Ecological Field Investigations
Field work
- Trends in ecology can be spotted by measuring the numbers and species of organisms in an ecosystem and how these change over time
- Knowledge of these trends can inform humans about how to take the best care of the land, lakes, rivers and oceans
- However, a typical ecosystem is a big place and we cannot count every single organism in it
- In order to have an idea of how many organisms occupy a particular ecosystem, we have to make sensible estimates, based on thorough sampling methods
- The idea is that a well-selected sample can be scaled up to give an accurate estimate of the whole population
- Field work involves scientists getting outside and carrying out work to sample and measure various aspects of ecosystems
- Abundance is the number of organisms of a particular species in a habitat
- Distribution is the geographical spread of a species
A field investigation into the distribution and abundance of organisms in a habitat
- Aim: To measure the population size of a common plant species in a habitat and use sampling techniques to investigate the effect of a factor on the distribution of this species
- Methods:
- Use a quadrat to estimate the population size of a plant species in a survey area
- Use a transect line and a quadrat to investigate the effect of a factor on the number of plants in a survey area
- Scientists can't put a quadrat on every single 50 cm × 50 cm patch of ground
- That would be very time-consuming and tedious work
- So a reliable sampling technique is required
- Essentially, parts of the population are counted, so sampling has to be done in a completely random manner
- The number of organisms in the sample is multiplied up to give a population size estimate
- Data can also be gathered on the distribution of species within the sampling area
- This may be due to abiotic or biotic factors which can also be investigated
Random sampling
- Scientists must go to great lengths to ensure that sampling is random and free from bias
- For example, in a quadrat study looking at thistle plant distribution in a field, scientists must not just focus on the areas where thistles are seen to be growing
- There may be valuable data about why thistles are not growing in a certain part of the field e.g. soil conditions are unfavourable
- Quadrat studies must use random number generators to identify co-ordinates within a marked-out area for quadrats to be analysed
Estimating Population Size Method
- Stratified sampling is also useful
- Divide a habitat into zones which appear to have different communities and take samples from each zone
- For example, if vegetation cover in an area of moorland is 70% heather and 30% grass, take 70% of the samples from within the heather and 30% of the samples from within the grass
- Divide a habitat into zones which appear to have different communities and take samples from each zone
- Systematic sampling is used e.g. with transects
- Systematic sampling is used where the study area includes an environmental gradient (change of conditions across the study area)
- For example, samples taken, every 10 meters along a line running from the sea shore, inland across a sand dune system
- Sampling mobile organisms (most animals) presents risks of missing certain individuals or double-counting others
- A technique called capture-recapture is employed in these cases
- Sampling the abiotic environment is also required to be random
- e.g. taking water samples from the right place in a stream/river system, rather than from the place that might be easiest to reach (the water)