Herbivore - Decomposer feedbacks

I am working to understand how animals that eat plants (herbivores) and animals that eat decomposing matter (detritivores) impact ecosystems.

Our experimental cages testing the combined impact of grasshoppers and earthworms on old-field plants and nutrient cycling.

Animals that feed on living plants and animals that feed on dead plants* are linked through shared resources and a common nutrient cycle. The identity of these types of animals changes, but their role in nutrient movements is analogous according to general ecological theories.

These theories predict that these, so-called, herbivores and detritivores, have interactive effects on ecosystem functions like plant growth and mineral nitrogen production. The idea is that the effects of these animals together push the ecosystem over a tipping point that neither can reach alone, where plant community and soil properties change. If this happens, it means that the whole ecosystem is not a sum of its parts (i.e. the two animals in this case). When I began my PhD, we had not tested this hypothesis in the field.**

Natural history

In New England old fields the dominant herbivores are grasshoppers and dominant decomposers are earthworms and rolly-polies. These animals feed on the plants before and after their death, respectively, so can both influence how much the plants can grow. Grasshoppers eat living plants and change the types of plants and the amount they can grow aboveground. Earthworms increase the decomposition of dead plant leaves and recycle more nutrients for plants.*** Rolly-polies also feed on dead leaves but their impact on plants and nutrient cycling is different from earthworms, because they also feed directly on the microorganisms that decompose leaves and produce low quality faeces that decompose slowly.

Our experimental design in cages with grasshoppers Melanoplus femurrubrum and earthworms in the genera Lumbricus and Apporectodea.

Experiment on grasshoppers and earthworms

I ran a four year experiment to examine how grasshoppers and earthworms jointly impact plant communities and the nutrients in soil to test the hypothesis that they have interactive effects on the ecosystem. I did not observe the interactive effects predicted by our theories. I built a new model of ecosystem nutrient cycling to figure out why. My results show that big pools of soil nutrients and established differences in the types of plants in each area can buffer feedbacks for decades. My work also contributes to understanding how non-native earthworms impact our ecosystems (e.g. 1).

One of our experimental cages with plants starting to grow in 2016.

Experiment on grasshoppers and rolly-polies

I have also studied the specific mechanisms that link grasshoppers and decomposers through plant leaf litter. An important hypothesis that has not be well tested is that herbivores make leaf litter difficult to eat because they induce the plant to make thicker leaves during the growing season. This litter slows down the rate at which decomposers can break down leaves into the nutrients that help plants grow in the next year. Unfortunately, we don't know much about how animal decomposers respond to the changes in leaf litter caused by herbivores or whether these changes in leaf decomposition actually have on plant growth.

I ran an experiment to explore both questions. 2 I found that decomposers, specificially rolly-polies, care more about the nutrients (nitorgen) in the leaves than herbivore defenses and that these decomposing leaves don't have an immediate impact (in 1 year) on whether plants have enough nutrients to grow in our fields. Using a simulation model, I tested different hypotheses about the reason why plant nutrient uptake and growth did not change even though litter decomposition did. I found that there was enough nitrogen available from the soil and plant root stores, so the relatively small increase coming from leaf litter was not enough to matter.

My big litter bags--cages where I decomposed different types of leaf litter to see how rolly-polies and decomposition changed the growth of plants.

* Dead plants and animals are called detritus. Animals that consume dead things are called detritivores.

** The theory had been tested in greenhouses and found to be correct under controlled conditions.
*** Many earthworms in northern North America are non-native species and some are having a negative effect on our forests!