Presentation on theme: "SAMPLING Sampling is indispensable, as we cannot study all individuals of a population or all species of a community Why ? Objectives come from ecological."— Presentation transcript:
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. surveying surveying 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. surveying 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
A B b c D number of species ☻ A B C D Ecotone effect
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
Representativity number of species number of species 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)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.
Relation area – number of species catches max number = 100% Total number of species Area experimental data Modelised curve (logarithmic system) 80% s2s2
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 Saprobiotic Diversity indicesDiversity indices [e.g. I=S(no of species) / N(total no of individuals.)] but they may mislead us Biotic indicesBiotic indices Multiparametric indices (Predictive models leading to a biolic index, e.g. RIVPACKS)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