Biology Reference
In-Depth Information
Resistance in an ecological sense refers to the ability of a community to
withstand encroachment by nonnative species. An overriding conclusion from the
literature on ecological resistance is that resistance is a probabilistic phenomenon
(D'Antonio et al. 2001b), varying across community types for a given invader
species (e.g., D'Antonio 1993) and among invader species within the same com-
munity type. Likewise, the relative importance of abiotic vs. biotic mechanisms of
resistance varies across landscapes and among years within the same landscape.
Thus, some communities are inherently more susceptible to invasion than others
while communities on the resistant end of the spectrum can be highly susceptible
to invasion after disturbance or a stress that opens up space in the community
(Davis et al. 2000). Despite the variation in resistance over space and time, experi-
mental studies have demonstrated that background vegetation plays an important
role in suppressing reinvasion of target weeds after initial top-down control
(McEvoy and Coombs 1999). D'Antonio and Thomsen (2004) have argued that
ecological resistance should be a more important part of invasive plant management
than it is currently.
In contrast to ecological resistance, resilience describes the ability of a commu-
nity to return toward its predisturbance, and presumably preinvasion, state after a
disturbance. It is a restoration goal because high resilience in a restored site means
less hands-on input is required by managers to keep vegetation within target
bounds. With relevance to invasive species, high resilience of native or desired spe-
cies will potentially reduce the need for immediate (postdisturbance) and future
control of invaders and for reseeding or replanting of native species after distur-
bances. Hence, high resilience increases the likelihood of resistance to invasion,
and both should be considered as management goals.
7.1.2
Fire-Enhancing Grass Invasions
The invasion of natural areas by introduced grasses is a widespread phenomenon
(e.g., D'Antonio and Vitousek 1992), and grasses are disproportionately represented
on virtually all published lists of high-impact natural area invaders (Daehler 1998).
Grasses can be difficult to control and manage for several reasons: (1) They
frequently have excellent mechanisms of resilience reestablishing from buds, roots,
or seedbanks after disturbance. (2) Many disperse readily across the landscape
either through effective passive dispersal or through attachment to animals.
(3) They are not easily controlled through classical biological control. (4) They can
transform ecosystems through their interactions with fire regimes (D'Antonio and
Vitousek 1992; D'Antonio 2000; Brooks et al. 2004). (5) They often play conflict-
ing roles in landscapes because within the same region they can provide forage for
livestock while also promoting fire and/or loss of native species. As a result con-
sensus for control is difficult to achieve.
In the Hawaiian Islands, perennial grasses from several other continents became
widely established by the mid 1900s (Smith 1985). Within the dry and mesic parts
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