Environmental Sampling
There are four
environmental sampling:
i)
Quadrat sampling
ii)
Capture-recapture or mark-recapture sampling
iii)
Transect sampling
iv)
Adoptive sampling
Quadrat Sampling
We start with a
general technique for environmental sampling particularly for ecological
studies, rather than with a specific sampling method if we wish to count the
numbers of one, or of several, species of plant in a meadow, we might throw a
Quadrat at random and do our count, or counts, within its boundary.
Correspondingly, for aquatic wildlife we might cast a net of given size into a
pond, river or sea and count what it trawls.
A quadrat is usually
a square (or round) light wood or metal frame of (often) a meter or several meter
side (or diameter). Where it lands
defines the search area in which we take appropriate measures of numbers of
individual plants, biomass or extent of ground cover.
Quadrat sampling can
be used with a variety of sampling designs, especially of the more classical type
such as simple random sampling or stratified simple random sampling. Such a
procedure can of course form part of many more-structured sampling methods,
such as capture recapture, mark-recapture or adoptive sampling.
Recapture Sampling
A wide range of
sampling methods are based on the principle of initially “capturing” and
“marking” a sample of the members of a closed (finite) population and
subsequently observing, in a later (or separate) independent random sample
drawn from the population, how many marked individuals are obtained. The term
capture-recapture is usually used for animals or insects, while mark-recapture
is often reserved for when studying plants.
Many forms and
variations are possible on this theme, and particular assumptions have to be
made to justify statistical methods for analyzing the resulting data:
i)
The marking process may be multi-stage, with separate
capture and recapture episodes.
ii)
Individuals in the samples may be chosen with or without
replacement.
iii)
The marking process may sometimes contaminate the
population, with the effect that market individuals may be selectively removed
from the population.
iv)
Individuals may be ‘trap shy’ or even ‘trap happy’; animals
or plants may even die due to the intervention process.
v)
The population may change from capture to recapture due to
births, deaths or inward or outward transfer.
In the simplest
approaches to capture-recapture we assume randomness of the samples with
constant capture probabilities in a fixed population and no capture-related effects
(of being ‘trap-shy’ or trap happy’ or ‘marks being lost).
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