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no fitness cost was detected in the Brassiceae (Ramachandran et al., 2000; Mason et al., 2003;
Di et al., 2009) or in sunflower (Snow et al., 2003). The population dynamics of introgression
could simply depend on the reproduction system, the rate of population turnover, the cost of
being a n in terspecific h ybrid, o r th e b enefit o f b eing r esistant. H owever, in h abitats w ith
limited resources, intraspecific competition for these resources is density-dependent (Verkaar,
1987). Ordinarily, insect- susceptible wild plant populations are regulated via the effects of
herbivory on ve getative and reproductive growth, but the population dynamic changes when
these populations are invaded by insect-resistant plants because the competitive landscape for
a g iven i ndividual va ries w ith t he g enetic composition of i ts ne ighbors. A s r esistant pl ants
have an advantage over susceptible under herbivory pressure, their prevalence would increase
relative to native plants, which in turn would face an increasingly competitive environment.
As a n e xample: t he di sadvantage t o chickweed plants ( Stellaria media) infected b y a v irus
increased w ith t he pr oportion of he althy competitors i n t he popul ation (Friess & M aillet,
1997).
The co mpetitive r elationship b etween i nsect-resistant a nd s usceptible pl ants c an be
partitioned i nto t wo c omponents: 1) i ntra-class c ompetition be tween i ndividuals of a s ingle
class; an d 2 ) i nter-class c ompetition b etween r esistant a nd s usceptible p lants. When i nsect
damage reduces the size of a susceptible plant, resistant neighbors can usurp its resources and
thus s uppress i ts g rowth ( Weis & H ochberg, 20 00). H owever, he rbivory da mage not onl y
affects the given individual, but also its neighbors and therefore regulation of the competitive
relationship might be influenced by herbivory (Chase et al., 2002). The interaction between
competition and herbivory could be: (1) additive, when the fitness disadvantage for an insectsusceptible
p lant is a mplified b y in creased competition f rom r esistant p lants ( Weis &
Hochberg 2000; A grawal, 2004) ; o r ( 2) antagonistic, w hen t he di sadvantage f or t he
susceptible pl ant de creases be cause of t he i mpacts of he rbivory on n eighbors ( Haag et al .,
2004; S chadler et al ., 2007). T here c ould a lso be no i nteraction, i n s ituations w here t he
disadvantage t o t he s usceptible pl ant doe s not c orrelate with uni que or combined pr essure
from competition and herbivory (Steets et al., 2006). The outcome of the interaction between
competition a nd herbivory w ill de termine w hether t he popul ation w ill e volve t o be 100 %
insect-resistant or whether it w ill reach some intermediate equilibrium between resistant and
susceptible plants.
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