Abstract
BackgroundCurrent 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.Methodology/FindingsWe assessed diet selection of sheep (Ovis aries) using plant functional traits in productive mesic vs. low-productivity dry species-rich grasslands dominated by resource-exploitative vs. 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 vs. 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.ConclusionsOur 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.
Highlights
The processes influencing patterns of diet selection have been brought together in optimal foraging theory which states that diet selection of a herbivore is influenced by the trade-off between the benefit of consuming a preferred diet, and the cost of handling and searching for it [1]
We addressed the following hypothesis: as mesic grasslands provide a sufficient amount of available biomass of high quality, sheep here selectively feed on high quality plots/plant species, in contrast, sheep grazing dry grasslands with generally low forage quantity and low quality select plots/plant species of greater biomass in order to fill their intestinal capacity and fulfill their basic metabolic requirements
Diet selection between plots At first, plot defoliation was compared across both grassland types using a mixed effect model where grassland type, community weighted means of quantitative traits, biomass proportions of grasses and legumes and their first order interactions with grassland type were used as fixed effects (AIC = 4890.03)
Summary
The processes influencing patterns of diet selection have been brought together in optimal foraging theory which states that diet selection of a herbivore is influenced by the trade-off between the benefit of consuming a preferred diet, and the cost of handling and searching for it [1]. In species-rich grasslands herbivores must solve two opposing problems: obtaining maximum quality and sufficient quantity (see review by Hejcmanova & Mladek [5]). This implies a trade-off in decision making which operates hierarchically at several spatial and temporal scales [6,7]. Two contrasting foraging strategies, maximizing intake (MI) or maximizing forage quality (MFQ), may be adopted by herbivores under different environmental conditions [8,9,10] The effectiveness of these strategies has been tested in models of plant – herbivore interactions [11,12] that each predicted the superiority of a MFQ over a MI strategy. There is a lack of evidence whether grazers comply with the model predictions under field conditions
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