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Bottom‐up effects of plant genotype on aphids, ants, and predators

Johnson, Marc T. J.
Ecology 2008 v.89 no.1 pp. 145-154
plant genetics, phytophagous insects, Aphis (Aphididae), plant-insect relations, regression analysis, field experimentation, Oenothera biennis, mutualism, genotype, population density, population growth, Crematogaster, predator-prey relationships, Ontario
Theory predicts that bottom‐up ecological forces can affect community dynamics, but whether this extends to the effects of heritable plant variation on tritrophic communities is poorly understood. In a field experiment, I contrasted the effects of plant genotype (28 genotypes; 1064 plants), aphid density, and the presence/absence of mutualistic ants in affecting the per capita population growth of a specialist aphid herbivore, as well as the effects of plant genotype on the third trophic level. Plant genotype strongly affected aphid population growth rate, explaining 29% of the total variation in growth rate, whereas aphid density and ant–aphid interactions explained substantially less variation (<2%) in aphid population growth rate. Plant genotype also had direct and indirect effects on the third trophic level, affecting the abundance of aphid‐tending ants and the richness of predators. Multiple regression identified several heritable plant traits that explained 49% of the variation in aphid growth rate and 30% of the variation in ant abundance among plant genotypes. These bottom‐up effects of plant genotype on tritrophic interactions were independent of the effects of either initial aphid density or the presence/absence of mutualistic ants. This study shows that plant genotype can be one of the most important ecological factors shaping tritrophic communities.