3.1 Population density and population size

Soil ecologists characterize populations usually not by their size but by their density.
Population density is estimated by delineating a plot in the field, taking a certain number of random samples from that plot, often in a stratified scheme, extracting the fauna from these samples, counting the animals and extrapolating these counts, using the dimensions of the sample, to estimate the average number per unit area.Statistical techniques are applied to quantify the confidence limits of such density estimates. An overview of sampling designs is provided in Jiménez et al. (2020).

A great variety of methods is available for reliable extraction of animals from soil samples. A classical method is to drive all mobile animals out of a soil core by heat and gradual drying. A funnel below each sample concentrates the animals in a vial with preservative. This method is due to the Swedish entomologist Albert Tullgren (1874-1958); a device with several replicate funnels is called a Tullgren extractor.
Depending on the species, type of soil and design of the extractor, efficiencies of 80% and higher can be reached with this method (Van Straalen and Rijninks 1982).

The principle of Tullgren is often equated with an extraction method described by the Italian entomologist Antonio Berlese (1863-1927), but this is not correct.
The apparatus of Berlese was actually quite different: it used a water bath to heat the sample from below, while Tullgren used a lamp to heat the sample from above (Berlese 1905, Tullgren 1918). The Tullgren method works best for small mobile arthropods (springtails, mites, beetles, spiders); inactive stages are not extracted.
Larger springtails, spiders and beetles may be collected by hand-sorting, after sieving the litter above a white tray and using a battery-driven aspirator to collect them (Van der Laan 1963).

For nematodes, two methods are available, one relying on motility, the other on the physical properties of the animals (size and density). A common method based on motility is due to the German medical doctor Gustav Baermann (1877-1950), working on human-parasitic soil nematodes in the Dutch East Indies (Baermann
1917). The apparatus consists of a glass funnel filled with water upon which a sample in a gauze is held floating. Such "Baermann funnels" are also used for tardigrades and enchytracids. Mobile animals escape from the sample into the water. The other method uses the so-called "Oostenbrink eluriator", which applies a two-step extraction. First the sample is gently flushed to wash-out nematodes and small soil particles; then nematodes are separated from soil particles by differential flotation in an upward flow (Oostenbrink 1954). The apparatus is named after the Dutch nematologist Michiel Oostenbrink (1921-1979), working at the Institute of Plant Pathology in Wageningen. Recovery of nematodes in the flotation step can be very high, up to 100%, but when using sieves to get rid of small soil particles, a significant fraction of the smaller animals may be lost (Verschoor and De Goede 2000).

For earthworms, a common method is to drive the worms to the surface by pouring a 0.5% formalin solution over the soil. This method, although cheap and popular, is not to be recommended since it has severe side-effects on other invertebrate populations and the recovery of worms is still modest, especially for the deeper burrowing species (Eichinger et al. 2007). Earthworms are best separated from the soil by (tedious) hand-sorting, after spreading samples on a white tray and carefully separating roots from soil.
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