Browsing by Author "James Gilliam, Committee Member"

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Coevolution Between Grasshopper mice (Onychomys spp.) and Bark and Striped Scorpions (Centruroides spp.)
(2004-11-29) Rowe, Ashlee Hedgecock; James Gilliam, Committee Member; John Godwin, Committee Member; Kenneth Pollock, Committee Member; Harold Heatwole, Committee Chair
Asymmetrical selection has been proposed as the strongest argument for rejecting hypotheses of coevolutionary arms races between predators and prey. In many cases there is evidence of increased investment by the prey in response to the predator, but no evidence of increased investment by the predator in response to the prey, thus producing asymmetry in selection. However, selection against a predator may be increased when the interaction is with a "dangerous" prey. Predators are most likely to respond evolutionarily to potentially lethal prey. This study employs grasshopper mice (Onychomys spp.) and bark and striped scorpions (Centruroides spp.) as a model to test the hypothesis that interspecific interaction between a predator and a potentially lethal prey will result in behavioral and physiological adaptations that reciprocally mediate their interaction (i.e., coevolution). Bark scorpions (Centruroides exilicauda) and striped scorpions (Centruroides vittatus) produce a potent venom containing neurotoxins that selectively bind to the ion-channels of vertebrates. Vertebrate-specific neurotoxins may produce lethal effects in mammals, especially small mammals. Southern grasshopper mice (Onychomys torridus) and Mearns' grasshopper mice (O. arenicola) are known to be voracious predators on scorpions. Southern grasshopper mice are broadly sympatric with bark scorpions in the Sonoran Desert, and Mearns' grasshopper mice are broadly sympatric with striped scorpions in the Chihuahuan Desert. The third species in this genus, the northern grasshopper mouse (O. leucogaster) is broadly allopatric with Centruroides spp. In a preliminary study, both southern and Mearns' grasshopper mice demonstrated resistance to bark and striped scorpion neurotoxins. The evolution of toxic venom and resistance to that venom strongly suggests a coevolutionary relationship between Onychomys spp. and Centruroides spp. To test this hypothesis, I evaluated the predator-prey relationship between grasshopper mice and their toxic scorpion prey during staged feeding trials. Additionally, I compared the geographic patterns of venom resistance in all three species of grasshopper mice with geographic patterns of venom toxicity in bark and striped scorpions. Results from the feeding study demonstrated that grasshopper mice do not distinguish between toxic and non-toxic species of scorpions; mice attacked, incapacitated, and consumed bark and striped scorpions without hesitation and as effectively as they attacked crickets and non-toxic scorpions in the genus Vaejovis. The feeding experiments indicate that grasshopper mice have the ability to prey on bark and striped scorpions in habitats where they co-occur. Venom resistance analyses demonstrated that all three species of grasshopper mice have evolved some resistance to the vertebrate-specific neurotoxins produced by Centruroides spp. The assays show that patterns of venom toxicity in Centruroides and venom resistance in Onychomys co-vary geographically, both within and among species; i.e., populations of Onychomys interacting with the most toxic populations of Centruroides were extremely resistant; populations of Onychomys interacting with only moderately toxic populations of Centruroides were only moderately resistant; and populations of Onychomys not sympatric with Centruroides were only weakly resistant. Such systematic covariation between venom toxicity in the scorpions and venom resistance in the mice is consistent with a coevolutionary, arms race hypothesis.
Epilithic diatom assemblages in headwaters streams of North Carolina and Virginia
(2003-07-01) Karoly, Cynthia Bell; James Gregory, Committee Chair; JoAnn Burkholder, Committee Member; James Gilliam, Committee Member
Using data collected by the Environmental Protection Agency during synoptic sampling for the Environmental Monitoring and Assessment Program, I studied the quantitative composition of diatom communities in 112 samples distributed in first through third-order streams within the Blue Ridge, Southeastern Plain, Coastal Plain and Piedmont ecoregions of the eastern U.S. The sampling sites studied were located predominantly in North Carolina and Virginia. The main objective was to identify dominant diatom species using both species density values and importance values incorporating estimated biovolumes. A secondary goal was to determine whether distinct diatom assemblages could be defined within the set of streams studied. Six diatom assemblages were defined based upon increasing diversity, richness and evenness of species within the groups. No trends could be detected in the cluster separation associated with ecoregions, sample years, and instream habitats, and paired riffle/pool samples. Diatoms were observed in all 112 samples, and 390 diatom species were identified. Achnanthes biasolettiana was the dominant species, comprising 54% of the relative density of the total dataset and observed in 89% of all samples. Only twelve additional species equaled or exceeded 1% of the total sample relative density or importance value. Use of estimated biovolume size classes for entire diatom genera did not alter the determination of dominant species. The same species were considered dominant using either measure of dominance. Within samples, the ten most dominant species consistently contributed > 50-75% of the total relative density of the sample.
Food Web Interactions of Larval Yellow Perch, Perca flavescens, in Lake Michigan: Implications for Recruitment
(2005-01-16) Fulford, Richard Stewart; James Rice, Committee Chair; Kenneth Pollock, Committee Member; Peter Rand, Committee Member; James Gilliam, Committee Member
Variability in annual recruitment for many fishes is correlated with survival during the larval phase. Yellow perch in Lake Michigan have experienced sustained recruitment failure since 1990 and this has been blamed on low larval survival. Direct examination of factors important to larval yellow perch survival in Lake Michigan is complicated by the large size of the lake (52,000 km2) and the short length of the pelagic larval period (30-40 days). Individual-based modeling is a valuable indirect method for assessing the importance of multiple factors to larval survival. I used an individual-based modeling approach combined with field data collection to test four hypotheses regarding factors limiting survival of larval yellow perch in Lake Michigan. I tested whether larval survival is limited by prey community composition, size-selective predation, advection of larvae into offshore habitat or an interaction of these factors. I sampled larval and zooplankton abundance in Lake Michigan along a transect from 1 – 32 km from shore in 2000 and 2001. I conducted laboratory experiments to quantify larval vulnerability to predation by three typical predators as a function of both predator and prey size. I also conducted laboratory experiments to quantify larval selectivity for different zooplankton prey as a function of larval size and prey community composition. I used the results of these experiments to develop an individual-based model specifically to describe growth and survival of larval yellow perch. Field data suggest that larval yellow perch are being transported from the nearshore to the offshore zone of Lake Michigan, but the timing of this transport varies between years. Model simulations in which the offshore prey community and the timing of larval advection were both varied suggested that larval survival will be highest in years when advection occurs within two weeks of peak hatch, allowing larvae to exploit offshore prey resources early during ontogeny. The model predicts that larvae will make foraging decisions based on prey availability as well as innate preference and they will change their diet if they are exposed to different prey communities. Model simulations also demonstrated that predation currently may not be an important factor for survival of larval yellow perch in Lake Michigan. This result is because alewife is the only fish abundant in Lake Michigan known to eat larval yellow perch. Experimental results suggest that alewife feeding rate on larval yellow perch is a positive function of larval density; yellow perch densities are currently too low to induce significant predation by alewife. Predation appears to be more important in smaller systems where larval densities are higher and larvae are exposed to other predator species. Larval survival in Lake Michigan appears to be primarily limited by an interaction of prey community composition and the timing during the larval period of offshore advection. Both factors vary between years and a good year-class is predicted when the offshore prey community is rich in cyclopoid copepods and larvae are advected offshore early. Early access to cyclopoid copepods results in an earlier transition from feeding on rotifers to feeding on copepods, which is predicted to increase larval growth and decrease mortality. These results suggest that survival of larval yellow perch in Lake Michigan is affected more by density-independent factors such as physical transport and prey community composition; Lake Michigan more closely resembles a marine environment than a typical lake with respect to larval recruitment processes. The unique aspects of larval yellow perch dynamics in Lake Michigan must be considered when applying lessons learned from analysis of larval yellow perch in other lakes to understanding survival of larval yellow perch in a large meso-oceanic system like Lake Michigan.
Hormonal Mechanisms Regulating Alternate Phenotypes.
(2003-12-08) Semsar, Katharine; John Vandenbergh, Committee Co-Chair; James Gilliam, Committee Member; Trudy Mackay, Committee Member; John Godwin, Committee Co-Chair
In the bluehead wrasse (Thalassoma bifasciatum), socially-controlled female-to-male sex change naturally decouples the brain from gonadal influences. This makes bluehead wrasses an excellent model for examining non-gonadal influences, such as social interactions, on the neural substrates of reproductive behavior. For my dissertation, I sought to further understand the role of AVT (a neuropeptide known to influence sexual and aggressive behaviors) relative to social context, gonadal input, and androgen influences in the mediation of the development and maintenance of male-typical behavior in this species. I examined these relationships by manipulating hormone levels and social context in the field and analyzing both the behavior and AVT neural phenotypes resulting from these manipulations. We found that changes in AVT neural phenotype are largely dependent on social influences not on gonadal input. In addition, AVT is necessary for both females and terminal phase (TP) males to display dominance on a spawning site. However, the ability of AVT to override social cues to induce male-typical behavior under conditions of social inhibition appears to be phenotype dependent. In the initial phase phenotype (either female or male) AVT treatment cannot induce male-typical territorial behavior. In contrast, in the TP phenotype, manipulations of the AVT system alone can override environmental cues to shift individuals between non-territorial and territorial social status. Finally, treating subordinate females with 11-ketotestosterone (11KT), a potent teleost androgen, induced male coloration and low levels of opportunistic courtship behavior typical of non-territorial males but did not alter responsiveness to AVT treatment. Together these results suggest there may be different hormonal mechanisms mediating courtship behavior under different social contexts.
The Influence of Temperature and Forage Availability on Growth and Habitat Selection of a Pelagic Piscivore
(2006-12-08) Thompson, Jessica Suzanne; Montserrat Fuentes, Committee Member; James Rice, Committee Chair; Joseph Hightower, Committee Member; James Gilliam, Committee Member
Habitat characteristics influencing growth of fishes often affect habitat selection because behaviors leading to rapid growth are under selective pressure. For pelagic piscivores such as striped bass Morone saxatilis stocked into southern reservoirs, temperature and prey density will likely be the most important factors influencing growth and spatial distribution. The traditional paradigm used to understand reservoir striped bass has focused on unsuitable physical conditions that may develop during summer stratification when hypolimnetic hypoxia forces fish into warm epilimnetic water. This study investigates how forage availability modifies the effects of physical conditions on this species. Striped bass in Badin Lake, NC, tagged with temperature-sensing transmitters tolerated temperatures above 27 degrees Celcius for two months to avoid hypolimnetic hypoxia but still displayed rapid growth. Bioenergetics models showed that high consumption rates allowed these fish to allocate energy to growth even during the summer and to grow rapidly during the fall as temperatures cooled. In contrast, striped bass in Lake Norman, NC, had slower growth despite experiencing warm summer temperatures for two to four weeks less. Lake Norman striped bass had lower consumption rates, and bioenergetics model simulations in which habitat conditions were exchanged between the reservoirs indicated that differences in forage availability had a greater relative effect on growth of striped bass than differences in thermal regime. These results suggest that criteria for determining the suitability of reservoirs for striped bass should incorporate a measure of prey availability. As with growth, habitat selection of Badin Lake striped bass was not solely dependent on temperature. Growth rate potential (the rate of growth of a predator occupying a particular location characterized by temperature and forage fish density) provided a better explanation of the spatial distribution of striped bass than did temperature or forage fish density alone. These results suggest growth rate potential can help us understand how fish integrate information on temperature and forage fish density, but it is important to note that growth rate potential was not a perfect predictor of the spatial distribution of striped bass so we cannot assume fish will optimize their patch choice with respect to this variable under all circumstances.
Local Dispersal and Coexistence in a Metacommunity Model with Trophic Structure
(2007-05-30) Hamilton, Matthew Williams; Kevin Gross, Committee Chair; James Gilliam, Committee Member; Alun Lloyd, Committee Member
One of the major goals of ecology is to understand the mechanisms which promote species coexistence. Much progress has been made in recent years in understanding how spatial processes influence diversity. Here we present a pair of models designed to investigate the role of one spatial process, localized dispersal, in promoting regional species coexistence when trophic structure is present. The model community considered is a two-predator, two-prey assemblage with cyclic endstates. Through analysis and simulation, we show that increasing the range of species dispersal can inhibit regional species coexistence. This result is at odds with the conclusions of some previous studies which considered locality within a single trophic level, and suggests that the relationship between dispersal distance and regional coexistence may be more complicated than previously realized.