Animals often compete with one another for food, but sometimes their actions can actually help other animals (Image Credit: Dennis Jarvis, CC BY-SA 3.0, Image Cropped)

Food and habitat provisions jointly determine competitive and facilitative interactions among distantly related herbivores (2019) Pan et al., Ecology Letters, https://doi.org/10.1111/1365-2435.13456

The Crux

Community structure, or the makeup of species within a given habitat, is largely determined by the interactions between the organisms that feed on plants. As such, the effects that different herbivores have on one another may impact how they feed, which would then feed back into which plants that are consumed, which would impact community structure. When one herbivore has a positive effect on the feeding of another, this is called facilitation.

Classically, facilitation has been studied as a one-way interaction (Species A facilitating Species B), but this ignores the reality of natural systems, where any interaction between species has the potential to act both ways. Today’s study investigated three different herbivores to investigate how they may interact with and/or facilitate one another.

What They Did

As their herbivores, the researchers used three animals from the Tibetan Plateau: yak (Bos grunniens), Tibetan sheep (Ovis aries), and tussock moth caterpillars (Gynaephora alpherakii). All of these animals have lived on the Tibetan Plateau and coevolved for thousands of years, making them ideal organisms to investigate the interactions and potential facilitation among herbivores. Yak and caterpillars both prefer to eat sedge, the dominant plant species in the community, and were predicted to compete. The sheep prefer to eat forbs (forbs are plants that are not woody or grass-like), which are normally outgrown by the sedge. The caterpillars likely facilitate the sheep, by eating the sedge, which releases the forbs from competition and allows them to grow more, giving the sheep more forbs to eat.

The authors set up 9 experimental plots and used three treatments; yak and caterpillar grazed, sheep and caterpillar grazed, and caterpillars only. Within each of these 9 plots, the authors sprayed a small area with pesticides to remove caterpillars without harming the plants. These combinations allowed the authors to tease apart the effects that the organisms had on one another and the plants. The authors then measured which plant each herbivore preferred, as well as how much of each they ate.

Did You Know: Indirect Effects

When we think of the effects that one organism has on another, we tend to think of direct effects, meaning that it is an interaction between two organisms. An example of a direct effect in this study would be the yak reducing the density of the sedge by eating it. Indirect effects, on the other hand, are those in which one organism interacts indirectly with another via a third organism. An example from this study would be the feeding of yaks increasing the densities of caterpillars. This is an indirect effect, because the yak eating the sedge allows other plants to grow more than they normally would (because the sedge would out-compete them). The caterpillars prefer these other plants to live on, so when these plants grow more as a result of the yaks eating sedge, the caterpillar population can increase.

What They Found

Yak and caterpillars did in fact prefer the dominant sedge, while the sheep preferred the forbs. The presence of the larger herbivores (yak and sheep) actually increased the densities of caterpillars. The presence of caterpillars increased the foraging rate of sheep, but it decreased the rate of foraging for yak. The larger herbivores changed the habitat that the caterpillars lived on, facilitating an increase in available food, while the caterpillars consumption of sedge facilitated sheep (by allowing the forbs to grow more) but competed with yak because they ate the same food.

Problems

The authors in the study only ran the experiment for ~1 year. While this was enough time to get an idea of what the different herbivores ate, it is only representative of a single season. The results they found may have been a fluke, and the patterns may be completely different over a longer time frame. I know I sound like a broken record, but these complicated ecological processes are just that, complicated, and it takes a LOT of work to understand them.

So What?

This study has shown that facilitative effects of herbivores go in both directions, confirming the need to consider both sides of the interaction. One of the difficulties in science is that experiments need to have strict controls and methodologies to ensure that confounding variables aren’t affecting the results. As a consequence, studies may only consider one part of the whole picture, and studies like today’s are perfect illustrations of why we need to investigate all aspects of ecology.