Agriculture Reference
In-Depth Information
active predation and epidemics (
e.g., Parisotoma notabilis
); as a result, they are highly
fecund. Others,
e.g., Orchesella
spp. escape more readily from predators and have a
lower oviposition rate. In the soil, Collembola may be better protected against predators
although some specialised predators (
e.g.,
Chilopoda, Geophilidae) have developed
adaptations for moving within the pore space. As soil-dwelling Collembola
cannot readily escape predators, some species have developed mechanisms for reducing
predation,
e.g.,
the production of toxic secretions in the subfamily
Onychiurinae.
Density and biomass of communities
As is normal in studies of microarthropods, Collembola are generally extracted by the
dry funnel technique, or an improved variant of this apparatus such as the 'high gradient'
canister extractor (Macfadyen, 1961). Extraction is never complete due to the variable
reactions of different species and stages to the extraction stimuli. The efficiency of
extraction may be as high as 85-89 % when using the best techniques (Marshall, 1972);
extraction using simple dry funnel methods may be 2 to 10 times lower than with
high gradient designs (Macfadyen, 1961). A further source of error may arise from an
inefficient separation and counting of the extracted material.
Population densities reported from more than 200 sites varied from 100 to 670,000
individuals (Petersen and Luxton, 1982) with most data falling in the range 10,000-
100,000. There is a strong thermo-latitudinal effect, with low values in colder tundra
sites (Devon Island, Canada) but maximum values in cold temperate grasslands (670,000
in an ornithogenic soil of Signy Island) and cold temperate forests (145-244,000 in
a Norwegian spruce forest). Densities in temperate forests range from 40,000 to 70,000.
In tropical forests, abundances may vary from a few thousands (Betsch and Betsch-Pinot,
1983) to 20,000-80,000 depending on soil type and drainage conditions. Tropical savannas
have significantly lower abundances (1100 to 3500 at Lamto, Ivory Coast)
and finally, a site in an American desert had the lowest densities (100
in a Californian
desert) (Figure III.32).
