Posts Tagged ‘2.3’

  • Construct simple keys and use published keys for the identification of organisms.

Keys called dichotomous keys are used to identify species, the key is written so that the identification is done in steps. At each step two options are given based on different possible characteristics of the organism you are looking at.  You go through all the steps until the name of the species is discovered. This is an example of a dichotomous key that divides 4 types of egg-laying species:

For the exams you need to have at least eight species in the key you construct. This can also be shown graphically:

  • Describe and evaluate methods for estimating abundance of organisms.
It is impossible for you to study every organism in an ecosystem, so limitations must be put on how many plants and animals you study. There are trapping methods which help obtain more samples, like:
  • pitfall traps
  • small mammal traps
  • light traps
  • tullgren funnels
You can either count them all or using percentage cover of an organism in a selected area or by using the Lincoln index and calculating the abundance.
Lincoln index:
This method allows you to estimate the total population size of an animal in your study area. This method includes collecting a sample from a population, then marking them like painting or attaching something to the animal, releasing them back into the wild, then resampling some time later and counting how many marked individuals you find in the second capture. IT is important to take into consideration that the marking methods are not harmful to the animal and clear so that they do not become easy targets for prey.
This method is also known as capture-release-mark-release-recapture techniques because of the processes involved. If all the marked animals are recaptured that is assumed to be the total population size of that species. whereas if half of the species is captured it is estimated to be twice as much as the first sample. The formula used to calculate population size:
N= total population size of animals in the study site
n1= number of animals captured of first day
n2= number of animals recaptured
m= number of marked animals recaptured on the second day
N= (n1 x n2) / m
Quadrats:
Quadrats are used to measure the percentage cover of a certain species. Ecologists want to find out how many organisms are living in a specific area, however they cannot count them all so they make a sample count. Percentage cover is the area within the quadrat being used by one particular species.
Percentage cover is worked out for each species present. Dividing the quadrat into a 10×10 grid helps to estimate percentage cover.
Sample methods must allow for the collection of that is scientifically representative and appropriate, and allow the collection of data on all species present. Results can be used to compare ecosystems.
Percentage frequency is the percentage of the total quadrat number that the species was present in.
  • Describe and evaluate methods for estimating the biomass of trophic levels in a community.
*Biomass:  the mass of organic material in organisms or ecosystems, usually per unit area. Biomass is calculated to indicate the total energy within in a living being or trophic  level. The greater the mass of the living material the greater the amount of energy present. Biomass is taken as the mass of an organism minus water content, like dry weight biomass. Water is not included in biomass measurements because the amount varies from organisms to organism, it does not contain energy and is not organic.
To obtain the samples, the biological material is dried to constant weight. It is then weighed. The specimens are then heated in a  oven which is not hot enough to burn the tissue and left for a certain amount of time. Biomass is usually measured per unit area so that comparisons can be made between the trophic levels present.
  • Define the term biodiversity.
Diversity is often considered as a function of two components: the number of different species and the relative numbers of individuals of each species. This is different from species richness, which refers only to the number of species in a sample area.
  • Apply Simpson’s diversity index and outline its significance.
There are many ways of quantifying diversity, one of the ways is using the Simpson’s diversity index:
D= diversity index
N= total number of organisms of all species found
n= number of individuals of a particular species
E= sum of
D= (N(N-1)) / (En(n-1))
*It is not important to remember the whole formula, but good to know the meaning of the symbols.
D is a measure of species richness. A high value of D suggests a stable and ancient site, and a low value of D could suggests pollution, recent colonization or agricultural management. The index is normally used in studies of vegetation but can also be applied to comparisons of animal diversity.
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