The influences of predation and resource subdivision on the coexistence of goldenrod aphids

Two abundant, widely co-occurring aphid species exposed to natural or reduced predation of goldenrod transplanted as either solitary stems or small, discrete patches of plants. Local interplant movement increased colonization rates for both species, resulting in more larger aphid colonies on patch plants than on solitary plants. Predator reduction increased aphid densities and produced evidence of increased resource limitation and interspecific competition, although the specific response differed between plant spacing treatments. On solitary plants, predator reduction resulted in a significant negative spatial association between the aphid species. Predator reduction in patches resulted in an increase in one species but a reduction in colony size of the other. In all cases, aphid abundance was negatively associated with plant health. Patterns of spatial association between the aphid species varied with experimental treatments. Aphids on solitary plants at reduced predation, aphids on patch plants showed no overall patterns of association. With natural predation, aphids on patch plants were positively associated, as were aphids within natural goldenrod patches in the field. The results show that predation and local interplant movement determine patterns of spatial overlap and mediate and impact of interspecific competition. Although resource limitation can influence aphid dynamics on a single goldenrod shoot, predation and interplant movements are considered to have a greater influence on community structure at a large spatial scale. These results are discussed in terms of several recent models of predator-mediated coexistence of a subdivided resource.