Dissertation/Thesis Abstract

The Effects of Stress on Communities: Using Modern and Fossil Data to Explore Community Response
by Webb, Amelinda Erin, Ph.D., Yale University, 2013, 220; 3578469
Abstract (Summary)

The current biodiversity crisis is challenging the ability of conservation biologists to both monitor ongoing declines and create effective management plans. Ongoing habitat destruction, pollution, introduction of invasive species, and the initial stages of climate change are only some of the anthropogenic stresses that face today's biosphere. On human time scales, these changes are unprecedented, curtailing the availability of knowledge regarding ecological responses to stress and disturbance. The fossil record provides numerous disturbances of varying magnitudes throughout the history of life, and yet this resource has been often overlooked or dismissed by biologists. The purpose of this study is to examine the effects of stress on communities using methods that allow integration of modern and fossil data. With this goal in mind, various levels of disturbance are investigated across increasing temporal and spatial scales.

At the smallest spatial and temporal scale, I examined the effect of recent lake acidification on plankton communities, using techniques commonly applied by ecologists, as well as introducing a new method based on a well-established technique. Throughout this thesis, I use the Buzas-Gibson evenness metric and Non-metric Multi-Dimensional Scaling analysis (an ordination technique), as well as applying Rank-Abundance Curve Kurtosis, which measures the shape of species-abundance distributions. Each of these techniques is a different way of representing community structure, with each metric providing slightly different information. Within the lake acidification system, all communities displayed a shift in community structure as pH dropped, and again when pH values returned to neutral, indicating a gradual recovery from acid stress. The timing of this change reveals the ability of different communities to resist acidification, and the resilience of those communities through the recovery phase.

To determine the feasibility of comparing modern and fossil data, I selected four unrelated datasets with distinct disturbance events to represent different time scales, from two decades to one million years. Each dataset displayed a similar pattern; the disturbance event created a distinct shift in community structure followed by a gradual recovery after the stress levels decreased. A major concern when comparing modern and fossil data is the difference in temporal resolution, and specifically the effect of time-averaging which is expected to obscure ecological signals. Instead, I found that applying a model of time-averaging across the community data reduced background noise, thereby clarifying the pattern of ecological change observed in the raw data.

Extending the temporal and spatial scale, I explored the ecological response of marine microfossil assemblages during three intervals of rapid global warming, as analogues for modern global warming. Four taxonomic groups were included, two benthic and two planktic. Overall, diversity within communities increased during global warming, however this was due to the response of the planktic groups, as both benthic groups showed decreases in diversity. These findings support the utility of the fossil record in examining past disturbances, by providing a useful prediction for biotic responses to global warming.

Representing the largest spatial and temporal scale is the Botomian mass extinction (mid-Early Cambrian). This mass extinction is the first recognized mass extinction in the history of life, and occurred during an interval of rapid evolution and faunal turnover. During the extinction interval, there was a distinct change in community structure and an associated increase in instability.

The findings of this study are unique; community structure displays a similar response to stress across various taxonomic groups, in different environments, and at multiple temporal scales. The commonality in community response to stress likely represents a fundamental feature of disturbed ecosystems. Not only is the comparison of modern and fossil data possible, such comparisons offer new discoveries relevant to conservation biology and about the very nature of life on Earth.

Indexing (document details)
Advisor: Briggs, Derek E. G.
School: Yale University
School Location: United States -- Connecticut
Source: DAI-B 75/05(E), Dissertation Abstracts International
Subjects: Conservation, Paleontology, Paleoecology
Keywords: Disturbance, Ecosystem health, Environmental change, Paleoecology, Recovery Community Structure
Publication Number: 3578469
ISBN: 978-1-303-71564-8
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