346
Pilar Andres, Eduardo Mateos and Carlos Ascaso
spend most of the year in the organic layers, but move to the S layer at the
end of the summer. For the small Prostigmata, which appear sporadically in
the soil, survival of the population is accomplished by massive egg hatching
coinciding with the wet period. The Astigmata can undergo drought as hypopus. Collembola are strictly dependent on the water availability and exhibit a large range of drought resistance strategies (Poinsot-Balaguer 1988).
In spite of these mechanisms, summer drought causes a decrease in their
density both in Montseny (down to 585 individuals m- 2 ) and in Prades
(down to 735 individuals m- 2 ).
Food supply also influences the mesofaunal dynamics, but as omnivory is
a common trend belowground (Wallwork 1983; Moore et al. 1988), many arthropods do not depend on a single food-type, and the availability or scarcity
of a given trophic resource must not be regarded as a determinant factor.
24.2.4 Main Hypogeal Decomposers: Oribatid Mites and Collembola
In Montseny, oribatids account for 58% of the hypogeal arthropods. Mean
annual density is 34 017 individuals m- 2 , mean annual biomass 1633 mg m- 2
(26% of the arthropod biomass), and cumulative annual specific diversity
(Shannon index) is 2.46 bits individual-I. Most of the oribatid mites (51%)
live in the F+H layers, 27% in Sand 22% in 1. The larger species live near the
ground surface, and, as a consequence, mean individual biomass decreases
from the shallower to the deeper soil layers (0.03 Ilg in L, 0.01 in F and 0.007
in S). Specific diversity is lower in S (1.6 bits individual-I) than in L (2.5) or F
(2.4). This low diversity of the mineral layer results from its low number of
species (18 in S, 34 in Land 35 in F) and from its low equitability (59% in S,
67% in Land 66% in F).
The maximum oribatid density occurs in autumn and the minimum at the
end of summer. Specific diversity also follows a seasonal pattern and falls to
1.27 in the dry season. Most animals move to the deeper layers at the end of
summer, so that in September 86% inhabit S and the litter layer is empty. In
the litter layer, where moisture is highly variable, there is a significant correlation between water content and population densities and distributional parameters for some species (Table 24.2).
The 35 species of Collembola collected in the holm oak forest of Prades
account for 6% of the soil arthropods, with a mean annual density of 13 939
individual m- 2 , mean annual biomass of 395 mg m- 2 , and cumulative annual
diversity of 3.35 bits individual-I. The relative abundance of Collembola is
highest in the humus layer (45% of the soil Collembola) and lowest in the
litter layer (10%). The mineral layer can be divided into two 5-cm parts: 32%
of the soil Collembola live in the upper one; and 10% in the deeper one.
There is a specific vertical stratification of Collembola at Prades. The most
abundant species can be ranked according to their depth preference. Some of
them inhabit exclusively the organic layers (Odontella vallvidrerensis, Heteromorus major, Pseudochorutella catalonica, Bilobella aurantiaca, Spaerida
Pilar Andres, Eduardo Mateos and Carlos Ascaso
spend most of the year in the organic layers, but move to the S layer at the
end of the summer. For the small Prostigmata, which appear sporadically in
the soil, survival of the population is accomplished by massive egg hatching
coinciding with the wet period. The Astigmata can undergo drought as hypopus. Collembola are strictly dependent on the water availability and exhibit a large range of drought resistance strategies (Poinsot-Balaguer 1988).
In spite of these mechanisms, summer drought causes a decrease in their
density both in Montseny (down to 585 individuals m- 2 ) and in Prades
(down to 735 individuals m- 2 ).
Food supply also influences the mesofaunal dynamics, but as omnivory is
a common trend belowground (Wallwork 1983; Moore et al. 1988), many arthropods do not depend on a single food-type, and the availability or scarcity
of a given trophic resource must not be regarded as a determinant factor.
24.2.4 Main Hypogeal Decomposers: Oribatid Mites and Collembola
In Montseny, oribatids account for 58% of the hypogeal arthropods. Mean
annual density is 34 017 individuals m- 2 , mean annual biomass 1633 mg m- 2
(26% of the arthropod biomass), and cumulative annual specific diversity
(Shannon index) is 2.46 bits individual-I. Most of the oribatid mites (51%)
live in the F+H layers, 27% in Sand 22% in 1. The larger species live near the
ground surface, and, as a consequence, mean individual biomass decreases
from the shallower to the deeper soil layers (0.03 Ilg in L, 0.01 in F and 0.007
in S). Specific diversity is lower in S (1.6 bits individual-I) than in L (2.5) or F
(2.4). This low diversity of the mineral layer results from its low number of
species (18 in S, 34 in Land 35 in F) and from its low equitability (59% in S,
67% in Land 66% in F).
The maximum oribatid density occurs in autumn and the minimum at the
end of summer. Specific diversity also follows a seasonal pattern and falls to
1.27 in the dry season. Most animals move to the deeper layers at the end of
summer, so that in September 86% inhabit S and the litter layer is empty. In
the litter layer, where moisture is highly variable, there is a significant correlation between water content and population densities and distributional parameters for some species (Table 24.2).
The 35 species of Collembola collected in the holm oak forest of Prades
account for 6% of the soil arthropods, with a mean annual density of 13 939
individual m- 2 , mean annual biomass of 395 mg m- 2 , and cumulative annual
diversity of 3.35 bits individual-I. The relative abundance of Collembola is
highest in the humus layer (45% of the soil Collembola) and lowest in the
litter layer (10%). The mineral layer can be divided into two 5-cm parts: 32%
of the soil Collembola live in the upper one; and 10% in the deeper one.
There is a specific vertical stratification of Collembola at Prades. The most
abundant species can be ranked according to their depth preference. Some of
them inhabit exclusively the organic layers (Odontella vallvidrerensis, Heteromorus major, Pseudochorutella catalonica, Bilobella aurantiaca, Spaerida