Predicting Insect Herbivore Distribution Using Climatic and Biotic Variables

A central goal of community ecology is understanding species distribution in nature. The abundance of insect herbivores can vary greatly spatially and temporally. These variations can be explained by abiotic variables such as precipitation, they can also be explained by biotic variables such as nutrient availability. Here I present two field observational and two laboratory experimental studies to better understand factors and mechanisms affecting insect herbivore distribution in nature. I examined 1) the climatic and biotic factors influencing the spatial and temporal variation in the density of a dominant salt marsh katydid. I found that katydid density is highly dependent on land elevation, and the availability of plant biomass for both dietary and reproductive purposes. 2) the effect of precipitation on plant and orthopteran community composition and functional traits. I failed to find a generalized trend describing the relationship between spring precipitation and orthopteran community composition or functional traits. However, various Orthopteran species responded differently to spring precipitation even if they are in the same subfamily. This suggests there is no generalized trend for orthopteran species and merits studying individual species. I also examined 3) biotic factors that determine orthopteran dietary choices by quantifying functional traits in a feeding choice experiment. I also found no single generalized trend predicting the relationship between plant toughness and nitrogen and grasshopper mandibular traits or dietary preference. But I did find some trends on a subfamily and species level. Finally, I performed 4) a feeding experiment where I examined elevated dietary protein and sodium as a mechanism driving insect herbivore distribution in nature. And as expected, elevated protein and sodium improved offspring production and both nutrients worked synergistically to improve the earlier stages of growth.