The second major class of interactions, indirect effects, can be defined as the impact of one organism or species on another that is mediated or transmitted by a third. In other words, A (donor) has an effect on B (transmitter), which then affects C (recipient). There are two main ways by which an indirect effect can occur (Wootton 1993, 1994). The first is known as an interaction chain, in which a donor species affect the abundance of a transmitter and has an effect on a recipient. When a bird species (donor) begins to aggressively prey upon a specific caterpillar (transmitter), the reduction in herbivory by the caterpillars can lead to increased growth or numbers of the plants the caterpillar was consuming (recipient) (Figure 3).