110
Community Structure
selected O. meridiatus (Chap. 3) can dominate in the satellite category. It
seems plausible that physiological constraints may underlie habitat selection
in exposed sites. In contrast, the community of sheltered sites appears homogeneous and numbers are evenly distributed among species in sheltered sites.
In these communities, where moderate climatic fluctuations occur, the effect
of the biotic components of the environment is, as anticipated, higher. It can
be observed that core species of open sites such as Folsomia quadrioculata
and O. meridiatus switch to satellite status in sheltered sites. Finally, in the
outer site no distinction between core and satellite species has been made,
while only the relationship between abundance and distribution is linear. In
accordance with Maurer (1990), it can be concluded that habitat in this site is
relatively simple with few resources. Argyropoulou et al (1994) suggested that
the partitioning of limited food resources probably caused spatial separation
of niches in simple habitats.
7.S
The Biotic Correlates of Habitat Selection
Although no detailed data are available on the feeding habits of arthropods,
most Mediterranean species are considered generalists. For example, Crawford et al. (1987) reported that no food specialisation was observed among
diplopods. Paris (1963) reported that, although the isopod A. vulgare shows a
net preference for the decaying leaves of Picris echioides, population density
and age structure in the field is independent of the distribution of plant
debris. He suggested that this species, which is broad food-selected, can easily spread and establish itself in new sites. In contrast, habitat selection among
oribatids within single shrubs is linked to their feeding habits {Sgardelis et al.
1981; Fig. 7.11}. Unspecialised feeders and panphytophages according to classifications by Berthet (1964) and Luxton (1972) are confined to the center of
the shrubs, while microphytophages and specialised feeders prefer the outer
borders.
Food correlates of habitat selection have been further explored in laboratory cultures of the oribatid Scheloribates cf.latipes. In Table 7.5 the effect of
diet on demographic parameters is shown. Feeding on lichens as well as on
decomposing litter enables the life cycle development of C. cf. latipes to take
place. Higher fecundity, lower mortality and lower CV% values were recorded
in cultures containing the lichen Xanthoria parietina, decomposing litter and
mosses. It must be noted that the latter are not palatable but constitute sites
for egg laying and preecdysial immobilisation of juveniles. Analogous nutritional preferences were exhibited by all other oribatid species tested (Asikidis, unpublished) and by the majority of collembolan species (Argyropoulou,
unpubl.). It seems that epiphytic and soil lichens, whose abundance is related
to the intensity of grazing pressure, are essential for the life cycle develop-
Community Structure
selected O. meridiatus (Chap. 3) can dominate in the satellite category. It
seems plausible that physiological constraints may underlie habitat selection
in exposed sites. In contrast, the community of sheltered sites appears homogeneous and numbers are evenly distributed among species in sheltered sites.
In these communities, where moderate climatic fluctuations occur, the effect
of the biotic components of the environment is, as anticipated, higher. It can
be observed that core species of open sites such as Folsomia quadrioculata
and O. meridiatus switch to satellite status in sheltered sites. Finally, in the
outer site no distinction between core and satellite species has been made,
while only the relationship between abundance and distribution is linear. In
accordance with Maurer (1990), it can be concluded that habitat in this site is
relatively simple with few resources. Argyropoulou et al (1994) suggested that
the partitioning of limited food resources probably caused spatial separation
of niches in simple habitats.
7.S
The Biotic Correlates of Habitat Selection
Although no detailed data are available on the feeding habits of arthropods,
most Mediterranean species are considered generalists. For example, Crawford et al. (1987) reported that no food specialisation was observed among
diplopods. Paris (1963) reported that, although the isopod A. vulgare shows a
net preference for the decaying leaves of Picris echioides, population density
and age structure in the field is independent of the distribution of plant
debris. He suggested that this species, which is broad food-selected, can easily spread and establish itself in new sites. In contrast, habitat selection among
oribatids within single shrubs is linked to their feeding habits {Sgardelis et al.
1981; Fig. 7.11}. Unspecialised feeders and panphytophages according to classifications by Berthet (1964) and Luxton (1972) are confined to the center of
the shrubs, while microphytophages and specialised feeders prefer the outer
borders.
Food correlates of habitat selection have been further explored in laboratory cultures of the oribatid Scheloribates cf.latipes. In Table 7.5 the effect of
diet on demographic parameters is shown. Feeding on lichens as well as on
decomposing litter enables the life cycle development of C. cf. latipes to take
place. Higher fecundity, lower mortality and lower CV% values were recorded
in cultures containing the lichen Xanthoria parietina, decomposing litter and
mosses. It must be noted that the latter are not palatable but constitute sites
for egg laying and preecdysial immobilisation of juveniles. Analogous nutritional preferences were exhibited by all other oribatid species tested (Asikidis, unpublished) and by the majority of collembolan species (Argyropoulou,
unpubl.). It seems that epiphytic and soil lichens, whose abundance is related
to the intensity of grazing pressure, are essential for the life cycle develop-