Drivers of grassland invertebrate communities: Effects of soil nutrient availability on invertebrate resource limitation

Background/Question/Methods

The soil-plant-insect interface is a key place to test the interactive effects of nutrient availability and herbivory. Soil nutrient amendment can indirectly alter invertebrate communities by changing plant tissue quality and quantity. The stoichiometric mismatch between plants and herbivores can constrain invertebrate herbivore growth and feeding rates. Invertebrate herbivores may control their own level of resource limitation by altering their feeding strategies between areas with high and low quality food, feeding selectively on high quality plant in areas with high resource availability, but switching to compensatory feeding (consuming higher quantities of low quality food) in areas with low resource availability. Here we examined whether (1) invertebrate abundance and community structure is affected by soil nutrient additions and (2) whether invertebrate feeding strategies (compensatory vs. selective) switch with soil nutrient additions. Invertebrate communities were characterized within a long-term chronic nutrient (N and P) addition experiment at three sites spanning the broad precipitation gradient of the US Central Great Plains. Additionally, damage by chewing herbivores was recorded for fourteen plant species to assess herbivore feeding rates.

Results/Conclusions

Overall, the invertebrate community varied across the precipitation gradient, with greater abundances of herbivores (e.g., leaf hoppers, aphids, grasshoppers, and thrips) and parasitoids (e.g., eulophid wasps) at the shortgrass steppe (SGS) site than the mixed-grass (MIX) and tallgrass prairie (TGP) sites. Nutrient additions, indirectly increased invertebrate herbivore, parasitoid, and predator abundances by increasing ANPP and decreasing plant C:N. Nutrient additions also impacted herbivore nutrient limitation. At MIX and TGP, the decrease in leaf tissue C:N with N addition resulted in an increase in chewing herbivore abundance, but no change in leaf tissue removed. As a result, herbivores at these sites fed more selectively (lower per capita rate of herbivory) with N additions. In contrast, N additions did not affect the number of chewing herbivores at SGS, but did increase leaf tissue removal. Thus, at SGS per capita rates of herbivory increased with N additions, indicating that chewing herbivores were feeding in a compensatory manner. This may be because herbivores are limited by another resource in this system, where plant C:N is naturally low. Ultimately, invertebrate resource limitation appears to vary across the broad precipitation gradient of the US Central Great Plains, thus altering invertebrate herbivore responses to chronic nutrient additions.