In this new post, Robin Heinen from the Netherlands Institute of Ecology presents his latest work ‘Foliar herbivory creates subtle soil legacy effects that alter future herbivores via changes in plant community biomass allocation’, discuss the importance of complex interactions to understand ecological communities and shares his worries about not being able to disconnect from nature when you are ecologist.

About the paper

In early 2018, while in the second half of my PhD project at the Netherlands Institute of Ecology (NIOO-KNAW), I was approached by a motivated MSc student, Jetske, who wanted to work together with me as part of her graduation project. In my PhD project I studied how plant-soil feedbacks (i.e., the impact that plants have, via changes in soil conditions, on plants that grow later in the soil), alter plant-herbivore interactions. I had just finished several large greenhouse studies that had already revealed the strong impacts of plants on soil microbiomes, and the effects that these soils often had on plants and associated caterpillars feeding on them. Until then, my own work had always assessed the effect of soils on plant-herbivore interactions. However, herbivory could potentially also alter the way plants interact with soils, something that had been shown in individual plants a couple of years prior to that. How herbivory by insects affected plant-soil processes in different plant species and how this would influence the resulting feedbacks in plant communities, had not been investigated.

At the time Jetske inquired with me about her project I was also supervising a BSc student, Jasper, that investigated how two different polyphagous insect herbivores performed on the 24 plant species I worked on for my PhD project, and – importantly – how plants responded to presence or absence of herbivores. The timing of these two projects could not have been more excellent, as Jetske could immediately start off using the already conditioned soils from Jasper’s experiment, which was nearing its end. After critically assessing Jasper’s data, we selected five grasses and five forbs that were experiencing the highest levels of herbivory, and used the soils from these plants, exposed to one of two insect herbivores, or no herbivore. After carefully separating the roots from the plants, we setup new mesocosms with a response plant community composed of all ten plant species. After several weeks of growth, these plants were all caged individually, and caterpillars were introduced – I was still interested in responses on herbivores, after all! In essence, a very simple study setup.

About the research

Our study combines aspects of plant-insect, plant-soil and plant community ecology, and is of interest to both above- and belowground ecologists (and those that study both, of course), and we share a couple of key findings of importance to the field. First of all, we find that both the plants and the insect herbivores that feed on them affect soil microbial activity, but that the two herbivore species, despite being similar and closely related, have opposite effects. Second, we find that although the plant species do affect the plant-soil feedbacks on the plant communities, the herbivores did not affect plant biomass, but did affect biomass allocation to roots and shoots. Interestingly, these effects of herbivory on conditioning plants were noticeable in caterpillar performance in the response plant communities. Our new paper highlights that herbivory on plants subtly changes the ways in which plant-soil feedbacks affect insect herbivores. An important question that remains is what role does this play in natural grassland ecosystems? Herbivory and plant-soil interactions are ubiquitous in most grasslands, but how they interact to shape plant community dynamics, or future herbivore communities, is simply not known. Remarkably, given the attention that herbivory and plant-soil feedbacks have received as independent processes, very few people have actually studied their interactive effects. The studies that do exist seem to suggest that effect sizes are reduced when complexity increases (i.e, from individuals to communities). It is not clear whether the increasing complexity of natural systems will mask the interactions between herbivores, plants, and soil, in grasslands. Future manipulative insect herbivore exclusion experiments in standardized grassland community plots in a classic two-phase plant-soil feedback study would pose an interesting – albeit a highly challenging way of testing this experimentally. Unfortunately, the short-lived nature of academic positions has caused me to slightly shift my own focus to other aspects of plant-mediated interactions. However, do not hesitate to get in touch if you are interested in taking this up further. I am happy to have a chat and share ideas.

About The Author

My name is Robin Heinen, an entomologist/ecologist (depending on who asks) based in Freising, Germany. To be honest, I never really planned to be a scientist, and certainly not in ecology. I obtained a bachelor’s degree in animal biology at Wageningen University in my home country of the Netherlands, with a curriculum mostly focused on animal anatomy and behaviour, and mostly towards the end, entomology. My curriculum had very little focus on classical ecology. My master’s degree was in bio-interactions, which touches on ecology a little more, but was heavily plant-oriented. At the end of my master’s degree, I was not sure if I was cut out for a career in academia, and actually did not plan on pursuing this either. To finish my master’s degree, in early 2015 I fulfilled an internship project, studying insect herbivores and parasitoid wasps at the Netherlands Institute of Ecology, supervised by Jeff Harvey. These were five months of experimental work on various projects, and served as a bit of an eye-opener for me. It was Jeff that eventually pushed me to apply for a PhD position with Martijn Bezemer, at the time based at the same institute. I’m glad I eventually did. Four exciting years with a wonderful group flew by, and I finished my PhD in 2019.

Since completion of my PhD, I have been employed as a research associate at the Technical University of Munich in the Terrestrial Ecology group with Wolfgang Weisser – where I teach ecology, and supervise various PhD projects that investigate plant interactions with bacteria, fungi, and of course insects. I am currently also developing my own research line that investigates the ecological side effects of light pollution on plants and the phytobiome, and I am a little bit proud of an opinion piece I recently published on this (https://doi.org/10.1016/j.baae.2021.10.007). I hope to soon share some of my experimental findings with the world, and I promise it will be interesting!

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