Abstract

WHEN studying interactions between trophic levels, ecologiste often restrict their attention to two levels as a means of simplifying the analysis; unfortunately, this simplification can be misleading if tritrophic interactions (such as plant-herbivore-predator) cannot be understood by simply adding together pairwise interactions (plant-herbivore plus herbivore-predator, for example)^1,2. We examined the significance of tritrophic interactions by asking how the morphology of the common pea (Pisum sativum) influences the population growth of pea aphids (Acyrthosiphon pisum) in the presence and in the absence of a third trophic level. We found significant interactions between the first trophic level (peas) and the third trophic level (ladybird beetles) in determining aphid population growth. Our results point out how simple genetic changes can yield morphological variants in plants that differ dramatically in their resistance to herbivores due to the effects of plant architecture on enemies of the herbivore.