New publication from Bröcher in Functional Ecology: Plant–herbivore interactions depend on plant richness, plant and soil history

Insect herbivores play a central role in grassland ecosystems. They depend on plants for their nutrition and exert feedback effects on plant communities. Although such reciprocal relationships are well recognized, how they are shaped by plant community diversity and history remains largely unexplored.

In a full-factorial mesocosm experiment in which plant species richness (1, 2, 3 and 6 species mixtures), soil and plant history conditions were manipulated, we used Spodoptera exigua (Hübner, 1808) caterpillars to inflict damage on Plantago lanceolata L. plants. We used herbivore performance, plant performance and various plant traits to investigate two main questions: (I) the interactive effects of plant traits, plant diversity and plant and soil history on herbivore performance, and (II) the impact of herbivory on plant traits and performance, considering varying plant diversity and plant and soil history.

Here, we found reciprocal effects between plants and herbivores that varied with plant species richness, plant history and soil history. Specifically, herbivores performed better on more nutritious plants, with the strength of this effect depending on both plant and soil history, while they performed worse with increasing plant species richness. However, the latter effect was not observed in a subsequent trial conducted a few weeks later. Additionally, herbivory reduced plant height in monocultures, altered nutritional quality and physical defences compared to undamaged controls depending on plant history, and increased chemical defence compounds.

Our study demonstrates that bidirectional interactions between plants and arthropods are key to understanding community-level responses, as the strength and direction of these reciprocal effects depend on plant and soil history as well as plant richness. This context dependence drives community dynamics and has important implications for ecosystem functioning, species coexistence and coevolutionary processes.

Conceptual framework illustrating the hypothesized reciprocal relationships between plants and herbivores, and the moderating roles of plant richness, plant and soil history. H1: We hypothesize that herbivore performance improves with higher plant quality and lower chemical and physical defences, and that these plant traits are influenced by the surrounding community’s diversity as well as plant and soil history. H2: We hypothesize that herbivory reduces plant fitness and induces both chemical and morphological defence responses. These responses are expected to vary depending on plant diversity, plant history, and soil history, resulting in more nuanced and context-dependent defences in plants from diverse or long-established communities. Blue arrows represent direct relationships (e.g. from plants to herbivores, or herbivory to plants), while green dotted arrows indicate hypothesized context dependencies. Green and brown symbols represent treatments *with* (green) or *without* (brown) plant or soil history, respectively.

Schematic representation illustrating (a) the experimental timeline and (b) the design of a single EcoUnit, including the manipulation of community history. In (a), the timeline outlines key experimental steps in chronological order, indicating the months in which each task was completed. In (b), the EcoUnit design is shown, divided into four subunits, each representing a unique combination of plant and soil history. Specifically, each subunit contains either plants grown from plot-specific seeds (*with* plant history) or plants grown from original seeds (*without* plant history), as well as plot-specific soil (*with* soil history) or soil from bare ground plots (*without* soil history).

Reference:

Bröcher, M., A. Ebeling, L. Bassi, P. Medina-van Berkum, N. M. van Dam, et al. (2025). Plant–herbivore interactions depend on plant richness, plant and soil history. Functional Ecology n/a. https://doi.org/10.1111/1365-2435.70191