Soil animal community responses to grasshopper defoliation under severe drought

Background/Question/Methods

Above- and belowground components of semi-arid grasslands can interact through shoot herbivory. Plant physiological responses to insect defoliation are often the primary linkage to soil biota at small spatiotemporal scales. Grasshopper grazing may enhance root exudation or alter the quantity or quality of root tissue, with direct impacts on soil microbes and herbivores, and knock-on effects to higher trophic levels. However, severe droughts can induce dormancy in host-plants, potentially limiting any root or soil biotic responses to defoliation.

We investigated the effects of grasshopper herbivory on the temporal patterns of soil nematode and microarthropod communities during an extreme drought in the shortgrass steppe of northern Colorado. Beginning mid-August 2012, 50 x 50 cm Bouteloua gracilis grass plots received either no grasshoppers or mixed species assemblages of 4 grasshoppers for 4 wk. Soil animal communities were sampled (two 6.35 x 10 cm soil cores per plot) at the start and end of the defoliation period and enumerated by trophic group. 

Results/Conclusions

Short-term grasshopper herbivory did not significantly affect the total density (ANOVA F_1,6=2.098, p=0.198), trophic group distribution (PerMANOVA pseudo F_1,6=1.283, p=0.312), or Simpson’s diversity of feeding groups (ANOVA F_1,6=0.180, p=0.686) of nematodes. Temporal variations of nematode communities also were not significant (all tests p>0.05). Microarthropods were not studied further. This study was conducted during the most severe drought in the shortgrass steppe of the last 48 years (annual precipitation: 206 mm (2012) vs. 320 mm (long-term average)). Therefore, our results add to the body of research on aboveground-belowground interactions and suggest that prolonged droughts may diminish soil food web responses to insect defoliation.