Insects in Variable Plant Patches
Merwin, Andrew Charles (author)
Underwood, Nora C. (professor co-directing dissertation)
Inouye, Brian D. (professor co-directing dissertation)
Cogan, Nicholas G. (university representative)
Winn, Alice A. (committee member)
Burgess, Scott C (committee member)
Travis, Joseph, 1953- (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Biological Science (degree granting department)
Animals move through landscapes where their resources are unevenly and often patchily distributed. When animals move and choose among their scattered resources in predictable ways, ecologists may be able to anticipate the spatial distribution of their populations and the relative strength of their trophic interactions (e.g. predation, or facilitation). Likewise, an understanding of animal movements can inspire the design and preservation of habitat for conservation or the promotion of ecosystem services. However, movement-based predictions of animal populations—and the human interventions these predictions inspire—are only as reliable as our understanding of the determinants of animal movement. The research presented here addresses three basic aspects of animals' environments and experiences that have the potential to influence our understanding of animal movement, population distributions, and ecological interactions: (1) animals' conspecific density (2) prior experience with resources, and (3) the composition and geometry of habitat patches. These topics are well-studied, but rarely in the context of spatially heterogeneous landscapes, and many prior studies have confounded important explanatory variables. For the first study, I performed a lab experiment using the bean beetle, Callosobruchus maculatus, and two of its host beans, Vigna unguiculata and V. radiata, to explore how consumer density influences resource choice and the relative distribution of damage among resources in a patch. My results demonstrated that the damage a focal resource type receives can depend on the frequency of neighboring resource types, and that this frequency dependence decreases with regional consumer density. These findings illustrate the importance of consumer density in mediating indirect effects among resources, and suggest that accounting for consumer density may improve our use of mixed‐crop pest management strategies. For the second study, I used a field experiment to test whether prior hostplant experience influences the distribution of offspring on and damage to hostplants among and within plant patches that varied in hostplant density and composition. Specifically, I reared diamondback moths, Plutella xylostella, on either collard or mustard plants (Brassica oleracea or B. juncea, respectively) and recorded the number of offspring on and damage to plants in three patch types within large field cages: two collards, four collards, and mixed patches of two collards and two mustards. I found that in cages with collard-reared moths, there were more offspring and damage per plant in four-collard patches than in two-collard patches, while mustard-reared moths did not respond to collard density. In contrast, I found no effect of natal hostplant experience on hostplant choice within mixed patches, and no influence of mustard plants on attacks on collards in mixed patches versus two-collard patches (i.e. there were no associational effects). These findings suggest that accounting for prior hostplant experience may improve our understanding of how some herbivores and their damage are distributed in patchy environments through time. For my final dissertation study, I used a correlated random walk model to make predictions for animal density in patches that vary in area and perimeter-to-area ratio. I then tested predictions from this model by manipulating the area and perimeter-to-area ratios of plant patches and observing the densities of two predaceous beetles: a relative habitat specialist, Calosoma sayi, and a relative habitat transient, Tetracha carolina carolina. The model predicted that as habitat specialists spend more time in patches relative to non-habitat, patches with lower perimeter-to-area ratios should have higher animal density. However, for relatively transient species, defined as spending more time within non-habitat than in the habitat patches, the model predicted slightly higher densities in higher perimeter-to-area patches or no difference between patch types. Area per se, in contrast, did not influence mean density. Contrary to the model, I found that in the field experiment patch area and perimeter-to-area ratio interacted to influence the movement and density of the relative habitat specialist, C. sayi. Their density increased significantly with patch size in high perimeter-to-area patches, but patch size had no influence on C. sayi's density within low edge-to-area patches. By contrast, densities of T. carolina were slightly higher in high perimeter-to-area ratio patches once the influence of plant height was accounted for, which was consistent with the model. These results underscore the importance of considering both patch area and perimeter-to-area ratio as well as species-specific movement behaviors for the management of habitat for ecosystem services. Together these projects highlight exciting new areas of consideration for the study of plant-insect interactions in heterogeneous habitats, which could improve our ability to predict insect distributions and interactions in natural and managed populations.
associational effect, natal habitat preference induction, plant-insect interactions, resource concentration
August 27, 2018.
A Dissertation submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Nora Underwood, Professor Co-Directing Dissertation; Brian Inouye, Professor Co-Directing Dissertation; Nick Cogan, University Representative; Alice Winn, Committee Member; Scott Burgess, Committee Member; Joseph Travis, Committee Member.
Florida State University