BIOLOGY 1100

Worm Sampling Pre-lab Assignment  - Fall 2010

Reading Assignment: questions will come from the reading below and Exercise 8 on Population Sampling.

Ecological Sampling

A biological population is defined as a group of organisms of the same species occupying a particular area at a particular time.  Ecologists are often interested in describing these populations, e.g., how many individuals, physical characteristics, genetic characteristics, etc.  In most cases, it would be difficult to count or measure all of the organisms in the population.  Instead, ecologists count or measure a portion of the population and infer from this portion the characteristics of the entire population.  A statistical population is the subset of the entire biological population from which one draws conclusions about the biological population.

The accuracy of the description of the biological population depends on how well the statistical population represents the biological population.  Any bias in the sampling would result in an inaccurate description of the population.  There can be several sources of error when collecting data, including:

1) sampling error – errors occurring when selecting subjects for the sample,

2) measuring error – errors conducted when measuring the desired characteristics or counting, and,

3) observer error – differences in measurements that might occur if different observers take the measurements.

While errors may not all be completely eliminated during a sampling collection, the ecologist should try to minimize the errors as much as possible.

            Sampling errors occur when the subjects sampled are more likely to be sampled than others in the population.  For example, if certain animals, such as females, are more likely to be trapped than others or brighter organisms counted more often than dull-colored organisms.  This error should be reduced if subjects are selected at random.  Randomness can be achieved several ways, such as, selecting subjects using a random numbers table as you encounter them or predetermining collecting sites before seeing them. 

            Measuring errors can be reduced by using the same method of measurement, having well-defined methods of measurement, becoming familiar with the measuring technique, and using well-maintained equipment.  When counting individual organisms, great care must be taken to not double-count or miss individuals.  Observer bias can be avoided by using the same observer for all measurements, and if that can not occur, training observers to measure in the same way.  Not all experiments can be perfectly designed to avoid all errors, but the errors should be recognized and can be compensated to some degree by increasing your sample size.

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