Browsing by Subject "Ecology, evolution and behavior"

Now showing 1 - 5 of 5
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    Ecological and anthropogenic drivers of giraffe (Giraffa camelopardalis tippelskirchi) population dynamics in the Serengeti
    (2014-04) Strauss, Megan Kate Louise
    Considerable changes in the abundance and demography of Serengeti National Park giraffes (Giraffa camelopardalis) provide insight into the drivers of ungulate population dynamics. When last systematically studied in the 1970s, the giraffe population was growing at a rate of 5-6% per year, in response to an increase in the biomass of palatable browse species. During three field seasons (2008-2010), I collected data on a significantly reduced population on giraffes: 5-7 times smaller than the 1970s peak. Interestingly, this drop in abundance corresponds to an increase in the density and cover of woody vegetation across the Serengeti. Using the Serengeti giraffe population as a case study, I explore the role of food supply, predation, parasites, and poaching on population dynamics. This investigation is facilitated by contemporary field studies, including vegetation surveys, aerial counts of giraffes, and regular observations of >900 individually identified giraffes. Recent observations are compared to results from the 1970s and combined with a range of other long-term data from the Serengeti (including data on lion kills from a >40-year dataset and data on poaching) to provide a multifaceted picture of the giraffe population and its environment. (I also devise a novel technique of studying lion claw marks on the skin of live giraffes as a measure of predation attempts.) My population estimates confirm that giraffe abundance has fallen significantly since the 1970s. Moreover, I find that recruitment and adult survival are lower now than in the 1970s. The data suggest that giraffe abundance in the Serengeti is currently limited by poaching and by a reduction in the relative dominance of palatable plant species brought about, in part, by the giraffe's own browsing behavior. Using a population model, I investigate how survival and fertility affect giraffe population growth and find evidence that population growth is most sensitive to adult survival. I recommend reducing the targeted poaching of adult giraffes as a conservation measure.
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    Ecological and evolutionary perspectives on bacterial resource use
    (2014-08) Weisenhorn, Pamela
    Bacterial metabolism mediates many biochemical transformations important to the stability and health of a diverse range of ecosystem types. In my dissertation, I examine the evolutionary and ecological context of a subset of bacterial metabolic pathways related to energy and metabolic precursor production that are crucial for bacterial growth. Specifically, I examine whether these pathways are conserved across a large, phylogenetically diverse set of organisms, whether related organisms respond similarly to differences in resource inputs, and whether knowledge of these pathways or phylogenetic relatedness can aid in the prediction of bacterial growth rates across a wide range of C substrates. While I found only a weak phylogenetic signal in the presence or absence of these pathways, there was strong evidence that constraints have limited the number of observed combinations of these pathways. Only 265 (6.5%) of the 4096 potential pathway combinations were found in this dataset of 8178 genomes. I propose this may suggest strong environmental selection acting to rapidly change pathway presence or absence, regardless of past evolutionary history. In order for this suggestion to be feasible, organisms must respond to their environment in a phylogeny-independent manner. To address this, I compared taxa response using 16S amplicon libraries from plots with substantial variation in C and N availability resulting from plant species identity in a long-term field experiment. I found an inconsistent response of soil bacteria at higher taxonomic levels to resource variation, in agreement with organisms responding to environment in a phylogeny-independent manner. I then cultured 56 bacterial isolates from these plots to examine the relative strength of phylogeny versus metabolic pathways in explaining growth responses of isolates across a range of substrates. Phylogenetic relatedness and similarities in energy metabolism each explained about 30% of the observed variation in patterns of bacterial growth, with about 50% overlap between the two approaches. Both phylogeny and energy metabolism are important in determining bacterial growth; however, environmental selection may lead to convergence towards a small number of ecotypes within a system despite high levels of phylogenetic diversity. The strength and consequences of such environmental optimization of metabolism warrant further study.
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    Experimental warming effects on soil organic matter dynamics at the temperate - boreal forest ecotone
    (2015-04) Eddy III, William Cyril
    Laboratory and field experiments have both shown that warming will increase soil CO2 flux to the atmosphere, but do not agree on the importance of this feedback as a future climate driver. As such, we lack the process-based understanding necessary to predict how warming will change soil carbon stocks in the future. Here we measured warming effects on soil organic matter (SOM) decomposition in a southern boreal forest warming experiment (B4WarmED) at two sites in northern Minnesota, USA. We used two laboratory incubations and measurements of soil extracellular enzymes to examine mechanisms that are predicted to alter the warming response of soil respiration, including soil microbial community thermal adjustment; differential temperature response of fast and slow cycling carbon pools to warming; and the potential for upland soil drying to reduce the response of respiration to temperature. We measured soil respiration in the first 5-yr of in situ warming at B4WarmED. Total soil respiration increased in the first three years of experimental warming (by 8 and 21%, for the +1.7 and +3.4°C treatments, respectively, relative to ambient temperature treatments), but warming responses decreased substantially in the fourth and fifth years of warming. In contrast, warming effects on root-excluded bulk soil respiration were relatively constant during the five years of treatment: the +1.7°C treatment showed little response to warming, whereas warming of +3.4°C increased bulk soil respiration by 13%. Warming treatments both decreased the long-term temperature response (i.e. Q10 parameter) and increased the soil moisture response for total soil respiration, but not for bulk soil respiration. Yet, in situ soils, even with warming decreases in soil moisture, were rarely dry enough to substantially alter the response of soil respiration to warming treatment. We used two laboratory incubations to test whether soil drying and soil microbial thermal adjustment reduced the temperature response of decomposition in B4WarmED soils. SOM decomposition was less responsive to temperature in dry soils, and in soils incubated at higher temperatures, suggesting that drying and thermal adjustment could reduce the warming effects on decomposition, although the effects of thermal adjustment were small. Finally, in both incubations, we found that the decomposition of slow cycling soil carbon responded more to warming than rapidly cycling carbon.Soil extracellular enzymes are important catalysts of SOM decomposition, as they break down organic matter polymers and increase the carbon substrates available to the soil microbial community. We examined warming effects on four soil extracellular enzymes at the B4WarmEd sites. In contrast to our predictions, we found little evidence of microbial thermal adjustment of enzyme kinetics to warming that would reduce the warming response of SOM decomposition. In summary, in both field and laboratory measurements SOM decomposition increased with experimental warming. Soil drying was found to decrease SOM decomposition and the response of SOM decomposition to increasing temperature, although warming-enhanced soil drying was not found to substantially decrease in situ soil respiration. Finally, in laboratory incubations, slowly cycling SOM was more temperature sensitive than fast cycling SOM. Together, these results suggest that warming will increase SOM decomposition, and that loss of large pools of slowly cycling SOM, in particular, could contribute significant carbon the atmosphere over the next century. Soil drying could, however, moderate the response of soil respiration to warming, especially if increases in evapotranspiration or changes in precipitation results in drier soils than were observed in the field component of this study.
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    Living with lions: spatiotemporal aspects of coexistence in savanna carnivores
    (2014-07) Swanson, Alexandra Burchard
    Top predators can suppress their smaller guild members and this can have profound consequences that cascade throughout the larger community. Suppression is mediated primarily through interference competition: (a) direct aggressive interactions, and (b) behavioral avoidance by mesopredators to minimize risks of encountering top predators. These avoidance responses can be costly, especially when they result in large-scale displacement that reduces access of the subordinate species to resources. However, fine-scale avoidance strategies may promote mesopredator persistence by minimizing risk without costly large-scale displacement. This dissertation explores the role of behavioral avoidance in driving intraguild predator dynamics. Specifically, I examine how African lions affect spotted hyenas, cheetahs, and African wild dogs in Serengeti National Park, Tanzania. Long-term lion monitoring by the Serengeti Lion Project provides a high-resolution understanding of how lions interact with each other and the landscape; I deployed a large-scale camera trapping survey to collect fine-scale spatial data on the broader carnivore community. Chapter 1 reveals that although lions displace African wild dogs from the landscape and suppress their populations, cheetahs persist with lions through space and time. Chapter 2 validates the camera trapping survey designed to study fine-scale carnivore avoidance and highlights the broad utility of citizen science for similar ecological projects. Chapter 3 applies the camera trapping survey to reveal that fine-scale avoidance does not always translate into costly spatial displacement for subordinate species. Together, these chapters identify large-scale displacement as a key driver of mesopredator suppression and fine-scale avoidance as a key mechanism for mesopredator persistence. This dissertation further establishes new methods to continue exploring community dynamics for long-lived, wide-ranging species.
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    The multi-trophic context of plant defense: ecological and evolutionary implications of variation in milkweeds
    (2014-08) Mohl, Emily Katherine
    Plants exhibit great diversity in defense-related traits, such as toxins and spines. Most explanations for this variation assume that such traits are costly because they require resources to produce and maintain; however, controlled experiments frequently fail to document costs of plant defenses. One explanation for this pattern is that ecological context matters. I develop the hypothesis that certain plant traits impose ecological costs in a multi-trophic context because they disrupt predation on herbivores. Because milkweed plants are toxic and harbor specialist herbivores that sequester the toxins to defend themselves against generalist predators, I expect plants to incur ecological costs of defense in this system. I investigate the impact of variation among milkweed species on the ecology and evolution of predators that consume herbivores, testing for costs of defense in a multi-trophic context. I show that plants can strongly impact the preference and performance of some predators and generate patterns consistent with ecological costs of defense. However, I also demonstrate that other traits and processes, such as plant tolerance of herbivory and predator tolerance of plant-derived defense, may have the potential to mitigate such ecological costs. Consequently, ecological costs of defense are unlikely to be fixed but are instead subject to coevolutionary dynamics. As introductions, extinctions, and range shifts change the pool of potentially interacting species, a more predictive understanding of the way individual traits affect, and are affected by, the community context in which they occur will improve our ability to prioritize and manage human impacts on these systems.