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1. Although plants and herbivores interact under varying soil resources
and natural enemy effects, little is known about how these factors
influence plant-herbivore interactions and shape the evolution of plant
and herbivore traits. Here we ask whether soil fertility and parasitoids
shape selection on fruit number imposed by a seed predator (SP) on the
perennial herb Ruellia nudiflora. 2. We used a common garden where half
the plants of 14 genetic families were fertilized, and recorded the
abundance of cleistogamous (CL) fruits and seeds, SPs, and parasitoids. We
calculated relative fitness per family based on CL seed number under the
following three scenarios: Three trophic levels (accounting for SP and
parasitoid effects), two trophic levels (accounting for SP but not
parasitoid effects), and one trophic level (fitness in absence of SPs),
and compared selection strength on fruit number between trophic scenarios
and fertility environments. 3. In unfertilized conditions, SPs selected
for increased CL fruit number, whereas parasitoids dampened (but did not
eliminate) this selective impact. With fertilization, however, selection
by SPs was reduced and unaffected by parasitoids. 4. Synthesis: Overall,
we show that parasitoids can shape herbivore selection on plants, but that
both herbivore and parasitoid selective impacts depend upon the abiotic
environment. These findings underscore how linkages between abiotic
factors and trophic complexity influence the ecological and evolutionary
outcomes of species interactions.
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