Population Ecology

A limiting factor is an abiotic or biotic factor that restricts the number of individuals in a population.

Limiting Factors: Biotic

Competition: results when organisms struggle to survive in a habitat with limited resources.
Predation: The predator-prey relationship is a balance between the two populations. As the prey population increases, the predator population increases. As the prey population decreases, then so does the predator population.
Diseases and parasites: can be dependent on population size. Larger populations mean more parasites and disease, therefore limits how big population can get.

In the UK, one of the most valuable crops planted by the Forestry Commission is spruce because its yield of timber is high. However, spruce trees grow very slowly when planted on land on where heather was also growing. a) Name the type of competition shown between spruce and heather; b) Give two resources for which spruce and heather are likely to be competing.

a) Interspecific; b) light; named nutrient, water

Limiting Factors: Abiotic

Populations can also be limited by abiotic factors, such as:

  • Space
  • Availability of nutrients
  • Pollution
  • Natural disasters
  • Extreme climatic events (drought, cyclones, global temperature change

Because of these limiting factors, each ecosystem has a finite capacity for growth connected to its carrying capacity.

Carrying capacity: The size of the population that can be supported indefinitely on the available resources and services of that ecosystem.

When a population is BELOW its carrying capacity, it will INCREASE in size. Birth rate exceeds death rates.

When a population is ABOVE its carrying capacity, it will DECREASE in size. Death rate exceeds birth rates.

Births, deaths, and net migrations determine the numbers of individuals in a population.

Population depends on the number of individuals added to the population from births and immigration, minus the number lost through deaths and emigration.
Calculating Population Growth

We can calculate the population size using the Lincoln Index and the capture-recapture method (also known as mark-release-recapture).

  1. Collect a sample of individuals, mark them and then release them.
  2. After a period of time, collect more individuals from the area and count the number that have been marked.
  3. We assume that a sample, if random, will contain the same proportion of marked individuals as the population does.

\fn_jvn N = \frac{M\; x \; n}{m}

Where

N = Size of whole population

M = number of individuals caught, marked and released initially

n = number of individuals caught on second sampling

m = number of individuals recaptured that were marked.

Explain how scientists could use the mark-release-recapture technique to estimate the population size of a species of insect.

  • collect a sample of insects in each area and mark unobtrusively/in a way not harmful to insects
  • release and allow time to re-integrate with rest of population
  • collect second sample and count number marked
  • estimate number in population using the Lincoln Index Equation : (number marked caught and marked initially x number caught on second sampling / number of individuals marked)

In a study of one population of phascogales, 72 animals were trapped and marked with ear tags. They were then released. One month later, fieldworkers examined 120 phascogale and found that 14 of these had ear tags. Use these figures to calculate the size of the phascogale population.

(72 x 120 / 14) = 617