Scale-dependent indirect interactions between two prey species through a shared predator

We investigated the potential for indirect interactions between two prey species, pea aphids (Acyrthosiphon pisum) and potato leafhoppers (Empoasca fabae), through a shared predator (Nabis spp.), and how these interactions may change across three spatial scales. In greenhouse experiments using small clusters of plants containing pea aphids and/or potato leafhoppers, the predation rates on both pea aphids and potato leafhoppers were independent of the presence of the other species, indicating no indirect interactions. In greenhouse experiments using cages containing 48 plants, when aphids and leafhoppers were confined to separate plants among which nabids could move, pea aphids had a positive effect on the survival of potato leafhoppers from predation. The positive effect of aphids on leafhoppers occurred because nabids spent more time on plants harboring aphids, thereby drawing nabids away from plants containing leafhoppers. Finally, we measured the abundance of nabids in a large-scale experiment designed to manipulate the abundances of pea aphids and potato leafhoppers in alfalfa fields. Fields with high aphid density contained more nabids, thereby suggesting that pea aphids will have a negative indirect effect on potato leafhoppers by increasing the density of nabids within fields. Potato leafhoppers had no indirect effects on pea aphids at any scale. This study shows that indirect interactions between prey species may depend upon spatial scale, because the factors affecting a predator's diet choice on a small scale may differ from those factors affecting a predator's distribution at larger scales.