Root herbivory in grasslands and savannas: the potential role of June beetle (Phyllophaga spp.; Scarabaeidae) larvae in central Texas plant community structure
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Aboveground herbivores play an important role in plant community processes; however the role of belowground herbivory has received little attention. This is a study on root herbivory by June beetle (Phyllophaga spp.; Scarabaeidae) larvae in grasslands and savannas in central Texas. I begin by presenting a descriptive survey of Phyllophaga spp. larvae in central Texas grassland and savanna sites (Chapter 1). Larval density was generally low and patchy within and among sites. Furthermore, I found that larval abundance was positively associated with forb vegetation and negatively associated with grasses. Stable carbon isotope analysis revealed that larvae selectively fed on forbs over grasses at one site, but larvae exhibited no preferential feeding at the other sites. I then describe a greenhouse study that investigated the feeding preference of Phyllophaga crinita, the most common Phyllophaga species in Texas, for co-occurring plant species (Chapter 2). Specifically, I observed larval feeding in a two-choice feeding environment that neighbored individuals of the native bunchgrass, Schizachyrium scoparium (little bluestem grass) with individuals of the exotic grass, Bothriochloa ischaemum (King Ranch bluestem), and with individual seedlings of the native woody species, Prosopis glandulosa (honey mesquite), and Juniperus ashei (Ashe juniper). Phyllophaga larvae preferred Schizachyrium over both woody species, but larvae showed no distinction between the two grass species. Schizachyrium size was also negatively related to the extent that it was preferred over its neighbor. Lastly, I present the findings of a garden study that examined the effect of root herbivory by Phyllophaga larvae on the competitive interaction between Schizachyrium and Prosopis seedlings (Chapter 3). In doing so, I conducted separate experiments in successive years that varied the density of larvae and Schizachyrium competitors in pots. There was no evidence that root herbivory altered the ability of Schizachyrium to competitively suppress Prosopis growth. Instead, at high density Phyllophaga larvae had a strong direct negative effect on both Prosopis and Schizachyrium performance. At lower densities, however, these herbivores caused similar moderate reductions in both plant species, and as a result Prosopis performance was primarily impacted by the strong competitive effect of Schizachyrium.