Do multiple herbivores maintain chemical diversity of Scots pine monoterpenes?
Iason, GR and O'Reilly-Wapstra, JM and Brewer, MJ and Summers, RW and Moore, BD, Do multiple herbivores maintain chemical diversity of Scots pine monoterpenes?, Philosophical Transactions of the Royal Society B-Biological Sciences, 366, (1569) pp. 1337-1345. ISSN 0962-8436 (2011) [Refereed Article]
A central issue in our understanding of the evolution of the diversity of plant secondary metabolites
(PSMs) is whether or not compounds are functional, conferring an advantage to the plant, or nonfunctional.
We examine the hypothesis that the diversity of monoterpene PSMs within a plant
species (Scots pine Pinus sylvestris) may be explained by different compounds acting as defences
against high-impact herbivores operating at different life stages. We also hypothesize that pairwise
coevolution, with uncorrelated interactions, is more likely to result in greater PSM diversity, than
diffuse coevolution. We tested whether up to 13 different monoterpenes in Scots pine were inhibitory
to herbivory by slugs (Arion ater), bank voles (Clethrionomys glareolus), red deer (Cervus elaphus)
and capercaillie (Tetrao urogallus), each of which attack trees at a different life stage. Plants containing
more a-pinene were avoided by both slugs and capercaillie, which may act as reinforcing
selective agents for this dominant defensive compound. Herbivory by red deer and capercaillie
were, respectively, weakly negatively associated with d3-carene, and strongly negatively correlated
with the minor compound b-ocimene. Three of the four herbivores are probably contributory
selective agents on some of the terpenes, and thus maintain some, but by no means all, of the
phytochemical diversity in the species. The correlated defensive function of a-pinene against
slugs and capercaillie is consistent with diffuse coevolutionary processes.