98
Community Structure
tered microsites as optimal exploitation of its microhabitat's potential for
shelter. Living in environments characterised by high spatial heterogeneity,
Mediterranean diplopods obviously share with desert species their high
capacity to use microsites such as crevices, the nests of social insects, or soil
burrows for shelter. Such behaviour involves horizontal aggregation, which is
commonly observed in these animals (Crawford et al. 1987).
Joose (1971) related the capacity of Collembola to obtain shelter to the
degree of moisture in the substrate. With increasing saturation deficit, locomotory activity increases. The more sensitive the species are, the more intense is their reaction to changes in saturation deficit. There is an inverse
relationship between activity and the degree of aggregation. Species with
high humidity requirements are forced to aggregate and therefore show less
activity in the field.
Finally, it seems plausible that, in addition to abiotic factors, biotic factors
such as circumscribed oviposition resulting in aggregations of eggs in
favourable and protected microsites as well as food partitioning may also be
important in determining the horizontal organisation of arthropod communities. In spatially uncertain Mediterranean habitats, phoresis, i.e. the attachment of a microarthropod to the outer surface of another arthropod for a
limited time, might be an efficient mechanism for easy establishment in
favourable microsites. Indeed, phoresis (which is of epigenetic origin) facilitates the dispersal of the individual or its progeny from unfavourable microsites (Athias-Binche 1995). However, phoresis in Mediterranean arthropods is
seldom reported. Athias-Binche (1984) observed phoretic dispersal in mesostigmatids (Uropodinae). The phoretic is always deutonymph in that a specific fastening apparatus has been developed at the end of its body (AthiasBinche 1981, 1984). During travel the phoretic enters a latent life, ceasing to
feed and develop. Activity is taken up again after detachment of the phoront
and is induced by stimulus either from the carrier or the site of arrival. Mediterranean microarthropods exhibit rather facultative phoresis stimulated
either by overcrowding or changing environmental conditions such as shortages of foods, air dryness etc. The carrier/phoretic relationship is infrequently species specific, and most phoronts can use a great variety of hosts {AthiasBinche 1995}.
Finally, mass migration of macroarthropods among different habitat types
is seldom observed (Warburg et al. 1984). Short displacements between adjacent micro sites within single biotopes are also rarely reported (Trihas and
Legakis 1991). Usually, no seasonal horizontal migrations are reported at all.
For example, within a pine canopy, litter of Pinus halepensis and from under
the understory shrubs of Quercus cocci/era, Phillyrea media and Arbutus
unedo was sampled for macro arthropods (Karamaouna 1990). No mass
migration of animals from one litter type to another was detected.
However, in spatially heterogeneous habitats such as the Mediterranean
one, populations usually split into interacting local populations with a finite
Community Structure
tered microsites as optimal exploitation of its microhabitat's potential for
shelter. Living in environments characterised by high spatial heterogeneity,
Mediterranean diplopods obviously share with desert species their high
capacity to use microsites such as crevices, the nests of social insects, or soil
burrows for shelter. Such behaviour involves horizontal aggregation, which is
commonly observed in these animals (Crawford et al. 1987).
Joose (1971) related the capacity of Collembola to obtain shelter to the
degree of moisture in the substrate. With increasing saturation deficit, locomotory activity increases. The more sensitive the species are, the more intense is their reaction to changes in saturation deficit. There is an inverse
relationship between activity and the degree of aggregation. Species with
high humidity requirements are forced to aggregate and therefore show less
activity in the field.
Finally, it seems plausible that, in addition to abiotic factors, biotic factors
such as circumscribed oviposition resulting in aggregations of eggs in
favourable and protected microsites as well as food partitioning may also be
important in determining the horizontal organisation of arthropod communities. In spatially uncertain Mediterranean habitats, phoresis, i.e. the attachment of a microarthropod to the outer surface of another arthropod for a
limited time, might be an efficient mechanism for easy establishment in
favourable microsites. Indeed, phoresis (which is of epigenetic origin) facilitates the dispersal of the individual or its progeny from unfavourable microsites (Athias-Binche 1995). However, phoresis in Mediterranean arthropods is
seldom reported. Athias-Binche (1984) observed phoretic dispersal in mesostigmatids (Uropodinae). The phoretic is always deutonymph in that a specific fastening apparatus has been developed at the end of its body (AthiasBinche 1981, 1984). During travel the phoretic enters a latent life, ceasing to
feed and develop. Activity is taken up again after detachment of the phoront
and is induced by stimulus either from the carrier or the site of arrival. Mediterranean microarthropods exhibit rather facultative phoresis stimulated
either by overcrowding or changing environmental conditions such as shortages of foods, air dryness etc. The carrier/phoretic relationship is infrequently species specific, and most phoronts can use a great variety of hosts {AthiasBinche 1995}.
Finally, mass migration of macroarthropods among different habitat types
is seldom observed (Warburg et al. 1984). Short displacements between adjacent micro sites within single biotopes are also rarely reported (Trihas and
Legakis 1991). Usually, no seasonal horizontal migrations are reported at all.
For example, within a pine canopy, litter of Pinus halepensis and from under
the understory shrubs of Quercus cocci/era, Phillyrea media and Arbutus
unedo was sampled for macro arthropods (Karamaouna 1990). No mass
migration of animals from one litter type to another was detected.
However, in spatially heterogeneous habitats such as the Mediterranean
one, populations usually split into interacting local populations with a finite