COS 111-6 - Species and population specific responses of plants and their associated arthropod communities to contrasting water availability conditions in a coastal sage scrub ecosystem

Abstract: Water availability is a key abiotic factor determining plant quality and resistance to insect herbivores, with plant responses varying both among and within species as a result of local adaptation to contrasting environmental conditions. Yet, whether species and population-specific responses to water availability drive changes in plant-associated multi-trophic arthropod communities has been seldom tested, specifically under experimental settings. Here we aimed to test the quantitative and qualitative effects of water availability on plant-associated arthropods and whether such effects differed among plant species and populations within species varying in their degree of adaptation to water stress. To that aim, we manipulated water availability levels in a common garden consisting of 12 plant species representative of the coastal sage scrub ecosystem of California and sampled arthropods in a subset of four shrub species, namely, Artemisa californica, Encelia californica, Eriogonom fasciculatum and Salvia melifera. Seeds of each plant species were collected from different genetic sources, including four coastal and four inland populations (i.e., mesic and xeric-adapted populations respectively). The experimental design consisted of a full factorial design of water availability (low vs. high) and plant genetic source (mesic vs. xeric) replicated in three blocks (12 plots). In April 2022, at peak biological activity, we vacuum sampled arthropods in three plants per species per plot (N=144), classified arthropods according to their order or suborder, and counted the number of individuals in each group and total arthropod abundance. Water availability effects on plant-associated arthropods depended on plant species and ranged between a 17% and a 12% increase in arthropod abundance with increasing water availability in A. californica and S. melifera respectively, and a 18% decrease in E. californica. Similarly, effects on arthropods were also contingent on plant genetic source, whereby mesic populations showed a 10% decrease in arthropod abundance with increasing water availability and xeric-adapted populations showed a 24% increase. Additional qualitative analyses also demonstrated that water availability affected the composition of arthropod communities, with plants with higher water availability showing higher abundance of insect herbivores (Hemiptera: Auchenorryncha), and plants in the low water availability treatment showing higher abundance of predatory ants (Hymenoptera), although compositional changes were also species-specific. Our results contribute to understanding water availability effects on plant-associated multi-trophic arthropod communities and suggests that different adaptive strategies to water stress in plants underlie contrasting inter- and intra-specific patterns in water availability effects