Assessing the impacts of Lepidium latifoliumon food web structure for Suisun song sparrows in a brackish marsh

Effects of a chronic increase in plasma corticosterone on the brooding behavior of the prairie skink, Plestiodon septentrionalis

Despite facing threat of extirpation in the eastern Pacific, the critically endangered leatherback turtle (<em>Dermochelys coriacea</em>) appears to be thriving in the Atlantic. The purpose of my dissertation was to establish a baseline of biological data for the leatherback at the northern limit of its nesting range in the northwestern Atlantic (Florida) to inform management. From six years of empirical field data collected on nesting female leatherbacks, I addressed four major objectives of the Federal (US) Recovery Plan for the east coast of Florida. 1) I determined the rate of increase in observed nest numbers since standardized nest surveys began in 1979. From these data collected at 70 beaches and using a multilevel modeling approach, I found a dramatic increase of 11.3 ± 1.9% each year in the number of nests. 2) By tagging and recapturing individual females as well as marking nests for inventories at one high-density nesting beach, I determined several critical population parameters. The annual survival rate was 95.6% for nesting females and the yearly nesting population size was 71 ± 23 turtles at my Juno Beach study site (19.4 km); statewide the estimate will be higher. The average remigration interval was 2.2 +/- 0.5 years, clutch frequency was 4.1 +/- 0.9 nests/season, and the average clutch size was 69.0 +/- 18.2 eggs/female. 3) I used novel GPS tags and conventional tag returns to discover the full nesting range and true clutch frequency of individual turtles. Leatherbacks in Florida exhibit weak nest site fidelity by nesting at broad spatial scales, distributing at least five nests within a single season up to 139.8 km from first encounter. 4) Finally, I used microsatellites and mtDNA to determine the relatedness of leatherbacks in Florida and found few family groupings, indicating that this rookery has been established through immigration and not through a founding event, as there is no evidence of a genetic bottleneck. Although the Florida population is relatively small compared to other Caribbean mainland rookeries, it has the potential to contribute to further increases of leatherbacks in the Atlantic as well as contributing genetic variation, thereby achieving objectives of the US Recovery Plan. / Dissertation

<p>This thesis examines the dynamics of cloud forest bird communities along an altitudinal gradient on the eastern slopes of the Ecuadorian Andes. Specifically, I examined the seasonality of breeding, investigated a novel tool for documenting altitudinal migrations, and compared diets among groups of hummingbirds. In Chapter 2, I compared the prevalence of breeding condition in mist-netted birds among elevations and months. Overall, birds breed less seasonally at lower latitudes, but there is substantial variation in the timing of breeding, which varies with both abiotic conditions like precipitation (Hau 2004; Tye 1992), photoperiod (Hau et al. 1998) and temperature (Wikelski et al. 2000), and the biotic variation in plant phenology (Komdeur 1996), and insect abundance (Poulin et al. 1992). Several of these biotic (e.g. canopy height, biological diversity) and abiotic (e.g. temperature, pressure) factors vary along elevational gradients. I compared the percentage of birds captured in breeding condition along an altitudinal transect in eastern Ecuador, and found that breeding is more seasonal at higher altitudes. There was a marked increase in breeding birds during Sept-Nov at higher elevations, but I found no such "breeding season" at lower elevations. </p><p>I also examined a novel methodology for tracing annual altitudinal migrations which takes advantage of the natural variation in deuterium abundance from the base to the peaks of the Andes (Chapter 3). Local migrations by birds in the tropics pose conservation problems, in part because the movements themselves are difficult to document. There is a theoretical relationship between Deuterium (or 18O) signature and elevation, because of fractionation during precipitation events and evapotranspiration. A previous study had suggested that if a bird had more or less deuterium in its tissues than theory would suggest, such discrepancy might be used to identify altitudinal migrants. Unfortunately, when I refined the methodology, I found that the variation within species and sites was too great to allow such applications. </p><p>In Chapter 4 I shifted my focus to comparing diet among hummingbirds. Hummingbirds rely on the sugars in nectar to meet their high metabolic requirements, but most nectars are extremely low in nitrogen. As a result, the birds must also consume arthropods to meet their protein requirements. In many hummingbird species, males use nectar resources differently from females. I hypothesized that the different genders might also differ in their intake of arthropods, because females have higher nitrogen requirements for breeding. I used stable nitrogen isotopic analysis of feathers and blood to demonstrate that females feed at higher trophic levels than males and adults at higher levels than juveniles, respectively. Feathers from female Coeligena torquata (Collared Inca) showed significantly higher 15N levels (one-tailed t20=1.73, p<.05) than males. The difference between genders in Heliodoxa leadbeateri (Violet-fronted brilliant) was smaller (one-tailed t16=1.63, p=.06). δ15N was significantly lower for juveniles (mean = 6.34, SD = 2.10) than for adults (mean = 7.53, SD = 1.24). It appears that females captured during the breeding season were also feeding at higher trophic levels than those captured outside of the breeding season, although the sample sizes were small. Finally, I also found a slight but unanticipated effect of elevation on δ15N values in feathers.</p> / Dissertation

<p> Many southern California coastal salt marshes are urbanized and heavily impacted, but still provide important ecosystem services, including carbon and nutrient cycling. Salt marsh community structure and functions, such as decomposition, are essential for marsh ecosystems and are potentially vulnerable to inundation impacts created by sea level rise (SLR). The saltmarsh communities driving decomposition are comprised of invertebrates, fungi, and bacteria, which may be susceptible to SLR. In this project, inundation of saltmarsh sediments with associated plants and rhizosphere were manipulated using a marsh organ to assess the impacts of SLR on decomposer activity (leaf litter decay, anaerobic respiration) and community structure (bacterial and benthic invertebrate). Marsh organ samples across all inundation treatments showed altered decomposer community diversity and function compared to controls, indicating disturbance. However in some cases there were no significant differences between communities among SLR treatments. However, inundation effects may have been obscured by marsh organ artifacts.</p>

I conducted call-response surveys for Yuma clapper rails in the Colorado River delta, Mexico during the breeding seasons of 1999 and 2000 to estimate abundance, determine distribution, and identify patterns of habitat use. The maximum estimate of abundance was 6,629 individuals (95% C.I. 4,859 to 8,399). Rails were widely distributed in the delta, occupying almost all marshlands dominated by cattail. Rail density was higher in the Cienega de Santa Clara than in the other wetlands of the delta. High densities of clapper rails were associated with increased water depth, high cattail coverage, high vegetation coverage, low saltcedar coverage, proximity to shoreline, and increased salinity up to 8 ppt. As this is an endangered subspecies shared by Mexico and the U.S., the conservation of the delta ecosystem should be the interest of both countries, especially when management decisions upstream in the U.S. have an impact over natural areas downstream in Mexico.

Describing processes that lead to the distribution of parasites in space is important for understanding disease transmission and spread. Similarly, describing plant distribution patterns is important to understanding ecological processes. Indeed, distinguishing between dispersal and establishment limitation is central in plant ecology. Mistletoes allow doing both because they are plant parasites with clearly defined spatial distributions among hosts. Most mistletoes are dispersed by birds that consume mistletoe berries and defecate seeds onto host trees in a mutualistic relationship. I studied desert mistletoes, Phoradendron californicum (Viscaceae), which in the Sonoran desert, parasitize legume trees and are dispersed by Phainopepla nitens (phainopeplas). I examined patterns of spatial distribution and dispersal of P. californicum and the processes and mechanisms underlying these patterns at multiple scales. By counting mistletoes and deposited mistletoe-seeds, and watching phainopepla behavior, I found that mistletoes were aggregated within host trees, and that seed deposition was highest in tall and infected hosts. Likewise, phainopeplas perched preferentially in these trees, creating a positive feedback in which highly infected trees received many seeds and were likely to become reinfected. However, phainopeplas spent less time in trees than it takes for a seed to pass through their guts, which suggested interhost seed dispersal. I conducted a mistletoe removal experiment that confirmed a high degree of inter-host seed dispersal. These observations suggested that mistletoes would be aggregated at scales larger than individual trees. By mapping mistletoes and defecated seeds within a 4-hectare plot, I found that mistletoes were spatially correlated to at least 145 meters. Sampling at larger scales indicated that mistletoe prevalence was spatially correlated to approximately 1500 m and at scales larger than 4000 m. I also found that seed deposition increased with mistletoe prevalence in local neighborhoods. In conclusion, mistletoes are dispersal limited plants and are spatially correlated at several scales. Desert mistletoes are aggregated within hosts and their prevalence is spatially correlated at <1500 m and >4000 m. At the individual and local scales, their distribution is shaped by where birds defecate, which is influenced by host and neighborhood characteristics. At larger scales, their distribution may be primarily influenced by abiotic effects.

Macromeria viridiflora is an herbaceous perennial in which floral traits vary geographically. In my dissertation research, I analyzed geographic variation in plant morphology and pollinator assemblages. I conducted experiments to determine the breeding system of the plants, and used visitation rate and pollen deposition to compare effectiveness of floral visitors as pollinators. I analyzed aspects of pollinator attractants and rewards in the flowers and placed this into the context of pollinator syndromes. In addition, I used phylogenetic analysis of the genus to determine polarity of change in corolla size within the species. Analysis of morphometric data from eight sites across the range of the species revealed significant among-population variation in vegetative and floral traits. Flower size variation is particularly strong and follows a latitudinal cline. Hawkmoths and hummingbirds were the main floral visitors throughout the range. The large-bodied hummingbirds visiting plants in the southern regions are not present in the northern regions, where flowers are visited by hummingbirds with barely half the body size and much shorter bills. This difference in bill size of hummingbird pollinators mirrors the geographic variation in flower size in M. viridiflora, suggesting that pollinator-mediated selection may be acting upon the species. Flowers of M. viridiflora have several characteristics that fit both the hummingbird and hawkmoth pollinator syndromes, namely copious sucrose-rich nectar and long floral tubes. However, they also have characteristics that correspond with a single major pollinator. This plant therefore presents a compromise floral syndrome that attracts two classes of pollinators. Breeding system studies showed that whereas plants are self-compatible and occasionally produce seed autogamously, pollinators are important for reproductive success in the plants. Combining visitation rate and pollen deposition as measures of pollinator effectiveness, hummingbirds were found to be the most effective pollinators at both sites. Phylogenetic analysis produced a single most parsimonious tree that supports the monophyly of the genus. Mapping of corolla size onto the phylogeny indicates that floral size has changed many times within the genus, and that very large corolla size in southern populations of Macromeria viridiflora has been derived from a smaller-flowered ancestor.

Over time, small local disturbances may result in large regional changes in landscape structure and function. For example, lightning strikes may lead to large-scale wildfire or land clearing to urbanization. In either case, landscape patterns change as the type and distribution of landscape elements change in response to disturbances. Additionally, changes in landscape patterns often affect ecological processes. For example, wildfires and urbanization affect succession and productivity, which changes the distribution of habitat features, and which may affect landscape connectivity for species inhabiting the landscape. I used rank-size distributions and their scaling exponents to illustrate landscape character and change in Yellowstone National Park and a portion of the metropolitan area of Tucson, Arizona, through patterns associated with the distribution of patch size. I found that natural and anthropogenic disturbances affected landscape organization similarly and thus produced similar distributional patterns of patch size. However, the magnitude of change created by natural and anthropogenic disturbances differed. Fires in Yellowstone National Park produced scaling exponents >1, suggesting that large patches affected the distribution of patch size disproportionately. Comparatively, urbanization in the Tucson metropolitan area produced scaling exponents ≈1, suggesting that large and small patches affect the distribution of patch size proportionately. To link changes in landscape patterns with changes in ecological processes I compared four commonly used landscape metrics with rank-size distributions and their scaling exponents. Rank-size distributions described the scaling properties of the landscape with regard to patch size, whereas other metrics did not. This is meaningful because there is an integral relationship between scaling properties of the landscape and scaling properties of species using the landscape. A species may perceive a landscape as connected when the patch-size characteristics of the landscape scale proportionally with the body-size characteristics of the species. As a result, the species may be more likely to move through and therefore persist in that landscape. I develop a theoretical relationship between natural and anthropogenic disturbances, describe landscape organization, and link landscape and species scaling characteristics.

This dissertation focuses on two patterns in macroecology. The first describes the distribution of abundances between species (SAD) within a single community. The second describes the structure of abundance across a species range (SAASR). The central result is that the SAASR, combined with some other assumptions, can be shown both theoretically and empirically to explain the SAD (as well as several other patterns such as the species area relationship or SPAR). Given the increased importance of the SAASR pattern, I then provide an extensive analysis of empirical data to test for the existence and exact nature of the SAASR as well as developing the first quantitative assessments of proposed mechanisms underlying the SAASR. I also clarify a current point of confusion about SADs: whether they are truly log left-skewed. I next present a philosophy of science paper on how best to test macroecological theories. Finally, I apply this approach to a well-known macroecological theory that is generally considered to be strongly tested and show that the existing tests are, in fact, weak.