Intraspecific phytochemical diversity increases with productivity but
has mixed effects on herbivory
Abstract
Resources play an important role in shaping the evolution of plant
defensive strategies. Much research has focused on how resource
availability impacts the overall concentration of specific defensive
compounds, particularly among-species. Yet, how resource availability
influences overall phytochemical diversity within species, and how this
in-turn might affect herbivore damage is less well understood. Using
Monarda fistulosa, a species that produces diverse terpene compounds and
distinct chemotypes (i.e., thymol or carvacrol), we addressed three
primary questions: 1) How do phytochemical components (i.e., total
concentration, Shannon diversity, and chemotypes) vary between distinct
populations from two large, widely separated regions that differ in
climate and productivity? 2) How do these components influence attack by
different herbivore species? 3) What are the growth costs of higher
phytochemical levels in different chemotypes? Seeds were collected from
12 populations, six from a low-productivity/resource environment in
Montana, and six from a high-productivity/resource environment in
Wisconsin. Seedlings produced from these seeds were transplanted to a
common garden in Wisconsin. Over two growing seasons, we analyzed
terpene concentrations, quantified the number of herbivores damage, and
measured plant above-ground biomass. Plants from Montana exhibited
higher terpene concentrations but lower diversity, whereas those from
Wisconsin displayed lower terpene concentrations but higher diversity.
Total terpene concentration emerged as the primary predictor of
herbivore damage, although some differences were observed between region
of origin and with terpene richness. Herbivores exhibited mixed
responses to phytochemistry; some herbivores were negatively affected,
while one species, a specialized leaf galler, responded positively.
Costs of producing defenses were evident, manifest by negative
correlations between plant biomass and total terpene concentration (but
not chemical richness or Shannon diversity). Our study revealed that
regional differences in resource availability and productivity can
influence the evolution of phytochemical concentration and diversity,
which, in turn, affects plant damage and incurs growth-related costs.