Frammentazione ambientale, connettività, reti ecologiche

134 Frammentazione ambientale
Abstract
Patchiness and habitat fragmentation - Biological communities spread in space
following a natural patchiness, according to environmental factors which may vary
along gradients or following an apparent casual trend at multiple scales.
Anthropogenic habitat fragmentation can operate on a preexisting patchiness,
altering the natural flows of individuals, matter and energy, affecting various processes
at multiple spatial/temporal scales and at every ecological level. Presently, it
is regarded as one of the main factors threatening diversity.
Habitat fragmentation may be defined as the dynamic process through which
comparatively homogeneous tracts of natural habitat are subdivided into progressively
smaller fragments and isolated. Such a process may be divided into different
components: habitat loss, decrease of fragment size, insularity, and increase in edge/interior
habitat rate.
Isolation of habitat fragments is similar, but not identical, to that seen in islands
sensu strictu: therefore, some ecologists have been induced to extrapolate the
theory of insular biogeography to terrestrial contexts. However, an one distinguishing
element of distinction is the presence of an anthropic matrix, surrounding the
fragments, that may have a meaningful impact according to its own characteristics
and to species-specific sensitivity.
Individual level - Size reduction and increase of isolation of fragments, besides
modifying the spatial pattern of the landscape, cause an alteration of the dispersal,
affecting behaviour, survival, and mortality of individuals with impact depending
on their age, sex, fitness and body size.
Population level - In fragmented landscapes, the populations of sensitive species
may be locally destroyed, reduced, or subdivided. The changes of dispersal rates
may induce an alteration of their social structure, distribution patterns, and
abundance, besides a reduction/interruption of the gene flow among them, with effects
on their viability and an increase of extinction risk at landscape/regional scale.
Also pointed out was the presence of a habitat fragmentation threshold at which
many species may start to show decreased abundance in response to reduced fragment
size and isolation.