Theses and Dissertations
This collection includes both ASU Theses and Dissertations, submitted by graduate students, and the Barrett, Honors College theses submitted by undergraduate students.
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- All Subjects: Ecology
The overarching question of this dissertation is: how do hydrology, soil conditions, and plant patches affect patterns of denitrification in accidental urban wetlands? To answer this question, I took a three-pronged approach using a combination of field and greenhouse studies. First, I examined drivers of broad patterns of denitrification in accidental urban wetlands. Second, I used a field study to test if plant traits influence denitrification indirectly by modifying soil resources. Finally, I examined how species richness and interactions between species influence nitrate retention and patterns of denitrification using both a field study and greenhouse experiment.
Hydroperiod of accidental urban wetlands mediated patterns of denitrification in response to monsoon floods and plant patches. Specifically, ephemeral wetlands had patterns of denitrification that were largely unexplained by monsoon floods or plant patches, which are common drivers of patterns of denitrification in non-urban wetlands. Several plant traits including belowground biomass, above- and belowground tissue chemistry and rooting depth influenced denitrification indirectly by changing soil organic matter or soil nitrate. However, several other plant traits also had significant direct relationships with denitrification, (i.e. not through the hypothesized indirect relationships through soil organic matter or soil nitrate). This means these plant traits were affecting another aspect of soil conditions not included in the analysis, highlighting the need to improve our understanding of how plant traits influence denitrification. Finally, increasing species richness did not increase nitrate retention or denitrification, but rather individual species had the greatest effects on nitrate retention and denitrification.
Results of this study showed that while body mass and body condition consistently diminished as the active season progressed, they were both greater in protected habitats compared to non-protected habitats, regardless of season. Dhubs surface activity and total body water decreased while evaporative water loss and body temperature increased as the active season progressed and ambient temperature got hotter. Total body water was also significantly affected by habitat protection.
Overall, this study revealed that, while habitat protection provided more vegetation, it had little effect on seasonal changes in surface activity. While resource availability in protected areas might allow for larger dhub populations, unprotected areas showed similar body morphometrics, activity, and body temperatures. By developing an understanding of how different coping strategies are linked to particular ecological, morphological, and phylogenetic traits, we will be able to make more accurate predictions regarding the vulnerability of species. By combining previous studies pertaining to conservation of protected species with the results of my study, a number of steps in ecosystem management are recommended to help in the preservation of dhubs in the Kuwaiti desert.
An understanding of the formation of spatial heterogeneity is important because spatial heterogeneity leads to functional consequences at the ecosystem scale; however, such an understanding is still limited. Particularly, research simultaneously considering both external variables and internal feedbacks (self-organization) is rare, partly because these two drivers are addressed under different methodological frameworks. In this dissertation, I show the prevalence of internal feedbacks and their interaction with heterogeneity in the preexisting template to form spatial pattern. I use a variety of techniques to account for both the top-down template effect and bottom-up self-organization. Spatial patterns of nutrients in stream surface water are influenced by the self-organized patch configuration originating from the internal feedbacks between nutrient concentration, biological patchiness, and the geomorphic template. Clumps of in-stream macrophyte are shaped by the spatial gradient of water permanence and local self-organization. Additionally, significant biological interactions among plant species also influence macrophyte distribution. The relative contributions of these drivers change in time, responding to the larger external environments or internal processes of ecosystem development. Hydrologic regime alters the effect of geomorphic template and self-organization on in-stream macrophyte distribution. The relative importance of niche vs. neutral processes in shaping biodiversity pattern is a function of hydrology: neutral processes are more important in either very high or very low discharge periods. For the spatial pattern of nutrients, as the ecosystem moves toward late succession and nitrogen becomes more limiting, the effect of self-organization intensifies. Changes in relative importance of different drivers directly affect ecosystem macroscopic properties, such as ecosystem resilience. Stronger internal feedbacks in average to wetter years are shown to increase ecosystem resistance to elevated external stress, and make the backward shifts (vegetation loss) much more gradual. But it causes increases in ecosystem hysteresis effect. Finally, I address the question whether functional consequences of spatial heterogeneity feed back to influence the processes from which spatial heterogeneity emerged through a conceptual review. Such feedbacks are not likely. Self-organized spatial patterning is a result of regular biological processes of organisms. Individual organisms do not benefit from such order. It is order for free, and for nothing.