Biological
influences on plants (plant-insect interactions):
Across the krummholz landscape in the Colorado Rocky
Mountains, I noticed that male and female plants were interspersed, but that a
particular hemipteran species seemed more often to be on female plants than on
males. This observation led me to
ask whether females differed from males in more than just physiology. I began
by looking at differences in the secondary chemistry (phenolics, carbon based)
of leaf tissue. Given that the two
sexes differed in resource use, then they are also likely to differ in defense
strategies. Male and female
willows did not differ, however, in leaf content, re-absorption, or
inducibility of phenolics. I
explored insect visitation more fully in an experimental array of willows that
I had grown from cuttings so that I could manipulate not only plant sex, but
also plant genotype. One frequent
visitor, a parasitoid wasp appeared to be attracted by flowering males but then
would congregate on neighboring females.
In addition, I surveyed larvae (Acleris
implexana) from male and female plants, and I found higher parasitism of
larvae from female plants than from male plants, indicating that females may be
enlisting the tertiary trophic level in their defense.
Given that male and female plants differ in secondary
traits, exhibit sex-biased interactions with other organisms and are often
spatially segregated, sexually biased interactions may occur on larger scales
than the individual plant. In the
Chilean Andes, I manipulated floral display of a cushion plant that dominates
the landscape so much that the vegetation zone has been named after this
species, Laretia acaulis. I removed inflorescences prior to mast
flowering in a design enabling me to manipulate neighborhood sex ratios and
then observed visiting insects on a focal plant within the manipulated
neighborhood. I found that male
plants had higher insect diversity compared to female plants, independent of
sexual neighborhood, but that females have highest fitness when surrounded only
by male plants. So at the patch
level, males seem to increase diversity and ensure population persistence by
increasing female fitness.
