A limiting factor is an abiotic or biotic factor that restricts the number of individuals in a population.
Limiting Factors: Biotic
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.
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.
Calculating Population Growth
We can calculate the population size using the Lincoln Index and the capture-recapture method (also known as mark-release-recapture).
- Collect a sample of individuals, mark them and then release them.
- After a period of time, collect more individuals from the area and count the number that have been marked.
- We assume that a sample, if random, will contain the same proportion of marked individuals as the population does.
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.
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.