Biology Reference
In-Depth Information
20.
Permanently Altered
Biotic Communities
“As we continue to manipulate biodiversity, our experience so far with the
evolution of virulence in diseases, short-term effectiveness of resistant crop
varieties, and rapid evolution of interactions within natural communities sug-
gests that the health and welfare of human societies will demand an increased
understanding of the ongoing evolutionary dynamics of species interactions.”
—T
HOMPSON
(1999c)
In the early 1900s, the southern flanks of the Alps in southern Switzer-
land and northern Italy were dominated by temperate deciduous forests
of oaks, chestnut, linden, and ash, growing under a strongly seasonal cli-
mate. Many human influences have affected these original forests, includ-
ing tree harvesting and planting, domestic animal grazing, modification of
the fire regime, and introduction of exotic plants, all of which have
changed over time.Added to these factors, however, is a changing climate,
especially during the last three decades of the twentieth century. Low
winter temperatures, especially those below freezing, have become much
less frequent, and the length of the growing season has increased. These
climatic changes have favored broad-leaved evergreen woody species over
deciduous species (Walther 2002).
Nonindigenous evergreen plants from several world regions have
invaded the forests of this region of the Alps (Walther 2000). More than
a dozen invaders are involved. Some, such as the European laurel (
Laurus
nobilis
), have simply migrated north from Mediterranean Italy. Another,
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