Insect outbreak and resource pulses in understory plants and soil

Background/Question/Methods

Island ecosystems are undergoing shifts in ecosystem resilience due to habitat conversion and invasive species. In particular, perturbations that lead to resource pulses may be particularly beneficial to invasive species.  Various hypotheses suggest that invaders are more able to take advantage of abundant resources, although these have not been explicitly tested in a resource pulse context. A massive outbreak of an endemic caterpillar (Scotorythra paludicola) on Hawaiʻi Island has led to large scale defoliation of native Acacia koa forests and large quantities of frass deposited on the forest floor.  The location and size of the defoliated area is unprecedented historically, and the outbreak is likely to impact forest communities and food webs as well as ecosystem structure, composition, and function.  We studied the resulting soil nitrogen (N) dynamics and native and exotic understory plant N uptake during and after this event at Hakalau Forest National Wildlife Refuge in Hawaii.   

Results/Conclusions

Although Acacia is an N-fixer, available soil N did not initially differ under Acacia compared to adjacent open areas.  During the defoliation event an estimated 4 to 11 kg/m² of nitrogen fell as frass, leading to a distinct pulse in soil N under Acacia canopies at two of our three sites approximately 1 month after peak caterpillar biomass.  Soil N reached 2-3 times ambient levels and did not return to pre-defoliation levels even after 8 months of sampling.  The variability in N pulse effect between sites was not related to estimated differences in frass deposition. This suggests that other ecosystem variables varying across sites controlled resilience to perturbation, such as understory plant diversity or biomass, degree of invasion by exotic grasses, or differing soil properties.