30
A. MACFADYEN
The third section is concerned with the special problems posed by the
patchiness of distribution which is usual among soil animals and the
application of ranking and other alternatives to a complete census in
community studies which do not necessitate the great labour of complete counts.
The fourth section is a systematic survey of the main types of collection and extraction processes employed in the study of soil arthropods. Three main types are recognized, those which are intended for the
study of animals within the soil mainly by means of sections, the “behaviour” methods in which animals are induced t o leave the soil by
attractant or repellent stimuli and the “mechanical” methods by which
animals are removed as a result of differences in physical properties from
the surrounding medium.
Among behaviour-type methods the use of pitfall traps and baited
“lobster pot” traps can be used in the field whilst various devices developed from the Berlese funnel and the wet or Baermann funnel demand that the samples be returned t o a laboratory for extraction.
Mechanical methods are based on sieving, sedimentation or differential wetting and flotation. These principles can be used successively and
material from behaviour methods can also be cleaned mechanically.
In general, the behaviour-type methods have the disadvantages, when
compared with mechanical methods, that resting stages (eggs, pupae,
etc.) are not removed, that, in any one apparatus the size of sampleunits is often fixed, and that changes in the populations of animals
within the samples can occur during extraction due t o hatching of eggs
or t o predation.
On the other hand the behaviour-type methods have certain clear
advantages. Sorting labour is reduced (because the animals walk out
of the samples by themselves) a factor of especial importance when
many small samples are to be treated. The simple behaviour-type
apparatus is cheaper and better adapted t o field use, the avoidance of
sieving presents loss of the very numerous small forms such as Prostogmata, Scutacaridae etc., and there is much less damage to specimens
due t o the more gentle handling of the soil.
It must be appreciated, however, that n o universal extraction method
exists at the present time. Some guidance as t o which methods work
best with which groups is given in Table 11, but even here such factors
as sampling design and the nature of the soil must be Considered.
As an example of the factors that determine the choice of sampling
systems three specific projects are considered in Section V.
A. MACFADYEN
The third section is concerned with the special problems posed by the
patchiness of distribution which is usual among soil animals and the
application of ranking and other alternatives to a complete census in
community studies which do not necessitate the great labour of complete counts.
The fourth section is a systematic survey of the main types of collection and extraction processes employed in the study of soil arthropods. Three main types are recognized, those which are intended for the
study of animals within the soil mainly by means of sections, the “behaviour” methods in which animals are induced t o leave the soil by
attractant or repellent stimuli and the “mechanical” methods by which
animals are removed as a result of differences in physical properties from
the surrounding medium.
Among behaviour-type methods the use of pitfall traps and baited
“lobster pot” traps can be used in the field whilst various devices developed from the Berlese funnel and the wet or Baermann funnel demand that the samples be returned t o a laboratory for extraction.
Mechanical methods are based on sieving, sedimentation or differential wetting and flotation. These principles can be used successively and
material from behaviour methods can also be cleaned mechanically.
In general, the behaviour-type methods have the disadvantages, when
compared with mechanical methods, that resting stages (eggs, pupae,
etc.) are not removed, that, in any one apparatus the size of sampleunits is often fixed, and that changes in the populations of animals
within the samples can occur during extraction due t o hatching of eggs
or t o predation.
On the other hand the behaviour-type methods have certain clear
advantages. Sorting labour is reduced (because the animals walk out
of the samples by themselves) a factor of especial importance when
many small samples are to be treated. The simple behaviour-type
apparatus is cheaper and better adapted t o field use, the avoidance of
sieving presents loss of the very numerous small forms such as Prostogmata, Scutacaridae etc., and there is much less damage to specimens
due t o the more gentle handling of the soil.
It must be appreciated, however, that n o universal extraction method
exists at the present time. Some guidance as t o which methods work
best with which groups is given in Table 11, but even here such factors
as sampling design and the nature of the soil must be Considered.
As an example of the factors that determine the choice of sampling
systems three specific projects are considered in Section V.