When most people see trees, shrubs, or other understory plants that are heavily damaged by herbivory, they think that one species is getting a meal, while the other has its fitness reduced. Many think this interaction ends there, with only two parties involved. Interestingly enough, these interactions not only alter communities of fungi and bacteria underground, but change the way these taxa compete. With around 90% of land plants forming mycorrhiza, the symbiotic union of sugar synthesizing plants and nutrient sequestering fungi, herbivory on aboveground plant tissues results in less sugars being sent to these underground assemblages. I try to stay away from hippy dippy innuendos, but ecosystems are truly interconnected. In a 2017 paper, David Eldridge on his team set out to understand how soil diversity around plant roots changes with vertebrate herbivory.
In so many mycological studies, scientists take a bottom-up perspective and analyze how abiotic factors like temperature and pH effect fungal activity, and how that translates to the plant community. In many cases top-down processes like herbivory are pushed aside. Over the past few decades, the scientific community has realized how easily herbivory alters ecosystem functioning. If you set up fenced areas to exclude just deer herbivory in the eastern United States, you will be flabbergasted to see not only what grows, but at what densities these plants occur in. With the local extinction of natural deer predators, these forest edge herbivores have multiplied by the millions and not only change plant communities, but underground environments surrounding plant roots, also known as the rhizosphere.
The rhizosphere is an intense place for microbial activity, as soil fungi and bacteria jostle to gain a foothold in these carbon rich settings. The authors here propose that the processes of competitive exclusion will be largely at play among the species living within the rhizosphere. Competitive exclusion has been historically described by plant studies that too focused on herbivory. Many of these studies found that herbivores feed on dominant plants, thereby reducing them. As this occurs, less dominate plants are excluded from competitive forces, and can survive in areas where the once dominate plant choked out other biota. It’s kind of amusing to realize that competitive exclusion is again playing a role at shaping communities interfered by herbivory. This time, it’s the microscopic organisms living in the rhizosphere.
This paper analyzed soil samples from three woodland communities in semi-arid eastern Australia. In each of their 54 plots, they quantified herbivore density based on the weight of oven dried herbivore dung per hector. By using recently updated genetic technologies, they were able to acquire results which show how the rhizosphere community changes with increased mammalian herbivory. Like they predicted, increased herbivory reduced the amount of sugars plants allocated to the forest floor. With fewer sugars, the dominate Actinobacteria become reduced, and as a result, bacterial diversity increased as other species were released from competition. Oppositely, the dominant Ascomycetes became even more dominant as fungal diversity decreased. With fewer sugars entering the rhizosphere, less fungal species can make a living.
Although this study was incredibly sound, these trends found here are not true for every ecosystem. Remember, this was carried out in a semi-arid woodland ecosystem in eastern Australia, a place notorious for ancient, nutrient poor soils. What you can take away from this study is just how dynamic the forest floor is. Aboveground herbivory several meters away from the soils surface, changes not only the community of fungi and bacteria living in the rhizosphere, but the way these organisms compete. In the face of voracious herbivores, taxa living beneath the forest floor are released from competition, while others succumb to it.