Alien Species and Fire - Fire in Mediterranean Ecosystems - page 334

Human activities conducive to the origin and spread of invasive species have
been present in the region throughout the Holocene. As a consequence, movement
of invasive species from one area to another has been going on for a very long
time and has prompted the distinction of archaeophytes and neophytes to
describe, respectively, old (before 1500) and new alien introductions (Pysˇ ek
1998 ). Neophytes often are found in similar habitats as those favoring archae-
ophytes (Chytry´ et al. 2008 ).
Despite being the source for many MTC invasive species, the Mediterranean
Basin has a number of naturalized alien species as well. However, considering both
the European and North African sides of the basin, these non-natives represent
only about 5% of the flora compared with, for example, about 20% in California
(di Castri 1989 ; Le Floc'h et al. 1990 ). For Spain, Italy, Greece and Cyprus
Arianoutsou et al. ( 2010 ) reported 782 neophytes, but only 30 were widespread
across all four countries. Most are roadside weeds or agricultural pests and not
aggressive invaders of natural ecosystems, although roughly half of all neophytes
do colonize fire-prone habitats including maquis, garrigue, phrygana and
grasslands.
The Mediterranean Basin represents a particularly interesting area to investi-
gate the relationship between invasive species and fire as it is the home to most of
the invasive annual grasses that produce prominent grass fire cycles ( Box 12.1 )in
other MTC regions. Adaptive modes of response to fire in woody species of
Mediterranean shrublands exhibit parallel patterns to those seen in the other four
MTC regions, and natural fire frequencies are similar to those of California (see
Chapter 4 ) . Many of the same genera of annual grasses and forbs that are highly
invasive in some other MTC regions are also present and become established after
fire. What makes the Mediterranean Basin dramatically different from the other
regions, however, is the remarkable resilience of shrub species to frequent disturb-
ance and seeming resistance to alien invasion.
Trabaud ( 1990 ) reviewed the resilience of a wide variety of Mediterranean Basin
shrublands to repeated fire and general resistance to alien plant invasion.
Although a number of naturalized alien species may briefly colonize recent burns,
they seldom persist even under regimes of frequent fire. Several factors appear to
be at work here. One is the limited number of aggressive alien species from other
regions of the world. Another is that many shrublands are dominated by vigorous
resprouting shrubs that quickly recapture space after fire. Relative to most other
MTC regions, the Mediterranean Basin has many fewer non-resprouting obligate
seeding shrubs (see Chapter 3 ). Thus, canopy cover increases rapidly, providing
fewer opportunities for herbaceous aliens to establish. Even on those sites with
obligate seeding shrubs, most of which are sub-ligneous, the vegetation exhibits
high resilience to short fire return intervals. Many such shrubs reach sexual
maturity in the third year after a fire, and some of them can sometimes produce
seeds at 1-2 yrs (Carreira et al. 1992 ; Roy & Sonie´ 1992 ). Similarly, pine forests in
the Mediterranean Basin have been found to be generally resilient to short fire
return intervals (Thanos & Doussi 2000 ; Eugenio & Lloret 2006 ).