Herbivory can increase plant fitness via reduced interspecific competition – evidence from models and mesocosms

Herbivores are generally considered to reduce plant fitness. However, as in natural communities herbivores often feed on several competing plant species, herbivores can also increase plant fitness by reducing interspecific competition among plants. In this study, we developed a testable model to predict plant fitness in the presence of an interspecific competitor and an herbivore that feeds on both plant species. Our model allows us to predict at which herbivore and competitor densities the focal species will benefit from herbivory. This can be estimated by quantifying the effects of the herbivore on the fitness of the focal plant and on its competitor, and by estimating the levels of intra- and interspecific competition in a pair-wise fashion, respectively. We subsequently validated the model in indoor microcosms using three interacting species: an aquatic macrophyte (the giant duckweed Spirodela polyrhiza), its native competitors (green algae), and its native herbivore (the pond snail Lymnaea stagnalis). Additional outdoor mesocosm experiments supported our model under natural conditions. Together, this study provides a conceptual framework to understand how herbivores shape plant fitness in a community context.

Methods
Files: 1_Exp2c_2d_algae_duckweed__duckweed_algae_day7.csv and 2_Exp3a,3b_snail_on_algae__snail_on_duckweed_24h_LB.csv

We tested within laboratory three-species microcosms under which starting conditions Spirodela polyrhiza benefitted from the presence of a snail herbivore when competing with Chlamydomonas reinhardii. We included a treatment with a high number of duckweeds competing against a low concentration of algae, and vice versa, as well as two treatments with intermediate densities. We grew these combinations in the with either no snails, one snail, or two snails. We excluded replicates in which the populations collapsed, or snails died within 14 days from the analysis, resulting in a data set of n = 3-8 per treatment. The details can be found in the Supporting Information (SI Text S2). On day 14 of the experiment, we counted the number of live fronds, measured the OD of algae, and calculated the per capita growth of both competing species. Data were analysed using Wilcoxon tests to compare per capita growth between the herbivory and control treatments.

ProcB_SuppInfo_R_Modelling_v3_LB.Rmd is an R-markdown file that describes data analysis processes.