Frammentazione ambientale, connettività, reti ecologiche

Frammentazione ambientale 135
Habitat fragmentation may directly or indirectly expose the populations to deterministic
factors (e.g., direct persecution, edge effect). When populations are
highly reduced in abundance, stochastic factors may come into play (demographic,
environmental, genetic); these may inhibit the population’s response ability to
changing environmental factors and, thereby, reduce viability.
An insidious effect of fragmentation is the delay induced in some (meta)populations
to die out at local/regional scale (lag effect and extinction debt): i.e.,
species with a long life-cycle may show a response only some generations after
fragmentation, giving rise to a misleading evaluation of the effects. Moreover,
following the decreased availability of habitats, a density increase of some sensitive
population in the fragments, utilized as refugees, can be observed (crowding
effect).
Community level - The alteration of ecological and spatial variables at fragment
scale (size, isolation, shape, quality, etc.) as well as at whole landscape scale can
affect qualitative composition and species richness as well as other parameters.
An increase of richness in fragments may occur due, for example, to colonization
of generalist species from the matrix and to edge effect. The gradual decline of
the more sensitive species, caused by changes in the extinction/colonization rates
(relaxation faunas) and the proportional increase of edge/generalist species, may
induce a species turnover in fragments (cascade effect).
Ecosystem and landscape level - Flows of energy and matter are affected by the
spatial pattern of the landscape. The changes in size, distribution, quality of the
fragments, and in the ecological processes may influence the functionality of the
whole ecomosaic.
Matrix surroundings may affect the edge effect, playing the roles of source area
(providing alternative habitats to generalist species) and of sink for interior species,
as well as modifying the rates of colonization and the between-fragments processes.
Fragmented landscape flows are prevalently controlled by the matrix.
Intra- and between-fragment effects - The main changes in the patterns of distribution/abundance
of the populations of sensitive species in the fragmented landscapes
are due to two classes of mechanisms acting at intra- and between-fragment
spatial scale.
Among the intra-fragment effects, alteration of the habitat quality is the most relevant
in the short term (edge effect). Instead, the between-fragment effect consists
mainly in alteration of the species-specific dynamics. These are related to indivi-