spec point:

(i) how the distribution and abundance of organisms in an ecosystem can be measured

(ii) the use of sampling and recording methods to determine the distribution and abundance of organisms in a variety of ecosystems.

M1.3, M1.4, M1.5, M1.7, M1.9, M1.10, M3.1, M3.2 PAG3 HSW4

keyword reminder

species richness: number of different species living in a habitat

species evenness: comparison of number of individuals of each species living in community

units

areas of land: g/m^-2 (grams per square metre) 

areas of water: g/m^-3 (grams per cubic metre)

sampling a habitat

Preparation

• Keys to identify the different species

• Camera – record specimens and locations

• Suitable clothing and footwear

• Apparatus for sampling

• Somewhere to record observations

measuring species richness and evenness in habitat

Plants 

Can count the:  

• Density number of plants per m²

• Frequency  estimate of no. of species

• % cover (number of each species)

Ground insects

• Insects fall in and can’t get out.

• capture mark release recapture

Flying insects’

• ‘sweep net’ method

• capture mark release recapture

Aquatic animals

• use a net

• capture mark release recapture

capture mark release recapture

capture and mark

1. Capture as many animals as you can in a defined time.

2. Count them and Mark them in a way that causes no harm

3. Release them to mix in the population.

recapture

4. After a few weeks/days capture the same species over the same time.

5. Count how many are marked and how many are not marked.

population size

limitations

Assumptions need to be made:

• No death, immigration or emigration

• Identical sampling methods

• marking does not affect the survival rate of the animals

random sampling

Each individual has an equal chance of being picked.

• Mark out a grid at right angles

• Use a random number generator to pick the x co-ordinate, then the y coordinate.

• Take a sample from that place.

advantage

Data not biased by selective sampling

disadvantage

• May not cover all areas equally

• Species with low presence may be missed

non-random sampling

systematic

Samples taken from fixed intervals across the habitat

Examples: line transect and belt transect

• line transect: line placed down across habitat + species in contact w line are recorded

• belt transect: 2 tape measures laid out and samples taken between 2 at set intervals along the tapes

• quadrat: square frame of given size, randomly placed in area being sampled species inside quadrat are identified + counted to determine abundance

advantages

• Useful when habitat shows a clear gradient in environmental factors

disadvantages

• Only species on the line/ belt are recorded leading to an underestimate

stratified

Divide habitat into areas which appear to be different and sample each section separately

advantages

• Ensures all areas of habitat are sampled

• Species  not under-represented

disadvantage

• This may lead to over-representation

• Could have a disproportionate number of samples taken in smaller areas that look different

opportunistic

Researcher takes samples based on prior knowledge or during the process of collecting data. May sample an area that they know contains a particular species.

advantages

• Easier and faster than random sampling

disadvantages

• Data may be biased

• Overestimate of biodiversity

simpson’s index of diversity (D)

way to measure species diversity that takes into account species richness and species evenness.

interpreting results

Always get a value between 0 and 1

High value (Closer to 1) = more diverse, can usually cope with change

Low value ( close to 0 ) = less stable and easily damaged by change