Small Furry Animals Influence Plant Species Size Diversity in Old Fields

Herbivores could be unwitting architects of plant communities in old fields, suggests a study by Riley Gridzak and colleagues. Their research, recently published in Plant Ecology, unveils a possible mechanism explaining why certain plant species dominate in areas grazed by herbivores.

Plant species size, often measured by mature height, is considered a critical functional trait contributing to the species’ ecological role. In particular, taller plants are usually more competitive and contribute significantly to the ecosystem’s function. Yet, evidence for size-based rules in plant community assembly has been limited.

In the study, Gridzak’s team examined an old-field plant community over five years, using “caged” plots to exclude herbivores while leaving other areas open for grazing. The authors write:

We enclosed caged plots with a 1-cm gauge fence that extends roughly 45 cm above the ground and 15 cm below the ground. We left the tops of cages open to allow for vertical plant growth. The purpose of the caged treatments was to eliminate or at least greatly reduce herbivory. Based on camera trap evidence and general observations, there are a significant number of meadow voles and meadow jumping mice that frequent our study site.

Gridzak et al. 2023.

Their findings showed a modest but consistent underrepresentation of smaller plant species in the herbivore-excluded areas, indicating that herbivore activity might be providing a slight advantage to larger plant species.

Some evidence suggests that smaller plant species may be more successful in colonizing gaps (Schamp and Aarssen 2010), perhaps due to their ability to survive and reproduce at smaller sizes (Tracey and Aarssen 2014). Therefore, even if seedling consumption is random and not size biased (but see Hulme 1994), seedling herbivory may still favor small species. For example, the observed herbivory on T. pratense, which is a relatively small species, will leave relatively small gaps that plant species with small maximum sizes may still colonize (Schamp and Aarssen 2010). We do not have data to confirm this mechanism, although we are confident that herbivory is benefiting smaller species and that this is not due to herbivores targeting the tall plant species. The mechanism for our results requires further study; for example, other herbivore effects such as those from trampling, burrowing, and changes in litter distribution may also contribute to the observed compositional differences between treatments.

Gridzak et al. 2023.

The result is that mice and voles are accidental gardeners by opening up small gaps in the vegetation. These gaps then provide spaces for the small plant species to colonize more effectively.

READ THE ARTICLE
Gridzak, R., Wylie, R., Bennett, W. and Schamp, B.S. (2023) “Size-biased compositional impacts of small mammal herbivores in an old-field plant community,” Plant Ecology, 224(5), pp. 513–521. Available at: https://doi.org/10.1007/s11258-023-01318-0.