Quantifying direct and indirect effects of early-season herbivory on reproduction across four brassicaceous plant species
Data files
May 01, 2024 version files 1.19 MB
Abstract
Insect herbivores can directly affect plant reproduction by feeding on reproductive tissues, or indirectly by feeding on vegetative tissues for which plants are unable to compensate. Additionally, early-arriving herbivores may have cascading effects on plant reproduction by altering the later-arriving community. However, the dynamic interplay between plant development and the assembly of herbivore communities remains underexplored. Hence, it is unclear whether non-outbreak levels of ambient herbivory early in the development of plants can impact plant fitness and to what extent these effects are mediated through changes in plant development and subsequent herbivory. By excluding the herbivore community in an exclosure experiment and by manipulating early-season herbivory in a common garden field experiment replicated across four Brassicaceae species and two years, we tested whether early-season herbivory by caterpillars (Pieris rapae) or aphids (Myzus persicae) affected development, reproduction, and the herbivore communities associated with individual plants. In addition, we tested a causal hypothesis to assess the relative importance and temporal interplay between variation in herbivore communities and variation in plant development in determining plant reproduction. Early-season herbivory affected plant reproduction in the exclosure experiment, with effects being highly dependent on the plant species, the herbivore species, and the year. However, we found no such effects in the field experiment. The exploratory path analysis indicated that variation in plant reproduction is best predicted by variation in plant development, explaining 80% of the total effect on seed production. This suggests that early-season herbivory had limited effects on later plant development, and plants were able to attenuate the impact of early-season herbivory. However, no clear compensatory mechanism could be identified. While early-season herbivory has the potential to affect plant reproduction through changes in plant development or the subsequent development of the associated community, these effects were small and varied across closely related species. This suggests that plant species may be exposed to different levels of natural selection by early-season herbivores through plant- or community-mediated effects on reproduction.
README: Quantifying direct and indirect effects of early-season herbivory on reproduction across four brassicaceous plant species
https://doi.org/10.5061/dryad.c866t1gf7
The data contains observations on the development of four different brassicaceous plant species and their associated herbivore community in a field experiment conducted in the summers of 2017 and 2018 in Wageningen, The Netherlands. These plants were challenged early in their life to herbivory by either caterpillars or aphids or were left untreated. In addition, we include the observed reproductive output on these plants, as well as that of plants that were exposed to the same treatment, but which were grown in a parallel experiment in which herbivore communities were excluded.
These data allow us to test whether our herbivory treatments affected plant development and reproduction and whether effects were predominantly mediated by variation in plant growth or variation in the plant-associated herbivore communities.
We found that the herbivory treatments affected plant reproduction in the experiment where the herbivore community was excluded. These effects were dependent on the plant species and the herbivore species used. However, no such effects were found in the open-field experiment. The subsequent SEM analyses revealed that variation in plant development was most likely to affect seed production. However, we found that our treatments had little effects on plant development later in the life of plants, indicating that plants were able to attenuate or compensate for these effects. No clear compensatory mechanism could be identified.
We conclude that, while early-season herbivory has the potential to affect plant reproduction through changes in plant development, these effects are limited and were similar for the species in this study. As plants are often able to attenuate and compensate the effects of early-season herbivores, the selective potential on plant life-history or defense strategies in our study system seems ubiquitously low.
Description of the data and file structure
The data collected is divided in four ready-to-use excel files to work in conjuncture with the supporting R-scripts (see Code/Software). Our Excel sheets contain a Read-Me explaining the contents of each data sheet, and the type of data collected. The Functional Ecology manuscript has a detailed methods section that outlines data collection and methodology. Identically named columns in the different datasheets link the different data together. For example, the unique identifier "Plant_Code" in the data on plant development relates to the same plant as the unique identifier "Plant_Code" in the data on herbivore community observations and the data on seed production.
We appreciate notification of data re-use. Please use the contact D.Mertens.sci@gmail.com and/or Erik.Poelman@wur.nl
Code/Software
All R-code used to analyze the data in this manuscript is published with zenodo and can be freely accessed by following the DOI.
Methods
Herbivore_Community
We recorded the development of herbivore communities on the focal plants planted in monoculture plots by monitoring individual plants from seedling to seed set. Recording of the herbivore communities started two days after the experiment was installed. We monitored the development of herbivore communities on individual plants by weekly counts early in the season and by biweekly counts later in the season. Insects were identified in situ to species or family level. If accurate identification was not possible, we included the observations as morphospecies in our data.
Plant_Parameters
In addition to observations of the herbivore community, we recorded a set of plant parameters as proxies for plant biomass and development: plant height (measured from the ground to the top of the plant), diameter (measured as the distance between the two most distal leaves), the number of true leaves, and the number of flowering and seed-carrying branches (aggregated as reproductive branches). The height and diameter of plants were used to derive the volume of a cone, representing plant biomass as a single volume parameter in the subsequent analyses.
Seedset_Field
We harvested plants after the start of plant senescence but before the siliques started losing seeds. From the harvested plants we collected the total number of seeds produced as a proxy for plant fitness. For each plant, the total number of seeds was estimated by extrapolating the weight of 100 seeds to the total seed biomass and rounding to the nearest natural number.
SeedSet_Tents
The same protocol as described above for the field experiment.
Analysis
All analyses and statistical approaches are detailed in the Functional Ecology manuscript. In summary: We applied linear regression to test for effects on the different growth and reproduction parameters and followed up on this analysis with piecewise structural equation modelling where we test a causal hypothesis on how effects of herbivory can cascade to affect seed production. Finally, we use multivariate analysis to describe differences between plant species in terms of the herbivore communities with which they interact.