Plant Trait Assembly Affects Superiority of Grazer's Foraging Strategies in Species-Rich Grasslands

<div><p>Background</p><p>Current plant – herbivore interaction models and experiments with mammalian herbivores grazing plant monocultures show the superiority of a maximizing forage quality strategy (MFQ) over a maximizing intake strategy (MI). However, there is a lack of evidence whether grazers comply with the model predictions under field conditions.</p><p>Methodology/Findings</p><p>We assessed diet selection of sheep (<i>Ovis aries</i>) using plant functional traits in productive mesic <i>vs.</i> low-productivity dry species-rich grasslands dominated by resource-exploitative <i>vs.</i> resource-conservative species respectively. Each grassland type was studied in two replicates for two years. We investigated the first grazing cycle in a set of 288 plots with a diameter of 30 cm, i.e. the size of sheep feeding station. In mesic grasslands, high plot defoliation was associated with community weighted means of leaf traits referring to high forage quality, i.e. low leaf dry matter content (LDMC) and high specific leaf area (SLA), with a high proportion of legumes and the most with high community weighted mean of forage indicator value. In contrast in dry grasslands, high community weighted mean of canopy height, an estimate of forage quantity, was the best predictor of plot defoliation. Similar differences in selection on forage quality <i>vs</i>. quantity were detected within plots. Sheep selected plants with higher forage indicator values than the plot specific community weighted mean of forage indicator value in mesic grasslands whereas taller plants were selected in dry grasslands. However, at this scale sheep avoided legumes and plants with higher SLA, preferred plants with higher LDMC while grazing plants with higher forage indicator values in mesic grasslands.</p><p>Conclusions</p><p>Our findings indicate that MFQ appears superior over MI only in habitats with a predominance of resource-exploitative species. Furthermore, plant functional traits (LDMC, SLA, nitrogen fixer) seem to be helpful correlates of forage quality only at the community level.</p></div

Linear mixed effect models where plot defoliation was dependent variable and every biomass proportion of taxonomic group or community weighted mean of quantitative trait was treated in the separate model and considered as fixed effect, while site code in a given year as random effect (degrees of freedom 1, 283).

<p>Linear mixed effect models where plot defoliation was dependent variable and every biomass proportion of taxonomic group or community weighted mean of quantitative trait was treated in the separate model and considered as fixed effect, while site code in a given year as random effect (degrees of freedom 1, 283).</p

Characteristics of sites within both grassland types, biotic parameters averaged over both years of observation.

<p>Note: MAT – mean annual temperature, MAP – mean annual precipitation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069800#pone.0069800-Tolasz1" target="_blank">[40]</a>, CSH – compressed sward height measured with rising-plate meter <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069800#pone.0069800-Correll1" target="_blank">[21]</a>: figure behind ‘±’ standard deviation.</p

Sheep selectivity (Jacobs' D) within plots in mesic and dry grasslands.

<p>Selectivity for (A) taxonomic groups of species; (B) sum of species possessing higher trait values than plot specific community weighted trait mean. Selectivity was evaluated with Jacobs' D selectivity index ranging from −1 to 0 (avoidance) and from 0 to +1 (preference). Squares show means with 95% confidence interval (CI). Selectivity for a particular group/trait was considered significantly positive/negative if CI did not involve a zero value.</p

Plot defoliation, biomass proportions of plant taxonomic groups and plot community weighted means of traits (including their abbreviations) averaged over both sites and years within grassland type, standard deviation in parentheses.

<p>Plot defoliation, biomass proportions of plant taxonomic groups and plot community weighted means of traits (including their abbreviations) averaged over both sites and years within grassland type, standard deviation in parentheses.</p

Relationship between plot defoliation and community weighted means of traits in mesic (A–B) and dry (C–D) grasslands.

<p>The four thin regression lines on each graph represent models for both sites in both years of observation (all levels of random effect), thick line (overlapping some thin lines) and its formula belongs to final mixed effect model.</p

Yoccoz_et_al_Table_S2

Supplementary Table showing the results obtained for the Tropical site

Tropical_sequence_data

Fasta file

Boreal_sequence_data

fasta file

Yoccoz_et_al_Table_S1

Supplementary Table with description of the results obtained for the Boreal site