COS 54-6 - Effects of plant ontogeny on arthropod communities are altered by drought

Abstract: Allocation towards growth and defense is expected to vary as plants develop (i.e., ontogeny), and these shifts can mediate changes in multi-trophic arthropod communities. Moreover, the effects of plant ontogeny on arthropods may also be contingent on abiotic conditions such as drought, but such effects remain largely unexplored. In a common garden we measured plant biomass and associated arthropods for plants sourced from eight populations of Artemisia californica over an 8-year period in which total annual precipitation varied between non-drought ( > 37 cm) and drought (< 18 cm) conditions. Populations were sampled in two common gardens established two years apart, allowing us to decouple covariation in annual precipitation and plant age. Plants grew ~6x more under non-drought (vs non-drought) conditions, leading to ~3.5x stronger effect of plant size on herbivore abundance that cascaded up to predators. Under drought conditions, plants grew little, but herbivores and predators declined with host plant age, suggesting an ontogenetic reduction in plant quality. Altogether, plants seemed to invest in greater defense as they aged under drought conditions, whereas plant defenses did not seem to vary under non-drought conditions. These findings highlight the role that population age structure may play in mediating drought impacts on multi-trophic arthropod communities.