Ant-Eating Bears Help Plants

by Matthew Roby

According to Dr. Grinath, ants can compose as much as one-third of a black bear’s diet in mountain meadow ecosystems.

Josh Grinath, a Visiting Assistant Professor of Community and Global Change Ecology at Idaho State University who studies global change impacts on ecosystems, spoke recently  for the University of Arizona School Of Natural Resources and the Environment seminar series held weekly in the Environment and Natural Resources – 2 building.

The bearded, first-generation college graduate from Maryland looks like an ecologist. He wears a plaid shirt and hiking pants with zippered pockets. Grinath’s research reveals the complex interplay among organisms. By studying connections between bears, insects and plants in a mountain meadow, he found that ant predation by bears can benefit plant reproduction by increasing seed growth. To understand this link you need to follow a cascade of species interactions.

In a mountain meadow ecosystem in Almont, Colorado, the population of treehoppers—tiny insects that feast on sap plants use for growth—is controlled by lady beetles. Predation by lady beetles limits how much sap treehoppers mine from plants. However, this interaction is modified by a synergetic relationship between ants and treehoppers.

Ants protect treehoppers from predation by lady beetles in exchange for a sweet liquid derived from plant sap that treehoppers secrete. Grinath found that when bears eat ants, more lady beetles are free to eat treehoppers. Fewer treehoppers means less sap loss, leaving plants with more nutrients to grow seeds.

“The bears were a surprise for me,” said Grinath, who noticed bear-damaged ant nests at his research site. By comparing plants near damaged nests with those near intact nests, Grinath had a natural experiment to ask how bears modify the relationship between plants and insects.

“Josh opens the black box,” said Yue “Max” Li, a Conservation Research Scientist at the Arizona-Sonora Desert Museum. Whereas most studies examine the net outcome, this work reveals the “internal mechanisms of interplay” that help us “pinpoint the direct and indirect effects of each player in the system,” said Li.

Studying multiple species interactions can help us understand global change impacts on whole ecosystems, said Grinath.

“Ecology is highly context dependent,” and by studying plants, herbivores and predators together—instead of in isolation—we can better understand that context and estimate global change impacts on real systems.

Now he is asking if humans modify this interplay. By adding small amounts of fertilizer to experimental plots, Grinath can test if mountain meadow ecosystems respond to the settling of excess airborne nitrogen linked to human activities.

Adding nitrogen appeared to make plants less responsive to ant predation by bears. Grinath suspects that plants used the extra nitrogen to grow faster and produce insect-defense chemicals.

This work helps “unravel how complicated the environment is” and highlights why “preserving as much as we can is important,” said seminar attendee Sierra Lauman, a graduate student in the School of Natural Resources and the Environment. Lauman said research results depend on study timing, location and scale. Whereas nitrogen addition benefits plant growth in the short term, large-scale nitrogen application can favor invasive species that exploit nutrients and outcompete native plants, Lauman said.

Grinath said long-term monitoring can help detect trends amid these complexities.

“Ecology is inherently messy,” said Grinath. But complexity is not an excuse to ignore the effects of global change on ecosystems. Even if we don’t understand the complete picture, we can still make informed conservation decisions, Grinath said.

Adding nitrogen appeared to make plants less responsive to ant predation by bears. Grinath suspects that
plants used the extra nitrogen to grow faster and produce insect-defense chemicals.
This work helps “unravel how complicated the environment is” and highlights why “preserving as much
as we can is important,” said seminar attendee Sierra Lauman, a graduate student in the School of Natural
Resources and the Environment. Lauman said research results depend on study timing, location and scale.
Whereas nitrogen addition benefits plant growth in the short term, large-scale nitrogen application can
favor invasive species that exploit nutrients and outcompete native plants, Lauman said.
Grinath said long-term monitoring can help detect trends amid these complexities.
“Ecology is inherently messy,” said Grinath. But complexity is not an excuse to ignore the effects of
global change on ecosystems. Even if we don’t understand the complete picture, we can still make
informed conservation decisions, Grinath said.

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