SOIL ARTEROPOD SAMPLINQ
27
basis. The size of sample-unit to be used will depend on the density of
the animal groups under study. In this case, parasitid mites for instance, which are often quite distinct indicators of the environmental
condition, might be considered and, since they might be expected to
occur at average densities around one or two per om2 for all species a
sample-unit of surface area not less than 25 om2 is indicated. On the
other hand the deliberate taxonomic limitation of the study to oribatid
mites with densities approaching 20 or more to the square centimetre
would permit sample-unit size to be reduced to, say 10 cm2 or even
less. These sizes are chosen assuming that complete counts are demanded, but if the purpose of the study is to detect general effects of
treatments on the relative importance of the different groups, the
analysis could be done on the results of ranking the different species
in the sample-units. This technique could well be satisfied with sampleunits only about a quarter of the size but might demand an increased
number of units to obtain statistically valid results. The methods for
determining the number and size of units to be used in the case of a
complete count have already been given. The use of an analysis based
on ranking would demand perhaps sixty-four sample-units or more
(4 treatments x 4 replicate blocks x 4 units per block) on each sampling
occasion. The actual weight of soil would be small but the number of
units obviously makes great demands on the labour of handling. This,
and the superiority of behaviour-type methods in grassland studies
(owing to the presence of much plant detritus) all point to the use of a
Tullgren-type extractor, and for groups other than adult oribatid mites
the canister type is indicated if some groups are not to be biased a t the
expense of others.
There remains the question of depth of sampling. Preliminary investigations would be required here, but in most rough grassland soils
the fauna is markedly concentrated near the surface. Unless large numbers of deeper forms such as rhodacarids, protura and Tullbergia species
were found, sample-units of 3 cm depth would probably be adequate.
Otherwise either a proportion of sample points or even all of them might
require a second unit from 3 cm to 6 cm depth. I n the latter case, of
course the number of sample-units would be doubled again.
2. An investigation into the changes in the total arthropod populations of a single arable field under different crop rbgimes and over an
extended period.
This is a further example in which absolute abundance figures are
not required. The main difficulties here acre the varying nature of the
soil under changing cropping rbgimes and the demand for a knowledge
of the complete arthropod fauna. On the assumption that the management would be largely arable and that grass crops would be in the form
27
basis. The size of sample-unit to be used will depend on the density of
the animal groups under study. In this case, parasitid mites for instance, which are often quite distinct indicators of the environmental
condition, might be considered and, since they might be expected to
occur at average densities around one or two per om2 for all species a
sample-unit of surface area not less than 25 om2 is indicated. On the
other hand the deliberate taxonomic limitation of the study to oribatid
mites with densities approaching 20 or more to the square centimetre
would permit sample-unit size to be reduced to, say 10 cm2 or even
less. These sizes are chosen assuming that complete counts are demanded, but if the purpose of the study is to detect general effects of
treatments on the relative importance of the different groups, the
analysis could be done on the results of ranking the different species
in the sample-units. This technique could well be satisfied with sampleunits only about a quarter of the size but might demand an increased
number of units to obtain statistically valid results. The methods for
determining the number and size of units to be used in the case of a
complete count have already been given. The use of an analysis based
on ranking would demand perhaps sixty-four sample-units or more
(4 treatments x 4 replicate blocks x 4 units per block) on each sampling
occasion. The actual weight of soil would be small but the number of
units obviously makes great demands on the labour of handling. This,
and the superiority of behaviour-type methods in grassland studies
(owing to the presence of much plant detritus) all point to the use of a
Tullgren-type extractor, and for groups other than adult oribatid mites
the canister type is indicated if some groups are not to be biased a t the
expense of others.
There remains the question of depth of sampling. Preliminary investigations would be required here, but in most rough grassland soils
the fauna is markedly concentrated near the surface. Unless large numbers of deeper forms such as rhodacarids, protura and Tullbergia species
were found, sample-units of 3 cm depth would probably be adequate.
Otherwise either a proportion of sample points or even all of them might
require a second unit from 3 cm to 6 cm depth. I n the latter case, of
course the number of sample-units would be doubled again.
2. An investigation into the changes in the total arthropod populations of a single arable field under different crop rbgimes and over an
extended period.
This is a further example in which absolute abundance figures are
not required. The main difficulties here acre the varying nature of the
soil under changing cropping rbgimes and the demand for a knowledge
of the complete arthropod fauna. On the assumption that the management would be largely arable and that grass crops would be in the form