Populations and Communities

A population is all the individuals of a particular species in a particular habitat at a particular time

Population growth exhibits four stages:

• Lag phase - the population is low and relatively stable as organisms adapt to their surroundings (nutrient assimilation, egg production/development, gestation period)
• Exponential phase - the population increases rapidly because there are abundant resources. Population is illustrating its biotic potential (reproductive capacity of population under optimum environmental conditions)
• Stationary phase -the population numbers are high and stable as births are matched by deaths, this is the carrying capacity (max number that it can support) of the environment
• Decline phase - overuse of resources or accumulation of waste products causes a decline in the population. If resources renewed, the population may recover, otherwise it crashes

Two types of population growth strategies are recognised:

• r-selected species - show rapid growth/colonisation, exhausting resources quickly, and move on. A 'boom-and-bust' strategy
• K-selected species - are more conservative, populations grow slowly though numbers remain relatively stable at the carrying capacity, unless distrubed

The factors that limit population growth are known as environmental resistance - can be abiotic or biotic.

Population numbers are influenced by a number of biotic factors:

• mutualism (+/+) - where both populations of different species benefit from the association
• predator-prey (+/-) - where one species (the predator) preys on another, oscillating population curves may result, with predator numbers lagging behind those of the prey
• inter-specific competition  (-/-) - where two species compete for a common resource, the outcome is the removal of one species by the other

Biological control is the use of predators, parasites or pathogens to limit the numbers of pest species. The control species should be highly specific, long-term and should not contaminate the environment.

A community = biotic component of an ecosystem involving interactions between autotrophic and heterotrophic populations

An ecosystem = a community of different species which are interdependent and interact with each other and their abiotic environment

Succession is the progressive change in the composition of a community over a period of time.

The first organisms to colonise a habitat are pioneers. Colonising organisms bring about changes that allow other organisms to colonise the habitat. A series of additions and replacement of species occurs.

Each distinct community in a succession is called a sere.

The final stable end stage that is in dynamic equilibrium with the environment is the climax community

If an area has never been colonised before (e.g. a newly formed volcanic island), the process is called primary succession.

If the area was previously covered with vegetation (e.g. a forest site that suffered a severe fire), secondary succession occurs.

There are different types of climax community:

• climatic climax composition is dependent on climate, such as temperature and rainfall
• biotic climax composition is influenced by biotic factors, such as grazing