A Preliminary Study On Sheep Foraging Selectivity And Plant Associational Defense

Animals were the main consumers in the natural grassland systems. The producer plants and animals constitute two ecosystem interface in the natural grassland ecosystems. Biological interactions in grassland ecosystem mainly occurred at plant-animal interfaces. The work on plant-animal interaction is of great importance not only for development of foraging theory, but also for maintaining biodiversity, sustaining the stability of grazing grassland systems. So the plant-animal (such as cattle, sheep, etc.) interaction has become the core of the ecologists'research.The traditional plant and animal relationship research focused on the individual level. Herbivores meet their nutrient needs through foraging. Plants form the simple morphological defenses (hard cuticle, needle, thorn, etc.) and chemical defenses (secondary metabolites, toxins, etc.) in order to survive and reproduce. However, the diet selection was the core of the plant-herbivore interaction, which was the first prerequisite and foundation of the plants-animals interaction. Foraging selectivity has been overlooked sometimes. When herbivore make decisions at the plant population or community level, plant individuals can gain protection not only by its own physical and chemical defense but also by that the neighboring plants. This theory was known as the plant associational defense.The aim of this study was to explore herbivore feeding selectivity at different foraging levels (individual and patch) under the different spatial and temporal scales, and determine whether the plant associational defense was affected by change of spatial and temporal scales. We selected three species with different quality of high, medium and low (Medicago sativa, Phragmitis australis, Leymus chinensis) as experimental plants. Three patches with high, medium and low quality, were constituted by three different proportions of forages. Three spatial scales (25m×25m, 35m×35m, 45m×45m) were set. The three species were aggregated distribution in each patch. Three grazing temporal scales (30min, 60min and 90min) were set.The results of experiment showed as follows:1) Sheep exhibited selectivity at plant individuals and patch levels. The changing of spatial and temporal scale has a significant impact on sheep's feeding selectivity in within-patch and between-patch. The within-patch selectivity significantly reduced with the temporal and spatial scale enlarged. At small spatial scale, the between-patch selectivity show an increasing trend with the increase of temporal scales, but at medium and large spatial scales, it was on the contrary. Overall, at any temporal and spatial scales, the within-patch selectivity of sheep was significantly higher than the between-patch selectivity.2) The results of the plants associational defense were significantly affected by the spatial and temporal scales. At the large spatial scale, sheep intake of the focal plant P. australis within low-quality patch was higher than that within high-quality patch. The focal plant was damaged within low-quality patch and protected within high-quality patch. The result at the medium spatial scale was not significant, but the same trend. The result was on the contrary at small spatial scale. Intake of the focal plant P. australis within low-quality patch was lower than that within high-quality patch, but it was not significant. The focal plant was protected in low-quality patches and damaged in high-quality patches. The two different results indicated that plant associational defense altered with the change of the spatial scale.3) The total intake was significantly affected by spatial scale. The intake of M. sativa was on the dominant at all scales, which indicated that sheep primarily selected the food with higher crude protein. As the spatial scale become larger, sheep improved feeding efficiency by reducing the traveling time and switching frequency between patches, thus reducing feeding costs.In conclusion, sheep feeding selectivity was higher at individual level than that at patch level in this experimental. The plant associational defense showed"neighbor contrast susceptibility"and"neighbor contrast defense"at the medium and large spatial scale, while"associational susceptibility"and"associational defense"at the small spatial scale.