Agriculture Reference
In-Depth Information
Density and biomass
Acari are often the most numerous group of soil arthropods. Their resistance to drought
and extreme temperature has allowed them to colonise almost all soils, from the south-
ernmost south polar cold desert to hot tropical deserts. In the 180 sites reviewed by
Petersen and Luxton (1982), population density varied from a few thousands in desert
soils and some arctic habitats, to a mean annual maximum of 792,000 ind.
in a
Swedish pine forest (Persson
et al.,
1980) and an absolute maximum of 1,783,000
in a late autumn sample from a beech (
Fagus sylvatica
) forest site at Soiling in Germany
(Schauermann, 1973). Most data fall into the range 20,000-200,000
There is a clear thermo-latitudinal gradient of population density with low abundances
in most tundra sites (2,000-80,000), high densities in subarctic tundra and temperate
coniferous forests (20,000-600,000), similar values in temperate deciduous forests
(25,000-250,000) and grasslands (10,000-200,000) and decreased densities in tropical
soils (20,000-70,000). Some known exceptions to this pattern include populations
densities of 213,000-242,000 ind. recorded from a rainforest in Zaïre and 187,000-
520,000 in a South African montane forest (Figure III.36).
At a regional scale, Acari are generally most abundant in soils with well-developed
O horizons with a mor type of humus, low pH and 'mesic' rather than dry or moist water
regimes. They are more numerous in wooded than non-wooded areas. Finally, abundance
varies in different cultural systems. Generally, both abundance and diversity are lowest
in cultivated annual crops and highest in pastures and crops with reduced tillage (see,
e.g.,
Winter
et al.,
1990; Hansson
et al.,
1990).
Biomass ranges from 10 to 1500 mg dry weight with most values ranging between
100 a n d 1000 mg. The mean weight of individuals tends to decrease from tundra sites
(10 µg in a number of sites) to the tropics (0.7 µ g in savannas and forests of Zaïre).
