Soil Arthropods
351
dry month. Many species show summer diapause, timing and duration of
which depend on the onset of soil water stress. As a consequence, vernal
populations must either have a very short biological cycle or exhibit summer
dormancy.
Eighty-five per cent of the annual biomass was collected from June to
August whereas only 1 % was collected in winter. This seasonality reflects
largely the activity patterns of the large predators and, specially, of the family
Carabidae which accounts for 90% of the summer biomass. In winter and
spring, the active predators are smaller and the effect of individual size disappears. Collected biomass is then proportional to the number of arthropods
and, specially, to the saprophagous and mycophagous activity density.
24.4.3 Epigeal Predators
Hypogeal communities are dominated by decomposers, but predators are the
most important epigeal· arthropods. Most abundant taxa are Chilopoda,
Phalangida, Araneae and Carabidae (Table 24.5). Mean annual specific diversity is 3.6 bits individual-I, maximum diversity 5.0, and equitability 70%.
These values reflect the co dominance of some species, whose activity varies
in space according to the plant cover phenology, and in time according to the
seasonal pattern. Climate is warm enough to allow some faunal activity in all
seasons. However, there are two well-defined subcommunities of predators
active in spring and in winter, respectively.
Spiders are the most abundant taxa throughout the year except in summer, when they are substituted by Phalangida. The summer abundance of
Phalangida has to be explained by the coincidence on the ground surface of
the adult stages of both litter-dwelling and shrub-dwelling species. Phalangida inhabit mainly the litter, where they prey on small insects and mites
(Rueda and Lobo 1984). Two genera (Troqulus and Nemastoma) belong to the
superfamily Troquloidea, associated with the epigeal environment. The other
species belong to the superfamily Phalangioidea, which inhabits the litter
sporadically and mainly in the adult stage. However, Phalangioidea includes
Nelima silvatica which accounts for 79% of the Phalangida.
TWenty-nine species of spiders were captured in the experimental plot at
La Castanya, and Tegenaria fuesslini and Lepthyphantes flavipes were the
most abundant. A great part of these species are not characteristic of holm
oak forests and can be found in the neighbouring herbaceous or bushy environments also. Most of the species strictly depending on the forest environment are not restricted to the soil surface and move along the vertical plant
strata, from soil to bushes and trees.
TWo species of Carabidae were collected: Carabus (Chrysotribas) rutilans
aragonensis and Ceuthosphorus (Actenipus) oblongus orientalis. The specific
diversity of the Carabidae in our forest is very low. On the one hand, summer
drought is not favourable for the species characteristic of the Pyrenees. On
the other, the shady conditions of the plot are not suitable for the Mediterra-
351
dry month. Many species show summer diapause, timing and duration of
which depend on the onset of soil water stress. As a consequence, vernal
populations must either have a very short biological cycle or exhibit summer
dormancy.
Eighty-five per cent of the annual biomass was collected from June to
August whereas only 1 % was collected in winter. This seasonality reflects
largely the activity patterns of the large predators and, specially, of the family
Carabidae which accounts for 90% of the summer biomass. In winter and
spring, the active predators are smaller and the effect of individual size disappears. Collected biomass is then proportional to the number of arthropods
and, specially, to the saprophagous and mycophagous activity density.
24.4.3 Epigeal Predators
Hypogeal communities are dominated by decomposers, but predators are the
most important epigeal· arthropods. Most abundant taxa are Chilopoda,
Phalangida, Araneae and Carabidae (Table 24.5). Mean annual specific diversity is 3.6 bits individual-I, maximum diversity 5.0, and equitability 70%.
These values reflect the co dominance of some species, whose activity varies
in space according to the plant cover phenology, and in time according to the
seasonal pattern. Climate is warm enough to allow some faunal activity in all
seasons. However, there are two well-defined subcommunities of predators
active in spring and in winter, respectively.
Spiders are the most abundant taxa throughout the year except in summer, when they are substituted by Phalangida. The summer abundance of
Phalangida has to be explained by the coincidence on the ground surface of
the adult stages of both litter-dwelling and shrub-dwelling species. Phalangida inhabit mainly the litter, where they prey on small insects and mites
(Rueda and Lobo 1984). Two genera (Troqulus and Nemastoma) belong to the
superfamily Troquloidea, associated with the epigeal environment. The other
species belong to the superfamily Phalangioidea, which inhabits the litter
sporadically and mainly in the adult stage. However, Phalangioidea includes
Nelima silvatica which accounts for 79% of the Phalangida.
TWenty-nine species of spiders were captured in the experimental plot at
La Castanya, and Tegenaria fuesslini and Lepthyphantes flavipes were the
most abundant. A great part of these species are not characteristic of holm
oak forests and can be found in the neighbouring herbaceous or bushy environments also. Most of the species strictly depending on the forest environment are not restricted to the soil surface and move along the vertical plant
strata, from soil to bushes and trees.
TWo species of Carabidae were collected: Carabus (Chrysotribas) rutilans
aragonensis and Ceuthosphorus (Actenipus) oblongus orientalis. The specific
diversity of the Carabidae in our forest is very low. On the one hand, summer
drought is not favourable for the species characteristic of the Pyrenees. On
the other, the shady conditions of the plot are not suitable for the Mediterra-