Biology Theses and Dissertations

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CRITICAL PATCH SIZES AND THE SPATIAL STRUCTURE OF SALT MARSH COMMUNITIES
(2009) Martinson, Holly Marie; Fagan, William F; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The size, connectivity, and quality of habitat patches can have multifaceted impacts on species and communities. In this dissertation, I combined a multi-year field survey, manipulative field experiments, and a literature review to investigate how spatial structure influences species and their trophic interactions in fragmented habitats. For all empirical work, I used as a study system the arthropod assemblage found on patches of the salt marsh grass Spartina patens. In Chapter 1, I conducted seven surveys of habitat patches over three years to examine the effects of patch size, connectivity, and local environmental conditions on a guild of specialist sap-feeding herbivorous insects and their natural enemies. I found striking differences among species in the effects of both patch size and connectivity, which led to differences in species' relative abundances and trophic structure among these patches. In Chapter 2, I manipulated host plant quality and predator density to experimentally examine mechanisms that might structure this arthropod community. I found that positive responses of herbivores to experimentally-elevated patch quality were limited by dispersal constraints and that predation by abundant generalist spiders may constrain the spatial distribution of certain species. Investigating systems beyond the marsh, I conducted a literature review and analysis in Chapter 3 wherein I examined whether the spatial structure of habitats generally influences trophic interactions. From the literature, I identified 171 studies of trophic interactions in fragmented habitats and found that the influence of fragmentation and related variables on the occurrence or strength of trophic interactions was largely predictable based on the habitat affinity of interacting species. With this dataset, I also identified key gaps in the fragmentation literature, including a heavy bias towards the study of two-species interactions. Therefore, in Chapter 4 I took advantage of my data from the salt marsh to identify how, in addition to the two-species interactions of parasitism and egg predation, more complex food web interactions might depend on variation in the size of habitat patches. Overall, my findings show that variation in patch size can have varied, but predictable, effects on patch occupancy, population density, and interactions between species in fragmented habitats.
MATING COSTS, MALE CHOICE DISPLACEMENT, AND THE EFFECTS ON HYBRIDIZATION AND SPECIATION IN THE HAWAIIAN CRICKET LAUPALA (SUBFAMILY:TRIGONIDIINAE)
(2009) Jadin, Jenna; Shaw, Kerry L; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Contact zones between two closely-related species provide unique laboratories for studying the processes of speciation. This is because, within these zones, species barriers will be reinforced and speciation will reach completion, or the barriers will break down, causing the two species to become one. Which of these two alternatives will occur depends on the degree of genetic differentiation and behavioral isolation between the species. If there is significant and non-combinable genetic variation between species, but behavioral isolation between the two incipient taxa is incomplete and allows hybrid offspring to be produced, these hybrid offspring will have lower fitness relative to parental types and selection should act directly to eliminate those offspring and indirectly against parents with broad mating preferences or traits. If however the genetic architecture is similar and behavioral isolation is incomplete, the populations would be expected to turn into a hybrid swarm and eventually become one species. Patterns of behavioral isolation and genetic variation in several Laupala species pairs suggest that contact zones between closely related species are marked by conflicting patterns of behavioral isolation and genetic differentiation. Evidence also suggests that the complex courtship system of Laupala may allow male choice to play an important role in sexual selection and speciation. Therefore I tested several hypotheses about the genetic differentiation, sexual selection, and behavioral isolation in a contact zone between the closely-related and morphologically indistinguishable L. tantalus and L. pacifica species pair. First, by using the mitochondrial COI gene and AFLPs as genetic markers, I demonstrated that there appears to be mitochondrial DNA introgression between sympatric, but not allopatric congeners, which suggests contemporary hybridization in the contact zone. Next, I found that males experience post-mating resource-limitation and show a significant tendency to invest less into a second mating, however, their investment is dependent upon female size. Finally, I found that there is apparent displacement of male choice, decreased variation in spermatophore production, and asymmetrical mating isolation within the contact zone. This evidence all suggests that there is increased behavioral isolation in this contact zone, which may be consistent with a hypothesis of speciation by reinforcement. However, this evidence also suggests that male costs may result in male choice conflicting with other isolating mechanisms. If so, this study may be another putative case of reinforcement, or it may be an entirely novel report of conflicting selection pressures within a hybrid zone. I suggest that further studies are needed to measure hybrid fitness as well as to evaluate relative male and female mating costs within the complex mating system of this rapidly-diversifying genus.
Predation by eastern mudminnows (Umbra pygmaea) on macroinvertebrates of temporary wetlands
(2009) Lombardi, Susan Elizabeth; Lamp, William O.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Fish play a substantial role in aquatic food webs, yet the effect of feeding activities of small stream fish that enter seasonally-flooded temporary wetlands during periods of hydrologic connectivity is not well understood. In this study, eastern mudminnows (Umbra pygmaea) were introduced to a fishless wetland in Caroline County, Maryland, and the aquatic macroinvertebrate community did not significantly change within two weeks. Gut contents of mudminnows collected from the wetland and a stream consisted primarily of dipteran larvae; ostracods were also a common food source for wetland mudminnows. Common prey not found in gut contents but present in the wetland were tested as food, and all taxa were consumed in a no-choice predation experiment. Mudminnows have the potential to directly affect multiple trophic levels and subsequent ecosystem functioning through predatory interactions with sustained hydrologic connectivity between fish sources and temporary wetlands.
ARTHROPOD COMMUNITIES IN RIPARIAN GRASS BUFFERS AND ADJACENT CROPS
(2008-05-31) Nelson, Jessica Lynn; Dively, Galen; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Riparian buffers composed of either native warm season or non-native cool season grasses are commonly planted alongside crop fields. Although their water quality function is well documented, few studies have examined grass buffers as habitat for arthropod communities. The aerial and epigeal arthropods were surveyed using pitfall traps and sticky cards to assess the effects of both grass types on community structure in the buffer and adjacent crop. I predicted that warm season grasses would provide favorable habitat for more diverse and abundant arthropod populations, particularly natural enemies. The weight of evidence did not support my prediction and suggests that cool season grass buffers provide equivalent, if not better habitat for arthropods than warm season grasses. Coupled with higher food quality, cool season species green-up much earlier than warm season grasses in the spring and provide food resources for many herbivores and natural enemies.
Geography and Genetics of Ecological Speciation in Pea Aphids
(2008-01-23) West, Joan Alette; Hawthorne, David J; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
During ecological speciation, divergent natural selection drives evolution of ecological specialization and genetic differentiation of populations on alternate environments. Populations diverging onto the same alternate environments may be geographically widespread, so that divergence may occur at an array of locations simultaneously. Spatial variation in the process of divergence may produce a pattern of differences in divergence among locations called the Geographic Mosaic of Divergence. Diverging populations may vary in their degree of genetic differentiation and ecological specialization among locations. My dissertation examines the pattern and evolutionary processes of divergence in pea aphids (Acyrthosiphon pisum) on alfalfa (Medicago sativa) and clover (Trifolium pretense). In Chapter One, I examined differences among North American aphid populations in genetic differentiation at nuclear, sequence-based markers and in ecological specialization, measured as aphid fecundity on each host plant. In the East, aphids showed high host-plant associated ecological specialization and high genetic differentiation. In the West, aphids from clover were genetically indistinguishable from aphids on alfalfa, and aphids from clover were less specialized. Thus, the pattern of divergence differed among locations, suggesting a Geographic Mosaic of Divergence. In Chapter Two, I examined genomic heterogeneity in divergence in aphids on alfalfa and clover across North America using amplified fragment length polymorphisms (AFLPs). The degree of genetic differentiation varied greatly among markers, suggesting that divergent natural selection drives aphid divergence in all geographic locations. Three of the same genetic markers were identified as evolving under divergent selection in the eastern and western regions, and additional divergent markers were identified in the East. In Chapter Three, I investigated population structure of aphids in North America, France, and Sweden using AFLPs. Aphids on the same host plant were genetically similar across many parts of their range, so the evolution of host plant specialization does not appear to have occurred independently in every location. While aphids on alfalfa and clover were genetically differentiated in most locations, aphids from alfalfa and clover were genetically similar in both western North America and Sweden. High gene flow from alfalfa onto clover may constrain divergence in these locations.
Terrestrial-aquatic linkages in human-altered landscapes
(2007-01-18) Menninger, Holly Lynn; Palmer, Margaret A; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Streams and adjacent riparian zones are intimately linked by the flow of resource subsidies between terrestrial and aquatic habitats. Landscape-level changes in land use can have profound impacts on riparian structure and stream health, and may alter the flow of resource subsidies across the stream-riparian boundary. Yet, terrestrial-aquatic linkages have not been well-studied in human-impacted landscapes. Here, I examine energy flows across the stream-riparian boundary in agricultural and suburban landscapes in Maryland. I study the effects of terrestrial resource subsidies (grass and herbaceous vegetation, periodical cicada detritus) on stream ecosystem processes and consumers and the effects of one aquatic subsidy (emerging aquatic insects) on agriculturally important consumers, wolf spiders (Lycosidae). I present strong evidence for terrestrial-aquatic linkages where large quantities of high quality, allochthonous resources subsidize stream ecosystems. Herbaceous vegetation and grasses growing along the edges of agricultural headwater streams provide significant quantities of organic matter that are rapidly decomposed and support a diverse macroinvertebrate community. Further, the dense vegetation appears to limit light to algae growing on the stream bottom. Detritus from 17-year periodical cicadas (Magicicada spp.) that falls into forested suburban streams provides an intense pulse of terrestrial resources that is unusual for the summer, but is locally utilized and causes dramatic increases in whole-stream community respiration. I provide weak evidence for a terrestrial-aquatic linkage between emerging aquatic insects and lycosid wolf spiders inhabiting agro-ecosystems in central Maryland. Results from field studies indicate that wolf spiders are generally more abundant in the riparian buffers adjacent to corn fields, and exhibit neither alternating abundance nor net movement between the field and buffer habitats throughout the year. While wolf spiders consume adult aquatic insects in the lab, I could not resolve the specific contribution aquatic insects make to the diets of field-collected wolf spiders. Terrestrial-aquatic linkages are important in human-altered ecosystems, and have significant implications for the conservation and restoration of impacted habitats and ecosystem services. The relative strength of these linkages, however, depends on the species involved, the direction of the subsidy flux, the nature of the land-water boundary, and the temporal context in which they occur.
Trade-offs within and between sexual traits in stalk-eyed flies (Diptera: Diopsidae)
(2006-06-05) Fry, Catherine L; Wilkinson, Gerald; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The allocation of limited resources to competing body parts during development may affect both the absolute and relative sizes of physical traits, creating potentially dramatic consequences for the evolution of morphology. While negative correlations between the sizes of body parts ("trade-offs") arising from the distribution of finite resources have long been anticipated, empirical support is relatively rare. In this study, I use two related species of stalk-eyed flies that differ in morphology to investigate whether eye-span exaggeration results in trade-offs with other body parts. One species, Cyrtodiopsis dalmanni, is sexually dimorphic, with males having exaggerated eye-span while the other, C. quinqueguttata, is sexually monomorphic with both sexes having approximately equivalent and relatively unexaggerated eye-span. I utilize complementary approaches including artificial selection, application of exogenous juvenile hormone, and diet manipulation to alter the absolute and relative length of the eye-stalks in order to reveal correlated changes in other physical traits. The results of these experiments suggest that exaggerated eye-span in male C. dalmanni is accompanied by a decrease in two other features of head morphology, eye-bulb size and eye-stalk width, as well as compromised testis growth and sperm production. No trade-offs were observed in females of either species or male C. quinqueguttata. These results are consistent with hormonally-mediated trade-offs arising from the allocation of limited resources to an exaggerated trait and suggest that those trade-offs may act as costs of developing exaggerated eye-span.
Ecological Effects of the Biocontrol Insects, Larinus Planus and Rhinocyllus Conicus, on Native Thistles
(2005-05-11) Dodge, Gary Jonathan; Inouye, David W; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Biological control of invasive weeds is, by nature, a delicate balance between introducing effective biological control agents and not introducing another invasive species. A disconcertingly similar suite of traits is used to describe invasive insect species and to identify appropriate biological control agents (or candidates): good control agents and invasive exotic species are good dispersers, they are good colonizers, they have high reproduction rates, and they are suited for broad distribution. It shouldn't come as a surprise that two previously released weed biological control agents can now be characterized as invasive species. The purpose of this dissertation is twofold: first, it is to explore the ecological relationship between predispersal seed predation and plant population dynamics, and second, it is to elucidate the risks to native plants involved with introduction and redistribution of exotic species. Rhinocyllus conicus and Larinus planus are Eurasian seed-head weevils, introduced and redistributed broadly across the western U.S. to control exotic thistles. Exclusion experiments on native thistles, including one that is rare and imperiled, at sites in Colorado present strong evidence that a decrease in seed production due to herbivory by both R. conicus and L. planus has lead to a reduction in recruitment of the thistles. The density of seedlings in both cases, even in the excluded units, was far below where density dependent effects may play a role in the dynamics of the thistle. Further, a survey of eight western states demonstrates established populations of L. planus and effects on seed production in multiple native species in four states. It also documents the near ubiquity and broad diet breadth of R. conicus. The process for approval of phytophagous biocontrol agents has become more cautious and more efforts are made to prevent nontarget herbivory. Nevertheless, land managers still routinely redistribute previously approved, non-regulated agent insects that appear to pose a higher risk to the native flora. The results of this research will benefit resource managers who wish to consider use of phytophagous insects as biological control agents as well as help ecologists and environmental managers understand the risk probabilities of biological control applications.
Use of Multiple Cues for Navigation by the Leaf-cutter Ant Atta cephalotes.
(2005-04-13) Vick, Kyle; Jeka, John; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In the first chapter, there is a brief introduction to ant navigation and a review of previous literature as well as a summary chapters 2-7. In chapter 2, I examine orientation of Atta cephalotes workers in the laboratory. Laden nest-bound foragers were moved from a "bridge" with or without trail pheromone present and placed on a parallel bridge with or without pheromone. In chapter 3, I continue to examine orientation of A. cephalotes foragers in the laboratory. Foragers walked on a single bridge and I altered various cues and contexts and recorded which manipulations caused the ants to reverse course. In chapter 4, I put orientation cues into direct conflict by letting the ants forage on a Y-maze. Foragers that were returning to a food source preferred visual cues to odor cues while recruited foragers consistently used odor cues. In chapter 5, I use a vertical T-maze to investigate the role that gravity plays in A. cephalotes navigation. The gravitational cue was put in direct conflict with odor cues and light cues. There was an asymmetry to the ants' response to the gravity cue in that ants returning to a food source had a tendency to go up regardless of the previous position of the food source or the position of the odor trail. Introducing a light cue changed the angle required to make the ants respond to the gravitational cue. In chapter 6, I investigate the anatomy of A. cephalotes eyes and brains. Based on tissue sections, I measured the angles between adjacent ommatidia in the eyes, and the volumes of sub-compartments of the brain. In chapter 7, I use the results from the other chapters to inform my speculations about the nature and neural basis of A. cephalotes navigation. I develop an hypothesis of navigation in the wild and a simple model of its neural underpinnings.
Stream macroinvertebrate communities in urbanizing watersheds in Maryland, USA
(2003-11-24) Moore, Aaron A; Palmer, Margaret A; Behavior, Ecology, Evolution and Systematics
The conversion of agricultural landscapes into residential developments is a prominent form of land use change in Maryland. Macroinvertebrates were sampled in Maryland headwater streams, and used to test for differences in diversity and abundance between agricultural and developed sites. Macroinvertebrate richness was highest in the agricultural streams, possibly due to a negative relationship between richness and impervious surface cover. The nature of farming operations and their proximity to the stream may contribute to increased richness values. In Chapter 2, I compare macroinvertebrate communities collected with single-habitat and multi-habitat sampling methods. Communities were compared using IBI scores, and the community variables comprising these IBI's. Several community variables differed between methods, however the IBI scores calculated using the two methods were strongly correlated. Single-habitat samples had a slightly stronger relationship with development, indicating that they may be better for monitoring important focal sites.

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