Community Structures Induced by Human Practices
99
lifetime, i.e. with a finite time of extinction (Hanski and Gilpin 1991). Accordingly, the maintenance of the genetic structure of such populations (favoured
by dispersion of arthropods among patches) is of great importance for their
persistence. Sheppard et al. (1992) studied the spatial distribution of oviposition in the sed predator Larinus cynarae (Curunlionidae). The arthropods
mate almost exclusively on the capitula of the Mediterranean thistle Onopordum illyricum illyricum. In agreement with the general scheme described in
previous paragraphs, the authors suggested that on a large spatial scale adults
are able to distinguish patch quality for oviposition. Indeed, they established
a relationship between capitula density (numbers of capitula/m2) in different
patches and reproductive effort (numbers of eggs laid/capitula). However,
within patches no correlation was found between microhabitat features such
as plant size, local capitula density and oviposition level. Moreover, within
patches reproductive effort is evenly distributed among different plants, irrespective of plant size or the availability of resources.
A mark-release-recapture experiment showed slight dispersion of arthropods among patches depending on patch size. However, even slight migration among patches (restricting risk) was effective in maintaining homogeneity in the genetic structure of the population. The overall conclusion is that
L. cynarae living in a highly heterogeneous habitat maintains a rather homogeneous genetic structure and the population is not split into local populations. Slight dispersion either for mating or oviposition is of great ecological
importance for the genetic stability of a population.
7.4
Community Structures Induced by Human Practices
Anthropogenic factors, namely fire and grazing, strongly affect the horizontal structure of Mediterranean ecosystems by increasing the heterogeneity of
habitats. Depending on irregularities of the landscape, fire results in
increased patchiness. In burned sites (local scale), both aboveground biomass and the litter beneath the canopies are completely destroyed, while
intact vegetation patches restricted to narrow catenas remain. For a few
months after fire, annuals, geophytes and resprouting shrubs cover the
burned soil. They produce large quantities of litter, evenly distributed throughout the burned area. In contrast, grazing pressure results in the creation of
paths and open sites between stands which are covered by grasses and soil
lichens. Thus, grazing results in the creation of environmental gradients from
the centers of shrubs towards exposed areas distant from them.
On a major scale, both fire and grazing result in increased horizontal heterogeneity of Mediterranean habitats. Resources are unevenly partitioned
and the climatic conditions vary among different sites. Indeed, in exposed
sites the available resources are limited and comparatively wider fluctuations
99
lifetime, i.e. with a finite time of extinction (Hanski and Gilpin 1991). Accordingly, the maintenance of the genetic structure of such populations (favoured
by dispersion of arthropods among patches) is of great importance for their
persistence. Sheppard et al. (1992) studied the spatial distribution of oviposition in the sed predator Larinus cynarae (Curunlionidae). The arthropods
mate almost exclusively on the capitula of the Mediterranean thistle Onopordum illyricum illyricum. In agreement with the general scheme described in
previous paragraphs, the authors suggested that on a large spatial scale adults
are able to distinguish patch quality for oviposition. Indeed, they established
a relationship between capitula density (numbers of capitula/m2) in different
patches and reproductive effort (numbers of eggs laid/capitula). However,
within patches no correlation was found between microhabitat features such
as plant size, local capitula density and oviposition level. Moreover, within
patches reproductive effort is evenly distributed among different plants, irrespective of plant size or the availability of resources.
A mark-release-recapture experiment showed slight dispersion of arthropods among patches depending on patch size. However, even slight migration among patches (restricting risk) was effective in maintaining homogeneity in the genetic structure of the population. The overall conclusion is that
L. cynarae living in a highly heterogeneous habitat maintains a rather homogeneous genetic structure and the population is not split into local populations. Slight dispersion either for mating or oviposition is of great ecological
importance for the genetic stability of a population.
7.4
Community Structures Induced by Human Practices
Anthropogenic factors, namely fire and grazing, strongly affect the horizontal structure of Mediterranean ecosystems by increasing the heterogeneity of
habitats. Depending on irregularities of the landscape, fire results in
increased patchiness. In burned sites (local scale), both aboveground biomass and the litter beneath the canopies are completely destroyed, while
intact vegetation patches restricted to narrow catenas remain. For a few
months after fire, annuals, geophytes and resprouting shrubs cover the
burned soil. They produce large quantities of litter, evenly distributed throughout the burned area. In contrast, grazing pressure results in the creation of
paths and open sites between stands which are covered by grasses and soil
lichens. Thus, grazing results in the creation of environmental gradients from
the centers of shrubs towards exposed areas distant from them.
On a major scale, both fire and grazing result in increased horizontal heterogeneity of Mediterranean habitats. Resources are unevenly partitioned
and the climatic conditions vary among different sites. Indeed, in exposed
sites the available resources are limited and comparatively wider fluctuations