In Insights we discover the story behind and beyond a recent publication in Functional Ecology. What inspired the authors to do the research, and how did the project develop leading to the final publication? What implications might their results have on the scientific community and on society?
This week, Bjorn talks to Martijn Vandegehuchte about his paper, Mammalian herbivores affect leafhoppers associated with specific plant functional types at different timescales (plain language summary here.) Martin recently moved from WSL (the Swiss Federal Institute for Forest, Snow and Landscape Research) to Ghent University in Belgium. With his colleagues in Switzerland, Martijn studied multitrophic herbivory interactions in the Swiss National Park.
What’s your paper about and how does it advance our knowledge of plant–animal interactions?
Our paper is about two very different and distantly related groups of herbivores: leafhoppers, which are small sap-sucking insects, and grazing mammals, ranging from red deer and chamois down to small rodents. We show that mammalian herbivores, through their effects on the plants on which they feed, affect leafhoppers. This is not new, as plant-mediated effects of mammalian grazers on insects have been shown in other systems. However, we show that because mammalian grazing affected different plant functional types in the short and long term, the leafhoppers specialized on these different plant types showed matching short- and long-term responses. The underlying mechanism was similar at short and long timescales, namely a reduction in plant biomass by the mammals that negatively impacted the leafhoppers’ abundance. In the long term, the small-sized sedge-feeding leafhoppers decreased in abundance in response to grazing, resulting in an increased average leafhopper body size. The key implication is that as different plants react at different timescales to mammalian herbivory, so do other animals feeding on these plants. This in turn implies that we cannot accurately predict the ecosystem-wide impact of mammalian grazers at a scale of decades or centuries by just extrapolating results from short-term experiments. While many mammalian herbivore species are currently at risk of extinction, a few are becoming overabundant due to the extinction of their natural predators. Our paper contributes to a better understanding of the long-term consequences of such changes in herbivorous mammal densities.
In your results, you mention that mammal herbivores avoid sedges. Surprisingly, the effect of mammal exclusion does not impact on sedge biomass as much as herbivory does on the other plants. Is there an easy explanation for this?
Actually, this is quite intuitive when you think about it. Plants that are preferred by mammals get grazed more, so when the mammals are excluded, they quickly increase in biomass. Sedges are less preferred, therefore less eaten by mammals, so that their biomass does not change much when mammals are excluded. However, in the long run, areas with lower mammal grazing become dominated by sedges. Grazing by mammals promotes grazing-adapted, palatable species. A germinating sedge would not have much chance there. However, over time, in areas that receive slightly lower mammalian grazing pressure, perhaps due to lower soil nutrient content, sedges have the chance to establish. This causes mammals to graze even less on the vegetation, eventually enabling the sedges to take over. Mammalian grazing thus affects different plants in the short and long term, and our paper shows that herbivorous insects specialized on these different plants therefore react at different timescales too.
This research much focuses on the role of mammal grazing on leafhopper populations. This is a very top-down approach for looking at herbivore interactions. Can leafhoppers also alter mammal herbivory? And if so, at what level – large, medium, or small mammals – would you expect this to work?
I remember a study from Spain by Gómez and González-Megías published in Ecology showing a negative effect of ungulates on leaf beetles, but no effect of beetles on the ungulates’ consumption of the study plant. This asymmetry was attributed to the large size difference between both types of herbivore. I doubt that leafhoppers, which are sap suckers, would have a large impact on mammals that consume entire leaves or even larger plant parts. Perhaps the removal of plant sap by leafhoppers makes plants less preferred by mammals to feed on, but I think only small mammals would discriminate between plant parts at such a small scale. If leafhoppers have any effect on mammals, I would thus expect it to be on the small rodents. As a side note, the difficulty of testing the effects of insects on mammals is that it requires a treatment that excludes the insects but not the mammals. The authors of the cited study managed to remove the leaf beetles by hand while ensuring that ungulates could access the plants. This may be feasible for a single, wingless insect species, but not for a group of highly mobile insects such as our leafhoppers.
Your results show that short-term and long-term effects of grazing on herbivore-herbivore-plant interactions are not dramatically different. In your set-up, the long-term ‘treatment’ – as I see it – is a corollary of shorter-term plant-animal interactions. Can you describe the differences in approach you took?
Both short- and-long term effects of mammals were mediated by changes in the biomass rather than the quality of the plants on which the leafhoppers fed, so the mechanisms were indeed similar. However, different types of plants, and therefore different species of leafhoppers, responded to mammalian grazing in the short and long term. In systems such as our subalpine grasslands, grazers prefer to feed on the dominant, grazing-adapted plants. These preferred plants will produce much more biomass if grazers are excluded in the short term, while pants that are avoided by grazers will initially not respond much. However, these less preferred plants tend to have a lower regrowth potential and when they are young they usually do not survive inadvertent grazing. In the long term, a lower grazing pressure may thus enable these less preferred plants to establish themselves and even become dominant and outcompete the grazing-adapted preferred plants. Our aim was to test how these contrasts between short- and long-term responses and between preferred and non-preferred plants would in turn affect other organisms associated with the plants. The Swiss national park was ideally suited to do so, because its subalpine grasslands have been monitored in detail for decades. We know that cattle and sheep were banned from the park when it was established in 1914. Wild ungulates such as red deer returned and started grazing most intensively in areas where the cattle and sheep usually rested and returned high levels of nutrients to the soil. This resulted in a short-grass vegetation. As other areas were less intensively grazed, less palatable plants such as sedges could get a foothold and start dominating, resulting in a tall-grass vegetation. These two vegetation types can thus be considered a long-term grazing “treatment”, as opposed to our exclosures, which were in the field for five growing seasons only.
You did your work in Parc Naziunal Svizzer (Swiss National Park), the only national park in Switzerland. Is it rewarding to be allowed into protected nature, and was it important for your study to do it in that setting?
The Swiss National Park is quite a special place. It is a strict nature reserve (IUCN category Ia), which means that it has an exceptionally low level of human disturbance. Hiking is only permitted on marked trails, whereas cycling, camping, dogs, and motorized vehicles are not allowed. In the winter, the park is closed to the public. This makes it a great place to study ecological processes and interactions in their natural state. Our field seasons started in May, when there was still quite a lot of snow, and continued into September. It was great to see the mountain landscape change over the course of the season. Because it is so undisturbed, wildlife thrives in the park, including large numbers of red deer, chamois, and alpine ibex. It was rewarding both professionally and personally.
This year’s British Ecological Society meeting, Ecology Across Borders, is in your home-town. Can you give us a little teaser what to expect, and where to go, in Ghent?
I am very excited about this meeting. It will be joint with the GfÖ, NecoV, and EEF, making it the largest gathering of ecologists in Europe. It will be a great opportunity for people from the different ecological societies to get to know each other and exchange ideas. I will actually give a talk at the meeting about our Functional Ecology paper, so if any readers have more questions about this study and are attending the meeting, they are welcome to ask me then.
Of course, I am entirely biased, but Ghent is a great town. The atmosphere in Ghent tends to be described as laid-back and a bit quirky. The city offers a mix of well-preserved medieval architecture, a vibrant nightlife that comes with the large student population, and a wealth of museums and cultural activities. Ghent is also a leading city regarding local sustainability projects. The city centre is car-free, and in December hosts a Christmas market and an ice rink. It is difficult to recommend any single place or activity, but the food and the beer are invariably superb. Readers are welcome to come find me at the meeting for more detailed suggestions.
You are an incredibly productive early/mid-career researcher. Have you followed the recent debate on work-life balance?
A person’s work-life balance is something personal and therefore difficult to generalize about. Research jobs are very performance-oriented, which can create the notion that one has never worked enough. However, I also have the impression that many scientists are driven by the enthusiasm about their research, so that they may not perceive a large amount of work as a burden. Nonetheless, dissatisfaction among young researchers is a genuine problem. Universities are incentivized to hire ever increasing numbers of PhD students. As there is no parallel increase in the number of faculty positions, this results in a relative oversupply of PhDs. This means that PhD students now have to work harder to outcompete one another for the few positions that may result in an academic career. At the same time, the work load of supervisors increases and the quality of the students’ training is put at risk. It is a positive sign that this current model is increasingly called into question, and solutions are being discussed, such as smaller labs or the creation of a type of permanent postdoc position to provide more continuous support to research labs. Furthermore, let us not forget that a good work-life balance has become a challenge for many people, not only researchers. In my lab, a healthy dose of free time is encouraged, and I am fortunate that I can balance my job with regular vacations to travel and disconnect from work entirely. I also try to carve out some time each week just to be lazy. Laziness seems underrated these days, even though good ideas often arise when the mind is left to wander.
More information on Martijn’s research can be found at:
Functional Ecologists 