By Walter Andriuzzi, Postdoctoral Fellow, Colorado State University
June 2011. I am standing near the edge of a vertical cliff above the Atlantic Ocean. Two puffins glide and land gracefully on the rocks despite their almost comically short wings. I am on the Isle of May, a wind-swept Scottish island with some of the biggest colonies of marine birds in Europe. Not far from the guano-streaked cliff there is a small plot of land enclosed by a fence. This is an exclosure, and its task is to keep out the rabbits that graze the vegetation on the island with the efficiency of a lawnmower. The grass outside, punctuated by cushions of forbs like Silene uniflora and Armeria maritina, is ankle-high; inside, the grass brushes my knees. And this is not the only difference. Outside the exclosures, the ground is littered with rabbit faeces, which fertilizes the soil. The plants grazed by the rabbits undergo changes in their chemical make-up, and tend to release more carbon into soil as root exudates. The rabbits also change the environment physically: with a reduced plant cover, the ground is more exposed to atmospheric fluctuations; and of course, rabbits make burrows. Unsurprisingly, excluding rabbits is a big deal for the plants. But what happens belowground?
This is a question that ecologists Richard Bardgett and Rene van der Wal have investigated since the early 2000s. They set up these exclosures on the Isle of May to test the idea that the effects of herbivores on a soil biological community depend on the fertility of the system. The Isle of May is an ideal natural laboratory to test this framework, because it has two levels of soil fertility: near the seabird colonies the soil is enriched in nitrogen of marine origin (courtesy of the birds’ guano and ammonia), whereas in the middle of the island it is not. And yet, three years after the exclosures had been established, rabbits had the same (weak) effects on the soil food web regardless of whether the plots were close to the coast or not.
In 2011, as a master’s student of van der Wal, I seek to find out if things have changed eight years since the experiment started. As a proxy for the soil food web, I study the nematodes, microscopic roundworms that live in the water films between soil particles. They occupy almost all trophic levels belowground: there are nematodes that feed on bacteria, nematodes that feed on fungi, nematodes that feed on microalgae, nematodes that feed on plant roots, and nematodes that feed on nematodes. Because they are so diverse and extremely abundant, they have a big impact on ecosystem functioning; in turn, of course, they are affected by what happens to the plants and the soil where they live. And yet, once again we find only small differences in the soil food web inside and outside the exclosures, despite obvious differences in plant biomass and plant diversity.
The difference between what happens above and below ground is in no way peculiar to this experiment. Many other studies found surprisingly weak effects of herbivore mammals on soil fauna and microbes. Other studies found big effects, but not in a consistent direction: in some herbivores increased the abundance of soil organisms or the rate of soil biological processes, while in others the opposite was observed. In an attempt to better understand general patterns in belowground responses to herbivory, I recently performed a meta-analysis, an analysis of results from numerous previous studies, on herbivore-plants-soil interactions to find global commonalities in how soil organisms and the biological processes they regulate respond to herbivores. Stay tuned for the next post to discover what I found out!