Nutrient dilution and climate cycles underlie declines in a dominant insect herbivore

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Welti, Ellen A.R., Roeder, Karl A., De Beurs, Kirsten M., Joern, Anthony, Kaspari, Michael
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Acrididae; Global change; Grasshopper; Grassland; Insect decline; Pacific; North Atlantic; North Atlantic oscillation
• Online Access: https://research.wur.nl/en/publications/nutrient-dilution-and-climate-cycles-underlie-declines-in-a-domin; https://doi.org/10.1073/pnas.1920012117

Evidence for global insect declines mounts, increasing our need to understand underlying mechanisms. We test the nutrient dilution (ND) hypothesis-the decreasing concentration of essential dietary minerals with increasing plant productivity-that particularly targets insect herbivores. Nutrient dilution can result from increased plant biomass due to climate or CO2 enrichment. Additionally, when considering long-term trends driven by climate, one must account for large-scale oscillations including El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO). We combine long-term datasets of grasshopper abundance, climate, plant biomass, and end-of-season foliar elemental content to examine potential drivers of abundance cycles and trends of this dominant herbivore. Annual grasshopper abundances in 16- and 22-y time series from a Kansas prairie revealed both 5-y cycles and declines of 2.1-2.7%/y. Climate cycle indices of spring ENSO, summer NAO, and winter or spring PDO accounted for 40-54% of the variation in grasshopper abundance, mediated by effects of weather and host plants. Consistent with ND, grass biomass doubled and foliar concentrations of N, P, K, and Na-nutrients which limit grasshopper abundance-declined over the same period. The decline in plant nutrients accounted for 25% of the variation in grasshopper abundance over two decades. Thus a warming, wetter, more CO2-enriched world will likely contribute to declines in insect herbivores by depleting nutrients from their already nutrient-poor diet. Unlike other potential drivers of insect declines-habitat loss, light and chemical pollution-ND may be widespread in remaining natural areas.