Indirect mutualism and commensalism involve a consumer-resource interaction coupled with either exploitative (Figure 1d) or interference (Figure 1e) competition. For instance, starfish and snails reduce the abundance of mussels, a dominant space occupier, and increase the abundance of inferior sessile species. The presence of grazers on oyster farms in Australia increases oyster recruitment by removing algae, who otherwise preempt the available spaces. In Figure 1d, an increase in species 1 should lead to a decrease in species 2 and an increase in species 3. The latter positive effect would propagate up the right branch of the diagram, increasing the abundances of species 4 and 5. This situation arises when, for example, planktivorous fish preferentially feeding on large zooplankton indirectly increase the abundance of small zooplankton. Cases involving interference competition are well known from, for example, the intertidal environment, where birds increase the abundance of acorn barnacles by consuming limpets that otherwise dislodge the young barnacles off the rock.
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