Plant herbivore defense syndromes mediated by canopy structural complexity

Plants defend themselves against attack by herbivorous (plant-eating) insects in many ways. For example, they make chemical poisons and construct mechanical defenses, like thorns. Plants also use predators of herbivores for protection, following the age-old adage that the enemy of my enemy is my friend.

But no plant can afford to invest in all types of defense. Investment in one strategy must trade-off against investment in another, leading to tremendous variation among plant species in the combination of defenses they employ. Understanding the environmental factors that dictate a plant species' defense strategy has proven difficult.

Yet, tackling this challenge is important for understanding why plants are made the way they are. That, in turn, is important for guiding plant breeding and pest management practices in agriculture. This study will test a new idea about how plants can make it easier for birds to pick off herbivorous insects.

Specifically, it is predicted that birds will be attracted to plants with few branches, where they can most easily move and find insects. Because birds remove lots of insect enemies, those plants should need to invest less in chemical and mechanical defenses. Thus, this study will thus reveal whether a plant's branching pattern sets the stage for predictable patterns of variation in how plants defend themselves.

The researchers will work with local teachers to use results from the project to promote an understanding of research practices to undergraduate and K-12 students.

The evolutionary ecology of plant defense against herbivores has been a major program of research for more than half a century. Both the types and magnitude of herbivore defenses vary dramatically with geography, among plant families and species, among populations and individuals within species, and even among tissues within individuals. Trade-offs are believed to lie at the core of this variation, but predicting how trade-offs play out has been difficult within the complex milieu of changeable ecological settings and the constraints imposed by evolutionary history.

This study tests a novel the hypothesis that plant structural complexity drives variation in multivariate herbivore defense strategy in plants. These predictions will be tested by characterizing co-variation among structural complexity, bird indirect defense, arthropod indirect defense, and direct defense traits for 30 species of sympatric plants (shrubs, large perennial herbs) from the California Coastal Sage Scrub ecosystem.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.