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Carnivore induced density- and trait- mediated indirect effects cause changes in plant resistance to herbivores.
Griffin, Celine*,1, Thaler, Jennifer1, 1 Department of Botany, Toronto, Ontario
ABSTRACT- Indirect effects are interactions between two species that occur due to the intervention of one or more species. These effects are determined both by changes in density and traits of the intervening species, such as behaviour, morphology and physiology. Such indirect effects are known to be important in many ecological systems. Their influence on plant resistance to herbivores, however, has never been tested. I present results demonstrating that the presence of the carnivore Podisus maculiventris caused changes both in the density and behaviour of the herbivore Manduca sexta, and these density- and trait-mediated indirect effects result in changes in resistance in Solanum ptychanthum. In the field, I tested the indirect effects of carnivores on plants using five treatments: Control, Herbivore only, Lethal carnivore and herbivore, Herbivore density control (herbivores were artificially removed at a rate to match removal on the Lethal treatment), and Non-lethal carnivore and herbivore (carnivores had their beaks snipped). In the field, M. sexta larvae feeding on S. ptychanthum in the presence of lethal P. maculiventris caused 18% less damage relative to control plants (Herbivore density control), whereas the presence of non-lethal carnivores caused a 39% decrease in damage relative to controls (Herbivore only). Consequently, both lethal and non-lethal carnivores indirectly caused significant decreases in the level of herbivore damage. This affected resistance to subsequent herbivores feeding on the plant, as seen by M. sexta lab performance bioassays. Furthermore, assays of several defensive proteins revealed that carnivore presence indirectly suppressed plant resistance. In the field, both lethal and non-lethal carnivores caused density changes in herbivores, but lethal P. maculiventris caused much higher mortality than did non-lethal carnivores. Behavioural observations in the lab showed that the presence of a carnivore (either lethal or non-lethal) caused M. sexta to feed less and spend more time in defensive postures. Overall, the results from this experiment indicate that both density- and trait-mediated indirect effects are important for plant resistance.
Key words: trait-mediated indirect effects, tritrophic interactions, density-mediated indirect effects, plant resistance