96
Community Structure
colonised by microbes. Hence, autumnal activation of macrofauna induces
the synchronous activation of the entire decomposition process, probably
resulting in an increase in available nutrients during the growth period
favourable to plants. The message from this example - as from many others
- is that synchronised processes are the response of living organisms to seasonally varying Mediterranean environments.
7.3
Spatial Patterns
Due to small- and large-scale irregularities in the landscape, the bushy vegetation, the shallowness of the soil, and organic layers, as well as to anthropogenic impacts, the horizontal heterogeneity of Mediterranean habitats is
higher than vertical heterogeneity. However, the vertical distribution of
arthropods is crucial for their survival, especially during dry periods. Thus,
the study of both the horizontal and vertical structures of soil communities
is of primary importance for a thorough understanding of the dynamics of
Mediterranean ecosystems.
7.3.1
Vertical Movement
Sgardelis and Margaris (1983) analysed data of meso fauna sampled in Mediterranean habitats using reciprocal averaging (RA). Although samples taken
during the dry period were to some extent differentiated from wet period
samples, the temporal pattern was rather complicated. Distortions were due
to samples taken after an unusually heavy rainstorm in July and to artificially watered samples taken in September. These were incorrectly combined
with samples taken in the wet period. To explain deviations from seasonality,
the authors suggested that in response to summer drought, animals migrate
to deeper soil layers where they either die after ovipositing or aestivate in an
inactive stage. In all probability, increasing moisture stimulates egg hatching
and/or reactivation of animals. Relatively high numbers can therefore be
found in samples taken after heavy rain or artificial watering of the organic
soil layers.
Due to low locomotory capacity, microarthropods rarely migrate (Metz
1971). By contrast, vertical movements have frequently been noted and are
considered as ways to escape desiccation or high or low temperatures in the
surface layers of the ground. Vertical movement to deeper soil is considered
by Poinsot-Balaguer and Sadaka (1986) to be a mechanism allowing some
Collembola to withstand summer drought. For example, during most of the
year Onychiurus zschokkei is abundant in the intermediate litter layer of a
Mediterranean Quercus ilex formation in southern France. However, at the
Community Structure
colonised by microbes. Hence, autumnal activation of macrofauna induces
the synchronous activation of the entire decomposition process, probably
resulting in an increase in available nutrients during the growth period
favourable to plants. The message from this example - as from many others
- is that synchronised processes are the response of living organisms to seasonally varying Mediterranean environments.
7.3
Spatial Patterns
Due to small- and large-scale irregularities in the landscape, the bushy vegetation, the shallowness of the soil, and organic layers, as well as to anthropogenic impacts, the horizontal heterogeneity of Mediterranean habitats is
higher than vertical heterogeneity. However, the vertical distribution of
arthropods is crucial for their survival, especially during dry periods. Thus,
the study of both the horizontal and vertical structures of soil communities
is of primary importance for a thorough understanding of the dynamics of
Mediterranean ecosystems.
7.3.1
Vertical Movement
Sgardelis and Margaris (1983) analysed data of meso fauna sampled in Mediterranean habitats using reciprocal averaging (RA). Although samples taken
during the dry period were to some extent differentiated from wet period
samples, the temporal pattern was rather complicated. Distortions were due
to samples taken after an unusually heavy rainstorm in July and to artificially watered samples taken in September. These were incorrectly combined
with samples taken in the wet period. To explain deviations from seasonality,
the authors suggested that in response to summer drought, animals migrate
to deeper soil layers where they either die after ovipositing or aestivate in an
inactive stage. In all probability, increasing moisture stimulates egg hatching
and/or reactivation of animals. Relatively high numbers can therefore be
found in samples taken after heavy rain or artificial watering of the organic
soil layers.
Due to low locomotory capacity, microarthropods rarely migrate (Metz
1971). By contrast, vertical movements have frequently been noted and are
considered as ways to escape desiccation or high or low temperatures in the
surface layers of the ground. Vertical movement to deeper soil is considered
by Poinsot-Balaguer and Sadaka (1986) to be a mechanism allowing some
Collembola to withstand summer drought. For example, during most of the
year Onychiurus zschokkei is abundant in the intermediate litter layer of a
Mediterranean Quercus ilex formation in southern France. However, at the