The indirect effect of above-ground herbivory on collembola populations is not mediated by changes in soil water content

Above-ground aphid herbivory of a shared host plant results in increased collembola populations within the rhizosphere. Three mechanisms potentially underpin this effect: honeydew deposition, aphid-induced reduction in root biomass and altered soil water content (as a result of root reduction inhibiting plant water uptake). This study focuses on the third mechanism, altered soil water content. This has the potential to influence collembola populations as, like most soil invertebrates, these organisms are highly susceptible to humidity levels. The indirect effects of leaf chewers (grasshoppers), phloem feeders (aphids) and manual (artificial) defoliation on soil water content, and hence on collembola population abundance, were compared. The different defoliation treatments significantly affected root biomass but not soil water content, and only aphid herbivory increased collembola abundance. Altered soil water content is unlikely to be the mechanism responsible for the increased collembola populations. The study demonstrated a strong negative relationship between soil water content and collembola abundance, confirming that soil moisture is an important factor in determining where collembola are found within the soil. The results also suggest that collembola have a higher tolerance for dry than for wet conditions.