Limiting Population Size: Examples (SL IB Biology)

• Consumers that kill and eat other animals are known as predators, and the animals that are eaten are known as prey
• In a stable community the predator and prey population sizes rise and fall in a predator-prey cycle that limits the population sizes of both predators and prey
• The graph below demonstrates some of the key patterns in predator-prey cycles:
  • The number of predators increases when there is more prey available
  • The number of prey decreases in response to an increase in the number of predators
  • The number of predators decreases in response to a decrease in the number of prey
  • The number of prey increases in response to a decrease in the number of predators
  • The cycle repeats
• The relationship between the Canada lynx and the snowshoe hare is a famous example of the predator-prey interaction
  • It is worth noting that relationships of this kind, with a single predator species and a single prey species, are unlikely to exist in this simple form in nature; there will be other predator and prey species, as well as additional factors that will affect the sizes of the respective populations

Predator-prey relationship graph

The predator and prey populations are closely linked in a predator-prey cycle

Top-down & bottom-up population control

• Populations in a community can be controlled by either top-down or bottom-up control factors
• A population that is limited by predators, e.g. the snowshoe hare in the example above, is controlled by a top-down control
  • Plant populations being limited by herbivory is another example of top-down control
• A population that is limited by the availability of resources, e.g. the lynx in the example above, is controlled by a bottom-up control
  • Plant populations being limited by light intensity is also a bottom-up control
• E.g. in the food web shown below, a change in the fox population could lead to a top-down cascade of effects as follows:
  • A decrease in the fox population could lead to an increase in the rabbit population, which could lead to a decrease in the growth of grass
  • Note that the grass → rabbit → fox food web does not exist in isolation, so this top-down effect will influence other parts of the food web as well

Food web diagram

The effects of a top-down control factor on a food web can be complex, as every food chain is connected to several others

• While it is possible for both top-down and bottom-up control factors to act on an ecosystem at the same time, the reality is that any one part of an ecosystem is likely to have one control type that is dominant at any given time, e.g.
  • A coastal seagrass ecosystem is likely to be mainly controlled by bottom-up nutrient availability
  • Overfishing by humans may reduce the number of marine predators, temporarily leading to a switch to top-down control dominance
• Note that top-down control may shape an ecosystem due to both lethal and non-lethal effects
  • Predators kill prey, influencing their numbers, and so their effect on the rest of the ecosystem
  • The presence of predators may affect the behaviour of prey organisms, affecting their choice of diet and where they choose to spend time; this can also alter the structure of an ecosystem

• Species compete with each other for resources; this is interspecific competition
• Some species have strategies which increase their ability to outcompete other species
• Such strategies can work by either increasing the survival chances of a species, or by decreasing the survival chances of a competing species, e.g.
  
  • Camouflage increases a species' survival chances
  • Secretion of harmful chemicals into the environment decreases the survival chances of a competitor
    • Such harmful chemicals are known as secondary metabolites, as opposed to primary metabolites which are molecules that are essential for survival
• Allelopathy is an example of a strategy that involves damaging the survival of a competing species
  
  • Antibiotic secretion in some bacteria is a well-known example of allelopathy

Allelopathy

• Organisms that carry out allelopathy secrete secondary metabolites that harm other organisms into their surroundings, e.g. in plants: 
  • Secreting harmful chemicals via roots into the soil
  • Releasing harmful gases via the stomata into the air
  • Storing harmful chemicals in the leaves which are released when the leaves fall and break down
• Examples of plant species that carry out allelopathy include:
  • Garlic mustard produces a chemical called sinigrin which reduces seed germination and root growth in other plant species
  • Bracken ferns are thought to release toxins into the surrounding soil, as well as containing toxic chemicals in their fronds which are released when they decay
  • Himalayan balsam is thought to secrete allelochemicals into the surrounding soil that limit the growth of other plants

CC BY-SA 2.0, via Geograph

Himalayan balsam shows allelopathy, a strategy that is thought to contribute to its success; it is a known invasive species in the UK, where it is often found along waterways

Antibiotic secretion

• The secretion of antibiotics is a form of allelopathy found in some microorganisms, e.g. the antibiotic penicillin was discovered in Penicillium fungus
  • Antibiotics are also secreted by some bacteria species
• Antibiotics kill bacteria by, e.g. preventing cell wall formation or inhibiting protein synthesis; this reduces interspecific competition, and so increases survival and reproduction in the species that produces the antibiotic