Spatial variation in herbivore pressure

Herbivores cause substantial damage to plants that can reduce plant growth, reproduction, and population size. Insect herbivores that consume seeds developing in the flower heads are particularly destructive to plants because they directly reduce the amount of seeds produced by a plant. However, there is tremendous variability in herbivore pressure — some plant populations experience very low herbivore damage, while others must withstand very high levels of attack. Predicting whether or not a plant might be attacked by herbivores is therefore critical to understanding plant population dynamics in natural ecosystems.


Insect herbivores attacking the seed heads of Monarda fistulosa: weevil larva (left) and a larva of the raspberry pyrausta moth (right).

Our research is focused on measuring herbivore pressure across grassland sites that differ substantially in rainfall and productivity. We hypothesize that more herbivore pressure is exerted on Monarda fistulosa in high rainfall sites compared to low rainfall sites. This is because herbivores tend to be more abundant in wet regions than in dry ones. To test our hypothesis we have constructed cages around Monarda fistulosa across four sites in dry grasslands in Montana and four sites in wetter grasslands in Wisconsin. Some of these cages are sealed around the stems of the Monarda fistulosa to exclude insect herbivores, while some cages have been left open to allow access by insect herbivores while still mimicking the possible effects of shading and humidity that the cages may have.


Building herbivore-exclusion cages in Montana (left) and Wisconsin (right).

The goal of our work is to better understand how biotic interactions change across resource gradients and to determine their importance for plant fitness, abundance, and community composition. Having a better understanding of these differences will help ecologists better predict the consequences of biotic interactions in different climates.

This work is funded by the National Science Foundation (DEB-1901552).  Additional support, including access to field sites, is provided by MPG Ranch, The Prairie Enthusiasts, Missoula Conservation Lands, and the University of Wisconsin-Milwaukee Field Station.