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experiment, harsh growth conditions increased investment in reproduction (Friess & Maillet,
1996), but i n s imilar e xperiments w ith c hickweed ( Stellaria m edia) r eproductive al location
decreased at h igher density, although it w as stimulated at moderate plant density (Friess &
Maillet, 1997). There could be also varied responses depending on t he a daptive strategy of
different genotypes: c ompetition c ould de crease r eproductive i nvestment be cause hi gh
investment in growth increases competitive ability (Pagan et al., 2009). The results suggest
different adaptive s trategies o f B. j uncea under di fferent growth c onditions, i ncluding
resource a vailability, c ompetition, a nd he rbivory, w hich c ould ul timately r esult i n different
population dynamics.
Population production and dynamics with increasing insect-resistant plants
Population pr oduction w as a ffected b y experimental c onditions, i ncluding bot h pl anting
procedure, p ercentage of c lipped l eaves a nd yearly conditions. In general, hi gh de foliation
caused a greater de cline i n popul ation pr oduction t han l ow de foliation. In a t wo-year
experiment, m ammalian he rbivores a ltered ve getative g rowth, i ncluding growth r ate a nd
timing of r eproduction, of Cirsium oc cidentale and u ltimately th is h erbivory a ffected th e
population dynamics (Palmisano & Fox, 1997). Spataro and Bernstein (2004) found that the
effect of competition on population dynamics was linked to the presence of parasitoids in the
system. However, estimating th e impact o f h erbivory i s complex b ecause h erbivory affects
neighboring plants in addition to target plants, and seed dispersal in addition to growth and
reproduction (Verkaar, 1987). In the current study, populations at the plot border had higher
production than those in the plot center, while the number of viable seeds of the populations
in pl ot c enters di d not differ be tween t he t wo e xperiments. T herefore, t he di fference i n
population production between the two experiments might be better explained mainly by the
different growth conditions than b y the different defoliation levels. In o ther words, like the
resource allocation of plants discussed above, growth conditions may play a more important
role in population production than defoliation, especially for reproductive production.
Maximum popul ation r eproduction w as obs erved a t T 75 at t he pl ot bor ders of t he
second e xperiment, a lthough popul ation pr oduction va ried l ittle a mong t he v arious
percentages o f he althy pl ants. R eproductive production w as i mportant f or popul ation
composition, s ince m ore s eeds f rom i nsect-resistant pl ants w ould be r eleased t han f rom
susceptible one s unde r high he rbivory. R eproductive pr oduction i s a lso i mportant f or t he
kinetics of popul ation demography, be cause e nhanced s eed pr oduction c ould m ake a
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