Environmental Engineering Reference
In-Depth Information
This long history of human pressure in the landscape
had several important consequences.
positive-feedback cycle. Only inferior genotypes,
phenotypes and, ultimately, species are left to
reproduce and contribute to the seed bank in the
areas subject to this short-sighted mining and mis-
management. An exception to this rule is the
dehesa system described in Box 14.1.
• Wars, imperial and colonial appropriation, plagues
and other catastrophes leading to social disintegration
may have contrasting effects on land use. They
favour destruction and intensification of uses, but
they may also have the opposite effect by preclud-
ing other uses, affecting human population density
and distribution.
• Original vegetation (e.g. virgin and old-growth
vegetation) is mostly absent in the Mediterranean
Basin, and semi-natural forest is confined to remote
and inaccessible, uncultivable zones. No forests
appear in flat and fertile soils, and most mid- and
low-altitude vegetation communities were trans-
formed to agriculture or degraded long ago to low
and sparse vegetation due to a long, indeed millen-
nial, period of overuse (grazing, fire, cutting). The
composition and structure of semi-natural forests
have been greatly modified by long-term uses (ter-
racing, fuelwood gathering, charcoal manufacture,
animal husbandry and grazing, etc.). However, due
to the complex geology and diverse topography of
the Mediterranean Basin, there is still a significant
portion of semi-natural vegetation, which is higher
compared to central Europe, that houses an import-
ant faunal diversity (including bear, lynx, wolf and
large raptors).
• Abuse and overuse have also affected the soils. One
general outstanding feature of dry Mediterranean
soils is their low organic-matter content. As a con-
sequence, low levels of microbial activity and low
aggregate stability are common. This produces a high
risk of soil compaction, surface sealing and crust
formation in silty soils, which greatly increases runoff
and soil erosion when plant cover is scarce (Vallejo
et al.
1999). In addition, soils developed from
calcareous substrates, very common all over the
Mediterranean, tend to have low P availability.
• Where water is an important limiting factor, as land
degradation increases, soil structure and loss of soil
through surface erosion lead to decreased efficiency
of rainfall capture, and thus to regression of vegeta-
tion cover (Thornes 1987, Thornes & Brandt 1994,
Whisenant 1999). This hydro-pedological negative-
feedback loop is common in the Mediterranean
Basin, especially in the drier regions.
• People have practised an 'artificial negative selec-
tion' on wild plants (Burkart 1976). This term
describes the short-sighted practice whereby people
selectively remove the most useful phenotypes and
genotypes of woody plants (and other organisms),
both within and among species and genera, in a
progressive fashion generated by some kind of
In summary, as a consequence of human activities over
millennia, carried out in a seasonally dry, unpredictable
climate, many Mediterranean ecosystems were, and still
are, affected by more or less irreversible desertifica-
tion, especially in the long-inhabited and cultivated
transition zones in the hotter, drier parts of these MCRs.
14.2.2 Recent changes and conflicts
During the past century, with the advent of industrial
and tourist development, European Mediterranean
countries have experienced an important change
related to the abandonment of rural livelihoods and
the sprawl of cities in coastal areas. These recent
changes have had strong ecological implications.
• Abandonment of large areas of former agricul-
tural and pastoral land and reduction of grazing
pressure, fuelwood gathering, fibre cropping, etc.
Abandoned fields and ungrazed land have been
recolonized by early secondary successional species
or, frequently, planted with pines. This results in
homogenization of large areas with flammable
even-aged stands of trees or shrubland and accu-
mulation of litter creating a continuous fuel bed
and increasing fire hazard (Pausas & Vallejo 1999,
Pausas 2004). Under dry to semi-arid conditions,
colonization by late-successional species seems to
be rather slow (Francis 1990, Albaladejo
et al.
1998).
Furthermore, the collapse of unmaintained terraces
results quickly in increased soil losses.
• A shift from exploitative to recreational use of
wildland, including urbanization of rural areas by
