This study addresses four hypotheses that may account for differences in the number of insect herbivore species among plant species. These hypotheses are based on the assumption that insect diversity is a function of the number, quantity, and distribution of plant resources used by herbivores. The study investigated predictions that herbivore species richness will increase as a function of increasing the following: (1) host plant distribution over the landscape (host plant geographical distribution hypothesis), (2) host plant density within a habitat (resource concentration hypothesis), (3) size of individual plants (plant size hypothesis), or (4) abundance of resources (resource abundance hypothesis). We tested predictions from these hypotheses by examining the species richness of insect herbivores on five sympatric species of fabaceous plants that varied in their local dispersion of individual plants and plant architecture. Among these five species, plant geographical distribution varied threefold, density varied 38-fold, plant size and food resources available to insect herbivores varied ≈100-fold. Plant geographical distribution, plant size, and the resource concentration hypotheses were not corroborated in this study. Resource abundance, measured as plant dry weight, accounted for the differences in number and abundance of insect species between host plant species. Leaf biomass accounted for 44.15% of the variation in number of insect herbivore species and 51.76% of the variation in abundance of insect herbivores using leaves as resources. Flower biomass accounted for 32.86% of the variation in the number of insect herbivore species using flowers. Therefore, plants that offered a greater quantity of resources, especially leaves and flowers, had greater species richness and abundance in insect herbivores.
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