Reduced precipitation dissolves apparent competition in a biocontrol system

Background/Question/Methods

While climate change may affect species directly, growing evidence suggests that indirect effects arising from altered species interactions may greatly influence the structure and function of ecological communities. We examined indirect effects of climate change in a Wisconsin, USA, agroecosystem consisting of alfalfa, two alfalfa pests (pea aphids and spotted aphids), and the aphidophagous Asian lady beetle. Asian lady beetles are highly mobile and preferentially seek fields with high aphid abundance. Although pea aphids can be abundant in alfalfa, previous research showed that reduced alfalfa water content significantly reduces pea aphid densities. Spotted aphids are generally less common than pea aphids, and their density is unaffected by alfalfa water stress. Therefore, we predicted apparent competition between pea aphids and spotted aphids under ambient conditions (i.e., high densities of pea aphids would attract predators that would consume both aphid species) but that this interaction would be reduced under drought conditions (i.e., low pea aphid densities would fail to attract predators). We tested this prediction in 1 m³ field enclosures using a 3x2 factorial experiment, crossing species (pea aphid, spotted aphid, or both) and precipitation treatments (ambient or reduced). 

Results/Conclusions

When predators were excluded, aphids in all treatments had positive growth rates. Consistent with previous studies, pea aphid growth rate in ambient conditions was approximately double that of pea aphids in reduced precipitation treatments, and spotted aphids had similar growth rates across both treatments. After 10 days, enclosures were opened allowing immigration of predators. Predators were slow to colonize spotted aphid only treatments, and consequently spotted aphid abundance remained high for the duration of the experiment (20 days). In contrast, predators were quick to colonize the ambient-precipitation treatments containing high densities of pea aphids (pea aphids only and both species together), with predator densities almost three times higher than reduced precipitation treatments. As a result, predators reduced the abundances of both pea aphids and spotted aphids in the ambient-precipitation/both-species treatments. However, in the reduced-precipitation/both-species treatments, spotted aphid density remained almost four times higher than the ambient-precipitation treatment because the low abundance of pea aphids did not attract predators. These results demonstrate the ability of the altered precipitation patterns to have wide-reaching, indirect effects within a community by altering interactions among species.