Transcript Intensive surveys

SAMPLING
Sampling is indispensable, as we cannot study all individuals of a population or all
species of a community
Why ?
 Objectives come from ecological studies of population, communities or/ and
ecosystems
 Surveying for information on evolution or for managing an ecosystem
 It can be also only a part of the above objectives.
The procedures used according to the two main objectives do not need the same
attention.
Where? / How long?
Where ?
As the objective is to have biotic data on flora or fauna of a biotope, we must well
define the biotope. We must choose an apparently homogenous place .
But for the mean level Homo sapiens as us an 'homogenous' biotope is in fact
'heterogeneous'. If our objective is surveying an ecosystem then the definition of the
biotope is easy and expectable. If the objective is studying scientifically an ecosystem
we must be very careful with all the surrounding biotopes because of ecotone effect.
How long ?
For surveying, according to the speed of change>
For studying, according with cycles of seasons (e.g. in temperate countries), life cycles
of species…
Sampling methods
There are several methods.
Each of them has advantage and disadvantages. The choose of the
method is linked the objective … and also the facilities we dispose.
Advantages (disadvantages = opposite)
● Great representativity of samples in relation to the whole population
or community (non selective)
● Meaningful data
● Short cost of time
● The least possible of destruction to species or the biotope
● Cost effective
Samplers
Sorbers, cylinders
Quantitative results possible for benthic organisms
 Hand nets
Qualitative results
Drift nets
Quantitative results on floating organisms (possibly according to the
depth).
 Emergence nets
Quantitative results on emergence fauna from aquatic larva.
Quantitative samplers
a.Sorber
b.Cylinder
c. Ekman
d. Corer
Artificial substrates
Representativity
Preliminary results are needed in order to decide the number of
samples. This is linked to the type of result we expect (e.g. number of
species)
If a conventional statistical analysis is needed then quantitative or
semi-quantitative results must be taken in a random way. In such a case
one sample is insufficient and the number of samples must not defined.
Extensive or Intensive Surveys
Surveys may be either extensive or intensive.
Extensive surveys aim to discover what species are present in an area,
usually with a measure of relative abundance, and are especially used
where the water quality over many sites is being monitored or compared.
Such surveys have been criticized, or even considered valueless (e.g.
Gray, 1976), because they are too superficial to detect or interpret subtle
environmental changes, such as alterations in species dominance due to
biological interactions (so that it is impossible to disentangle natural
changes from those caused by pollution). Such a view is undoubtedly
too pessimistic and these surveys may detect natural changes
without a detailed foreknowledge of the ecology of a site. It is
probably true, however, that sampling design is frequently given
inadequate thought in extensive surveys.
Intensive Surveys
Intensive surveys usually aim to determine population densities.
Populations of organisms are usually highly aggregated so that a large
number of samples are frequently required to obtain a population
estimate that is statistically meaningful. Chutter and Noble (1966) and
Chutter (1972), using benthic macroinvertebrate data from streams in
South Africa and California, considered that over fifty (50) replicates are
required to attain an estimate of population size to within 20 per cent. For
some species, many more samples would be needed. Ninety-eight (98)
samples were required to obtain an estimate of mean density (±40 per
cent), with 95 per cent confidence limits for the limpet Ancylus fluviatilis
(Edwards et al, 1975). Such sampling intensity would be impossible in
an extensive survey and even in an intensive survey the rarer species will
be inadequately sampled. Edwards et al (1975) have shown that,
sampling a riffle, only 44 per cent of the species taken would be
common to both of any two random samples. The number of samples
required for a specified degree of precision can be readily calculated if an
estimate of the mean abundance is made from a pilot survey.
Different types of results
Data on communities help us to evaluate the biodiversity. Indices are
good tools for such evaluations (see Daniel CDrom) : richness,
Shannon index, Margalef index…
Patrimonial index translates also the global degree of rarity of
species.
 Data on population are good for the study of evolution population,
survival curves,…
 Data for surveying must be standardized in order to assess evolution in
ecosystems.
Abiotic factors are also needed in order to be able to make
comparisons with the biotic data. The latter can be analysed by
multivariate analyses.
Indices- scores or other
•
Saprobiotic
• Diversity indices
[e.g. I=S(no of species) / N(total no of individuals.)]
but they may mislead us
• Biotic indices
• Multiparametric indices (Predictive
models leading to a biolic index, e.g.
RIVPACKS)
Thank you very much for Angela to be present so
early in the morning