Ecological Archives E096-256-A1
Anne Kempel, Mialy Razanajatovo, Claudia Stein, Sybille B. Unsicker, Harald Auge, Wolfgang W. Weisser, Markus Fischer, and Daniel Prati. 2015. Herbivore preference drives plant community composition. Ecology 96:2923–2934. http://dx.doi.org/10.1890/14-2125.1
Appendix A. Detailed description of the assessment of herbivore preference in a “plant tournament” and mode of the plant tournament.
Assessment of herbivore preference in a “plant tournament”. We assessed which of the 28 plant species were most preferred and which disliked by the generalist caterpillar Spodoptera littoralis, and by the generalist slug species Arion vulgaris Moquin-Tandon (syn. Arion lusitanicus Mabille; Arionidae). Similar to Spodoptera littoralis, Arion vulgaris is known to feed on a variety of plant species and is often used in bioassay studies (Pérez‐Harguindeguy et al. 2003; Dirzo 1980).
For each herbivore species, we performed a series of pairwise choice-tests, which we called a “plant tournament”, where we allowed herbivores to choose between two plant species. In each pairwise choise-test, we randomly placed two plant species next to each other and connected their pots with tape and a sand ramp, allowing herbivores to walk easily from one plant to the other. Thus, all plant cues (olfactorial, visual, mechanical, and taste) could influence herbivore choice. At the beginning of each choice-test, we placed a petri-dish lid filled with seven caterpillars (third to fourth instar) in the tournament for caterpillars, or five slugs in the tournament for slugs, respectively, in the middle of each playing field and enclosed both pots together with a nylon gauze. Herbivores were now allowed to explore the playing field and choose their preferred plant species (Fig. 1 c). After 24 hours, we counted the number of herbivores on each opposing plant species as “goals”. Herbivores that stayed close to the half-way line were not counted. Similar as in association football, we awarded three points to the plant species winning a choice-test (having highest number of goals), one for a draw, and no points to the losing species. To decrease the probability that a choice-test is won by chance, we decided that a choice-test is won only if the goal difference is more than one.
The fairest way to obtain a preference ranking of our plant species would have been a round-robin tournament in which each plant species plays each other species once. However, this would have required a lot of single choice-tests (n/2 *(n-1)). We therefore modified the round-robin tournament in such a way that allowed us to reduce the amount of total choice-tests but still provide a reasonable ranking from the least to the most preferred plant species. In a first round, we randomly assigned the 28 species into seven groups of each four species. Each group played a round-robin tournament, where every plant species played each other plant species once (giving three choice-tests per species and six choice-tests per group). We ranked the species within a group based on points. In a second round we assigned species to four new groups of each seven species with the condition, that each group contains one species from each previous group, and is comprised of equally powerful species (see Fig. A1). This was important, because we wanted to assure that each species played against the same number of preferred and less preferred species. A random assembly could have by chance enabled groups with only preferred or only non-preferred species. A species being, e.g., the least preferred in a group of only preferred species would lose against all other species in its group and would end up having a very low rank, although it actually would be moderately preferred overall.
Each of the four new groups played a round-robin tournament (six choice-tests per plant species and 21 choice-tests per group). We recorded goals and points of each choice-test and ranked species according to their scores. Doing so, we obtained two rank lists, one based on goals, and one based on points, with the most preferred species having the highest number, and the least preferred species the lowest number, of goals, respectively points. Caterpillar and slug preference measured as number of goals was highly correlated to caterpillar and slug preference measured as number of points (Table 1). We therefore used preferences measured as number of goals in the following correlations.
Fig. A1. Mode of the plant tournament. We assigned the 28 plant species to seven groups of each four species. In round 1, each group played a round-robin tournament and species were ranked using points. Based on the results of round 1 we formed four equally powerful new groups of each seven species (1st rank of group A, 2nd rank of group B, 3rd rank of group C, 4th rank of group D, etc.).
Literature cited
Dirzo, R. 1980. Experimental studies on slug-plant interactions: I. The acceptability of thirty plant species to the slug Agriolimax caruaneae. Journal of Ecology 68:981–998.
Pérez‐Harguindeguy, N., S. Díaz, F. Vendramini, J. H. C. Cornelissen, D. E. Gurvich, and M. Cabido. 2003. Leaf traits and herbivore selection in the field and in cafeteria experiments. Austral Ecology 28:642–650.