Plant traits across space: Does co-occurrence of related host plants drive variation in herbivory and defenses across populations?

Katherine Holmes, Ecology and Evolutionary Biology, Cornell University and A.G. Power, Ecology and Evolutionary Biology, Cornell University, Ithaca, NY

Background/Question/Methods
Plant susceptibility to herbivore enemies is known to vary dramatically with community composition at small spatial scales. Spatial variation in herbivore risk has been proposed as a driver of diversity in plant defensive traits, particularly phytochemical diversity; however, this has seldom been tested at the scale of multiple plant populations. Two marsh asters provide an opportunity to test this question: at small scales, spotted Joe Pye weed (Eutrochium maculatum) is more susceptible to herbivore damage when growing near common boneset (Eupatorium perfoliatum) in upstate New York. These two species commonly co-occur, express similar physical and chemical defenses (i.e., trichomes and pyrrolizidine alkaloids), and share multiple specialist herbivores. To explore whether or not defensive traits change with the relative frequency of these two host plants across a broader spatial scale, we surveyed twelve isolated marsh populations of Joe Pye weed and boneset in the Finger Lakes region of upstate NY in the summer of 2020. Data collected included the relative abundance of each species, the abundance of key herbivores, plant traits related to performance (height, flowering), and leaf damage from herbivores.
Results/Conclusions
Populations varied from 0 to 44% in the proportion of boneset plants. The two species were negatively correlated in their flowering rate across sites. Herbivores that specialize primarily on boneset (Ophraella notata, Eupatorium plume moths) were found at greater frequency in boneset-dominated sites, while herbivores that specialize on Joe Pye weed (Haploa confusa) were more abundant at Joe Pye weed-dominant sites. Our data show that Joe Pye weed shares one isomer of the toxic pyrrolizidine alkaloid lycopsamine with boneset, but expresses two other isomers that are not shared; this may provide an opportunity for convergence or divergence in plant chemistry with changes in relative frequency of the two species. Broadly, we suggest that variation in co-occurrence with risky neighbors at population scales may drive divergence in plant defense traits and contribute to species-level phytochemical diversity. Forthcoming common garden data will address whether differences in herbivore abundance and plant damage correspond with divergence or convergence in trichome density and the relative concentration of different isomers.