Unequal Impacts: USU Ecologist Explores Variability in Plant, Herbivore Interactions

For plants, it’s a try-not-to-get-eaten world, where photosynthetic organisms have evolved many different physical and chemical defenses to avoid attack from hungry plant-eating animals and insects. But herbivore damage on plants can be highly variable — even among plants in the same population.

“Herbivory on plants is one of the most important processes in nature,” says Utah State University ecologist Moria Robinson. “Yet, we find puzzling patterns within this common interaction type. We observe highly uneven distributions of herbivory within plant populations, with some plant individuals getting lots of damage and others escaping entirely. But we have limited data to explain this common pattern.”

Robinson, assistant professor in USU’s Department of Biology and Ecology Center, is among leaders of an international collaboration of scientists, founded in 2019, which seeks to better describe and understand how and why patterns in plant-herbivore interactions vary across the globe. The group, known as The Herbivory Variability Network, reports findings from its research, supported by the National Science Foundation, in the Nov. 10 online edition of Science.

The amount of variability within an ecosystem is thought to influence most aspects of plant-herbivore interactions, says Robinson, lead author on the paper, but less is known about what shapes that variability.

“Two plant populations can have the same average herbivory, but in one population each plant could get about the same amount, while in another, all the damage could be concentrated on a few individuals,” she says.

Why the difference?

“That’s why this multi-institution collaboration was formed,” Robinson says. “We sought to look across some of the largest factors that differentiate plant-herbivore systems, including large-scale gradients between temperate and tropical systems, across plant growth forms such as woody and non-woody systems, and across the tree of life. Our network grew and we ended up working with 127 research teams in 34 countries, who surveyed more than 500 species of plants at 790 sites across 116 degrees of latitude.”

And what did they discover?

“We found the amount of variability in plant-herbivore interactions is a defining feature of ecological systems, just like the average amount of herbivory,” she says. “We found that plants in tropical populations tend to each get a similar amount of herbivore-feeding, whereas in temperate populations, herbivory can vary widely from plant to plant, with a few individual plants sustaining most of the damage.”

Robinson adds the team found herbivory is more even on plant species that are larger in size or woody, and more closely related plant species have more similar distributions of herbivory.

What does this mean?

“When we think about these findings in conjunction with evolutionary theory, we can draw connections between these results and explanations for why certain plants have evolved certain types of defenses, and why rates of evolution may be higher in some areas of the globe compared to others,” she says.

Robinson says the timing of the consortium’s efforts presented unique challenges, because the world was headed into pandemic lockdown as the research got underway.

“In hindsight, I think of this project as creating global community in a time when many of us were uncertain, isolated and experiencing loss,” she says. “Going outside to look at holes in plant leaves was something all of us could do safely, and I was privileged to be able to meet and interact with so many excited scientists around the world and across career stages.”

Her collaborators ranged from a high school student in New York to a graduate student in New Zealand to a seasoned research professor in Nigeria.

“We found human connection and shared interest in the midst of a trying situation, and laid the groundwork for a thriving and enduring research network,” Robinson says.

More than 150 institutions collaborated on the Science paper, including Michigan State University, Montana State University, Florida State University, University of Florida, University of Nevada, Reno; Universidad Nacional Autónoma de México, Case Western Reserve University and the University of Vermont.