Cirsium altissimum performance in response to nutrient availability and spider density

Both abiotic factors and biotic interactions are key components driving community level processes. Nutrient availability (abiotic) and herbivore damage (biotic) are two direct factors that can affect plant performance at the population scale.  Nutrients have direct positive effects on plants’ survival and reproductive output, and herbivores have direct negative effects by feeding on photosynthetic tissues, destroying the conducting tissues, and reducing viable seeds produced after pollination.  Indirect interactions also occur, stimulated by predators on the herbivores or by the herbivores in response to variation in nutrients availability. The dynamics and outcomes of such direct and indirect interactions currently are difficult to predict.  The first objective of this study was to examine experimentally the direct effects of nutrients, insect herbivores, and their interaction on performance and dynamics of Cirsium altissimum (tall thistle) along a soil moisture gradient within its tallgrass prairie ecosystem.  Plant performance was analyzed by measuring total plant biomass and reproductive effort (flower heads) for each plant.  The second objective was to examine the indirect effect of soil moisture and nitrogen availability on invertebrate predator (spider) abundance through the growing season and spider effects on plant performance. Tall thistle plants downslope (high soil moisture availability) have 2x more plant biomass compared to the upslope tall thistle plants (low moisture availability). As expected, nitrogen addition stimulated plant growth; tall thistle in the N+ treatment has 1.5x more plant biomass than tall thistle in N- treatment.  Gradient position also influenced both overall spider abundance and herbivore load.  This study provides new evidence that both resources (nutrient availability and moisture) and topdown interactions (spider density) can interact to effect native plant performance.