying crops (e.g. cabbage).
  • Population dynamics

Population dynamics describes the factors that cause changes in population numbers. Predator-prey numbers interact due to:

  • availability of food, which increases predator numbers when high but reduces them when low;
  • concealment, which means that some prey survive by hiding from predators;
  • predator movement to new areas when prey numbers are low.

Human population continues to increase despite famine, disease, war and contraception. Rapid population growth in the 20th century was due to reduced infant mortality and people living longer because of improved sanitation and medicines and disease eradication programmes.

  • Famine can occur when an acute shortage of food affects a population and can cause starvation, death and mass emigration. Famine is often associated with war as war zones have reduced agriculture.
  • Reduced disease and the ability to cure diseases through vaccines, antibiotics, improved sanitation, insecticides, safe anaesthetics, improved surgical methods and new drugs) have helped to reduce the death rate and increase the human population.
  • Wars normally reduce population numbers temporarily due to death. However, birth rates often increase following wars.
  • Contraception has helped to reduce birth rates.

3. Parasitism

Parasitism occurs when two organisms of different species live in close proximity and one organism (parasite) obtains its food from, and to the disadvantage of, the second organism (host).

A parasite is an organism that feeds from and harms another organism. Parasites sometimes reduce the numbers in a population e.g. potato blight or disease-causing bacteria, but often have little effect on host numbers.

Endoparasites feed in a living host e.g. liverfluke, potato blight fungus, bacteria of disease in human body.

Ectoparasites feed on the outside of the host e.g. fleas, athletes foot fungus, mosquitoes on human skin, greenfly on a rosebush, blood-sucking leeches on human skin, lice on hawks

Parasites are often thought of as predators but they differ by:

  • Being smaller, often attacking from within, being dependent on one particular host, and only do a small amount of damage to host so that they do not harm their food source or home.

4. Symbiosis

Symbiosis is a close association between two organisms of different species in which at least one of them benefits. Symbiosis also increases the numbers of both species.

In one type of symbiosis, mutualism, both organisms benefit.

  • Lichen - consists of a fungus and an alga.  The alga makes food for both and the fungus absorbs minerals and water for both and give protection and support.
  • Nitrogen - fixing bacteria in root nodules of legumes e.g. clover. Bacteria provide nitrates for plant to make protein and the plant provides carbohydrates, shelter and anaerobic conditions for bacteria.
  • Bacteria - living in large intestine, supply us with vitamins B and K and protect us from pathogenic bacteria. We supply bacteria with food, water and protected site to live.
  • Niche describes the role a species plays in the ecosystem
    • e.g. what it eats, what it is eaten by, how it interacts with other organisms and with its abiotic environment (The Physics Teacher, 2018)

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