Soil Arthropods
343
other Mediterranean holm oak forests. According to Di Castri and Vitali di
Castri (1981), the Prostigmata density increases towards the xeric parts of
the environmental gradient.
In the soil arthropod trophic pyramid of Montseny, predators account for
18% of the individuals and 37% of the live weight. Decomposers (i.e. microphytophagous, mycophagous, and detritivorous arthropods) 1 account for
82% of the individuals and 63% of the dry weight. The Mesostigmatic families Rhodacaridae and Parasitidae are the most abundant predators. Despite
their abundance, these small individuals (58 jlg individual- I in Rhodacaridae) contribute less biomass to the trophic web than the large predators like
Chilopoda (4026 jlg individual-I). The trophic pyramid changes along the
soil profile. In Montseny, the biomass ratio of decomposers to predators is 3
in the litter layer (L), 2 in the humus layers (F and H; these two layers were
intermixed in the Montseny site and were sampled as a unit) and 1 in the upper mineral soil (S). This ratio is higher when applied to the number of individuals: 11 in L, 6 in F+Hand 3 in S.
24.2.2 Spatial Structure
Fifty per cent of the soil arthropods in Montseny and 46% in Prades inhabit
the humus layers The litter layer is less suitable for animal life, and shelters
only 18% of the soil mesofauna in Montseny and 16% in Prades. The harshness of the edaphic microclimate and the low trophic quality of the fresh
holm oak leaves can explain the poor nature of this layer. The conditions of
the underlying mineral soil are intermediate between the humus and the litter layers.
Taxa exhibiting a great surface to volume ratio or soft teguments (such as
Pauropoda, Symphyla, Protura and Diplura) seem unable to endure drought
and do not colonize the L layer of Prades. In Montseny, under a smaller water
deficit (Chap. 2), they sporadically appear. The same reasons can explain the
scarcity of Collembola in the L layers of our two forests. On the opposite side,
Psocoptera are the only arthropods showing preference for the L layer. According to Garcia Aldrete (1991), this behaviour is characteristic of forest
litter-dwelling insects, whose presence in the underlying layers is merely sporadic. Nearly all of the Acari families occupy mainly the humus layer, with
the exception of Rhodacaridae and Rhaphignatidae, two families that share
characteristics of small size, light teguments and predatory behaviour.
In each layer, arthropods usually occur in tridimensional aggregates
whose form, size and distribution are related to factors such as water availability, soil temperature, animal behaviour, seasonal and daily specific
rhythms, and patterns of food and plant distribution (Butcher et al. 1971). In
Montseny, the Lexis Index of aggregation (Greig-Smith 1983) is lowest in the
1 Editors' Note: alhough in ecology "decomposer" is usually applied to microorganisms, we
have respected the authors' meaning, which is widespread among soil zoologists.
343
other Mediterranean holm oak forests. According to Di Castri and Vitali di
Castri (1981), the Prostigmata density increases towards the xeric parts of
the environmental gradient.
In the soil arthropod trophic pyramid of Montseny, predators account for
18% of the individuals and 37% of the live weight. Decomposers (i.e. microphytophagous, mycophagous, and detritivorous arthropods) 1 account for
82% of the individuals and 63% of the dry weight. The Mesostigmatic families Rhodacaridae and Parasitidae are the most abundant predators. Despite
their abundance, these small individuals (58 jlg individual- I in Rhodacaridae) contribute less biomass to the trophic web than the large predators like
Chilopoda (4026 jlg individual-I). The trophic pyramid changes along the
soil profile. In Montseny, the biomass ratio of decomposers to predators is 3
in the litter layer (L), 2 in the humus layers (F and H; these two layers were
intermixed in the Montseny site and were sampled as a unit) and 1 in the upper mineral soil (S). This ratio is higher when applied to the number of individuals: 11 in L, 6 in F+Hand 3 in S.
24.2.2 Spatial Structure
Fifty per cent of the soil arthropods in Montseny and 46% in Prades inhabit
the humus layers The litter layer is less suitable for animal life, and shelters
only 18% of the soil mesofauna in Montseny and 16% in Prades. The harshness of the edaphic microclimate and the low trophic quality of the fresh
holm oak leaves can explain the poor nature of this layer. The conditions of
the underlying mineral soil are intermediate between the humus and the litter layers.
Taxa exhibiting a great surface to volume ratio or soft teguments (such as
Pauropoda, Symphyla, Protura and Diplura) seem unable to endure drought
and do not colonize the L layer of Prades. In Montseny, under a smaller water
deficit (Chap. 2), they sporadically appear. The same reasons can explain the
scarcity of Collembola in the L layers of our two forests. On the opposite side,
Psocoptera are the only arthropods showing preference for the L layer. According to Garcia Aldrete (1991), this behaviour is characteristic of forest
litter-dwelling insects, whose presence in the underlying layers is merely sporadic. Nearly all of the Acari families occupy mainly the humus layer, with
the exception of Rhodacaridae and Rhaphignatidae, two families that share
characteristics of small size, light teguments and predatory behaviour.
In each layer, arthropods usually occur in tridimensional aggregates
whose form, size and distribution are related to factors such as water availability, soil temperature, animal behaviour, seasonal and daily specific
rhythms, and patterns of food and plant distribution (Butcher et al. 1971). In
Montseny, the Lexis Index of aggregation (Greig-Smith 1983) is lowest in the
1 Editors' Note: alhough in ecology "decomposer" is usually applied to microorganisms, we
have respected the authors' meaning, which is widespread among soil zoologists.