Direct and indirect effects of alkaloids on plant fitness via herbivory and pollination

Herbivores and pollinators can simultaneously exert selective pressures on plant traits via direct and indirect effects. Net selection on plant traits , such as defensive chemistry, may be difficult to predict from studying ei ther of these interactions in isolation. In this study, alkaloids were mani pulated experimentally in the hemiparasitic annual plant Castilleja indivis a (Scrophulariaceae; Indian paintbrush) by growing these parasites with bit ter (high-alkaloid) or sweet (low-alkaloid) near-isogenic lines of the host Lupinus albus (Fabaceae) in the field. To evaluate the effect of herbivore s. half of the Indian paintbrush plants were randomly assigned to a reduced -herbivory treatment using insecticide, and the other half to a natural-her bivory treatment. Floral traits. bud and fruit herbivory, pollination, alka loids, and plant performance were measured. These variables were used in a path analysis to dissect the direct and indirect effects of herbivory and p ollination on lifetime seed set, and the direct and indirect effects of alk aloids on seed set via herbivory and pollination. Bud herbivory and fruit herbivory directly decreased seed production, where as pollination had a direct positive effect. In addition, bud herbivory had negative indirect effects on seed set by reducing the number of open flowe rs, which reduced pollinator visits. Alkaloids directly reduced bud herbivo ry but did not significantly affect pollination or fruit herbivory directly . However, because bud herbivory indirectly reduced seed set by reducing po llinator visits to flowers. alkaloids also had additional indirect benefits for plants by increasing pollination. Overall. the net benefit of alkaloid uptake was due to both reduction in herbivory and an increase in pollinato r visits to flowers. This study demonstrates the importance of considering multiple interactions simultaneously when attempting to understand the mech anisms underlying correlations between plant traits and fitness.