Date of Award

Spring 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Entomology

First Advisor

Ian Kaplan

Committee Chair

Ian Kaplan

Committee Member 1

Clifford Sadof

Committee Member 2

Douglas Richmond

Committee Member 3

Kevin Gibson

Abstract

Weed pressure is the most costly challenge that vegetable growers face, requiring more labor investment than other production inputs. Vertebrate and invertebrate seed predators destroy a large percentage of weed propagules on the soil surface, and their ecosystem services may ease labor requirements for farmers in herbicide-free systems. Cover provided by living vegetation is an important predictor of seed predator activity, and my dissertation takes a comprehensive approach to understanding the behavior, predator, and environment-mediated mechanisms by which cover impacts weed seed predation in crop environments. ^ First, I performed a meta-analysis of 27 studies to quantitatively evaluate what is currently known about seed predation by vertebrates and invertebrates across weed species, crop environments, and seasons (Chapter 1). I found that that seed predators impact some weed species more than others, depending on taxa-specific seed preferences, and that predation rates are minimal in environments entirely void of vegetation. Next, in Chapter 2, I examined the role of vegetative over in determining oviposition preferences of Harpalus pensylvanicus , the most common carabid seed predator in Midwestern crop systems. I found that while adult beetles were strong dispersers and foraged in a variety of habitat types, larvae were less mobile and more vulnerable to disturbance than adults. They were almost exclusively captured in environments characterized by long disturbance intervals and abundant living biomass, emphasizing the importance of cover as perennial refuge for maintaining stable populations of natural enemies. ^ In Chapter 3, I examined omnivorous predator assembly around basic biological resources (cover, seeds, and prey). I found that both predaceous and omnivorous carabid species aggregated in patches of vegetative cover and omnivores assembled in seed patches. None, however responded to prey availability. Using food-specific protein markers, I found that cover doubled the likelihood of detecting seed material, but not prey material in beetles’ digestive tracts. This implies that omnivorous carabids are competent biological control agents of weed seeds, and that provisions of plant cover will not only attract more seed predators, but also induce their seed-feeding behavior. Even though cover directly facilitates seed-feeding, it may also increase the likelihood of intraguild predation on invertebrates by small mammals, as both taxa utilize the same refuge environments. In chapter 4, I quantified the cascading effects of behavior mediated predator-prey interactions over four trophic levels. I found that use of cover by small mammals avoiding predation risk by nocturnal avian predators reduced the activity of carabid seed predators by 50%, but the net effect of small mammals on seed removal was neutral. ^ Finally, in chapter 5, I directly evaluated the utility of seed predation by measuring the effects of seed predators on weed emergence. I simulated seed rain of common lambsquarters, and found that seed predators overcame intense propagule pressure and reduced the germinable seedbank. I found 38% fewer seedlings in seed-augmented plots where seed predators had access, compared to plots where they were excluded. Minimal differences between differential exclusion of vertebrate and invertebrate seed predators suggest that the effect of vertebrates on seed predation is neutral, corroborating evidence from chapter 4. ^ Together, these five chapters enumerate multiple interacting drivers of tropic cascades, with insights of both basic and applied importance. I learned that predator avoidance and intraguild predation interact and simultaneously shape trophic ecology, with distant downstream implications. Because each process is common in nature, it is important to integrate both in future predictions of trophic dynamics. Provisions of vegetative cover can promote weed biological control by both attracting more seed predators and facilitating per-capita seed consumption. While vegetation may also facilitate intraguild predator events, these effects are minimal compared to the strong positive effects of cover on seed predation overall. Thus, cover crops and forage crop rotations can be powerful tools to promote weed biological control, among the numerous other benefits they provide.

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