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Integrating Ecological and Evolutionary Theory of Biological Invasions 85
species suffer less from enemies relative to native competitors (and how this
affects competitive interactions) remains uncertain (e.g., Settle and Wilson
1990; Calvo-Ugarteburu and McQuaid 1998; Blaney and Kotanen 2001).
The biotic resistance hypothesis (BRH) focuses on the communities into
which species are introduced, and aims to understand how they differ in their
ability to resist invasion (Elton 1958; Tilman 1997; Levine and D’Antonio
1999; Maron and Vilà 2001), and to suppress population growth of invaders
(Levine et al. 2004). Most studies of biotic resistance have focused on competitors
and the role of species diversity in shaping competitive regimes. However,
there is growing appreciation that the importance of this process varies
strongly with the setting in which competitive interactions occur. For example,
the relative abilities of species to compete successfully is influenced by
their interactions with higher trophic levels (e.g., Schierenbeck et al. 1994;
Courchamp et al. 2000; Chase et al. 2002). Invading species also experience
resistance directly from enemies that are able to use the novel organisms as
prey or hosts. Introduced species invading communities with close relatives
are more likely to accumulate natural enemies and experience stronger competition
than is the case in the absence of relatives (Strong et al. 1984; Mack
1996; Torchin and Mitchell 2004). Hence, the enemy release and the biotic
resistance hypotheses are fundamentally linked.
6.2.2 Evolutionary Hypotheses
Most research on the mechanisms underlying biological invasions has
focused on the ecological explanations outlined above, despite the pioneering
symposium of Baker and Stebbins (1965) addressing the potentially critical
role of evolution in the success of colonizing species. For evolution to contribute
to the success of introduced species, it must increase relative fitness of
individuals in the population, and thereby increase population growth rates.
A challenge is understanding whether and how evolution may lead to more
rapid increases in fitness in introduced populations than in native populations,
leading to strong invasions.
The evolution of increased competitive ability (EICA) hypothesis is an evolutionary
corollary to the enemy release hypothesis that proposes that escape
from natural enemies alters the selection regime, such that costly defenses
against enemies no longer enhance fitness. The evolutionary loss of defenses
enables resources to be directed toward growth and reproduction, or other
traits influencing performance (Blossey and Nötzold 1995). Thus, EICA
addresses a specific scenario in which the selective regime changes for introduced
species but not for native species, and thus can explain strong invasion.
Research on this hypothesis has principally focused on plants, but the basic
premise of EICA should be valid for other taxa, too. The EICA hypothesis
leads to two key predictions. First, introduced populations should exhibit a