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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Understory Plant Responses to Uneven-Aged Forestry

Smith, Kimberly J. 12 September 2007 (has links)
In northern hardwood-conifer forests, alternatives to conventional forest management practices are being developed in order to maintain biodiversity and ecosystem functioning while providing for timber revenue generation. The understory layer of vegetation encompasses the majority of plant species diversity in forested ecosystems and may be sensitive to timber harvest disturbance. Thus, monitoring the response of forest understories to new silvicultural techniques may provide a means for evaluating their intensity. In this study, we hypothesize that i) uneven-aged, low-intensity silvicultural systems can maintain understory plant diversity and support latesuccessional species through harvest disturbance; ii) retaining and enhancing stand structural complexity can increase understory plant diversity in northern hardwoodconifer forests; and iii) plant responses are influenced by interactions between canopy structure, soils, and exogenous climate processes. Experimental treatments include two conventional uneven-aged prescriptions (single-tree selection and group selection) modified to increase structural retention, and a third technique designed to promote late-successional forest structure and function, termed structural complexity enhancement (SCE). Four replications of each treatment were applied to 2 ha management units at three sites in Vermont and New York, U.S.A. Understory vegetation was monitored over 2 years pre- and 4 years post-treatment. We used a linear mixed effects model to evaluate the effects of treatment, soil properties, and drought stress on understory diversity and abundance. Compositional changes among treatments were assessed with non-metric multidimensional scaling (NMS), an ordination technique. Model results show that over time, understory responses were strongly affected by overstory treatment and less influenced by soil chemistry and drought stress. All treatments were successful in maintaining overall composition and diversity. However, late-successional diversity increased significantly in SCE units compared to group selection units. These results indicate that while conventional uneven-aged systems are capable of maintaining understory plant diversity, variations that retain or enhance structural complexity may be more efficient at retaining latesuccessional species. Increased microsite heterogeneity as a result of these techniques may also increase understory plant diversity, at least during the initial post-harvest recovery period.
2

Outplanted Acropora cervicornis enhances the fish assemblages of Southeast Florida

Goldenberg, Ellen Dignon 03 May 2019 (has links)
Acropora cervicornis, commonly known as the staghorn coral has historically been a major contributor to reef structural complexity, providing habitat for many functionally important fish species throughout Florida and the Caribbean. Unfortunately, due to disease, bleaching, and local anthropogenic stressors, A. cervicornis populations have suffered drastic declines that have negatively impacted associated reef fish populations. In order to promote recovery, A. cervicornis fragments can be cultivated in nurseries and outplanted back onto reefs. This practice can effectively increase A. cervicornis abundance, but the long-term effects on local fish assemblages, and specifically functionally important grazing fishes, has not been assessed. Fish assemblages at natural (control) sites were compared to outplanted A. cervicornis sites in Southeast Florida. Fish surveys were conducted each summer at four locations from 2012 to 2017. Each location contained three outplanted A. cervicornis and one or two control sites. Outplant sites were defined by 50 A. cervicornis colonies in a 36 m2 area. Control sites occupied the same area but did not contain outplanted colonies. The fish assemblage structure was assessed in terms of composition, demography, and functional temporal trends as well as with the increasing structural complexity of the outplanted corals, defined as total linear extension (TLE). Significant temporal trends were recorded for total fish abundance, grazer abundance, and diversity. Structural complexity (outplanted A. cervicornis measured in TLE m-2) was found to be a significant predictor of total fish abundance, grazer abundance and diversity. Fishes 2-5 cm total length were most numerous indicating that the outplant sites may be providing habitat for juvenile reef fishes, particularly algae consumers. These findings suggest that A. cervicornis restoration may be creating a positive feedback loop in which outplanted corals create habitat for grazing fishes that in turn reduce algae competition, potentially providing new habitat for coral settlement.
3

On the Influence of Structure and Complexity in Perceived Duration

Zeigler , Derek E. January 2013 (has links)
No description available.
4

Pests and pest controlling organisms across tropical agroecological landscapes in relation to forest and tree-cover

Lemessa, Debissa January 2014 (has links)
A major challenge in agroecosystems is how to manage the systems so that it reduces crop pests and enhances natural pest control. This thesis investigates patterns of crop pests and top-down effects of birds and arthropod predators in relation to land-use composition across spatial scales. In paper (I) I examined the crop distribution and land-use types in relation to the crop raiding patterns in 15 transectsin sites close to and far from forests along with a questionnaire survey at household level. I found severe crop raiding close to forests, but it had no impact on crop composition growing between the two sites. In paper (II) I examined the effect of forest and tree cover, at local and landscape scales, on the abundance of arthropod predators by collecting specimens from 40 home gardens. My result showed higher abundance of arthropod predators when either the home garden or the surroundings had a high tree-cover, compared to when tree-cover at both scales was similarly either high or low. In paper (III) I investigated the effect of excluding birds and arthropod predators on leaf damage on rape seed in 26 home gardens. I found stronger top-down impacts from arthropod predators on crop pests in tree-poor gardens than in tree-rich gardens. There was no effect of birds. In paper (IV) I explored the effect of landscape complexity on bird and arthropod predation using plasticine caterpillars in 36 home gardens across landscapes. The rate of arthropod predation on caterpillars was higher in simple than in complex landscapes. The rate of bird predation did not vary between complex and simple landscapes. In simple landscapes, arthropod predation was higher than that of birds. The overall results suggest that simplified gardens/landscapes still have enough habitat heterogeneity to support arthropod predators for the significant top-down controlling effect on crop pests. However, I did not find clear effect of complexityon the top-down effect of birds. / <p>At the time of the doctoral defence the following papers were unpublished and had a status as follows: Paper 1: Manuscript; Paper 3: Manuscript; Paper 4: Manuscript</p>
5

Dimensão fractal e métodos quantitativos aplicados ao estudo de comunidades do macrobentos marinhos / Fractal Dimension and quantitative methods applied to the study of marine communities

Rodrigues, Carina Waiteman 19 October 2017 (has links)
A macroalga Sargassum C. Agardh é de reconhecida importância ecológica nos ecossistemas costeiros, particularmente nas comunidades de costões rochosos de regiões tropicais e temperadas quentes. Está amplamente distribuída na costa sudeste brasileira, sendo frequente em costões rochosos de locais moderados ou protegidos do embate de ondas. Nesses ambientes pode formar bancos densos e extensos, estruturalmente complexos, capaz de prover microhabitats variados para uma grande diversidade de organismos. Os bancos de Sargassum são suscetíveis a mudanças sazonais na sua biomassa e/ou estado fisiológico relacionados a fatores abióticos e bióticos, sendo que essas variações podem influenciar drasticamente a distribuição e densidade dos organismos associados às algas. A complexidade desse substrato tem sido avaliada por meio de várias medidas, sendo quantitativas, como peso, as mais utilizadas. O presente trabalho tem como objetivo avaliar o desempenho de diferentes medidas quantitativas da complexidade estrutural de bancos de Sargassum filipendula i) no desenvolvimento temporal da complexidade estrutural da alga em dois ambientes hidrodinamicamente diferentes e ii) na composição e distribuição dos organismos epifaunais. Foram coletadas 15 frondes de S. filipendula por mês, durante 13 meses, nas praias da Fortaleza e do Lamberto, Ubatuba, SP. A fauna presente nessas frondes foi obtida através de lavagem e peneiramento contínuo. O período de amostragem caracterizou-se por ser atípico, apresentando altas temperaturas ao longo de todo o ano e um verão com baixa pluviometria. Este fato influenciou fortemente a variação sazonal do S. filipendula e epífitas associadas, ocasionando elevados valores de peso seco entre a primavera e verão. Os resultados das comparações das medidas analisadas mostraram que um único parâmetro não é representativo da complexidade estrutural da alga, uma vez que cada medida apresentou diferenças em relação à abundância e diversidade da fauna. Estes dois indicadores também mostraram correlação positiva com todos os parâmetros de complexidade do substrato. Houve diferença significativa entre as praias, e as frondes do Lamberto foram estruturalmente mais complexas, suportando a maior abundância. Contudo, foram as frondes do Fortaleza que exibiram os maiores valores de riqueza de grupos. Discute-se o emprego de mais uma de uma medida quantitativa para mensurar a complexidade estrutural do habitat. / The Sargassum C. Agardh macroalgae is of recognized ecological importance in coastal ecosystems, particularly in the rocky coastal communities of tropical and warm temperate regions. It is widely distributed on the southeast coast of Brazil, being frequent in rocky shores of moderate locations or protected from the impacts of waves. In these environments can form dense and extensive banks, structurally complex, capable of providing microhabitats varied for a great diversity of organisms. The Sargassum banks are susceptible to seasonal changes in their biomass and/or physiological status related to abiotic and biotic factors, and these variations can drastically influence the distribution and density of organisms associated with algae. The complexity of this substrate has been evaluated by means of several measures, being quantitative, as dry weight, the most used. The present work aims to evaluate the performance of different quantitative measures of the structural complexity of Sargassum filipendula banks i) in the temporal development of algae structural complexity in two hydrodynamically different environments and ii) in the composition and distribution of epifaunal organisms. Fifteen fronds of S. filipendula were collected per month, during 13 months, on the beaches of Fortaleza and Lamberto, Ubatuba, SP. The fauna present in these fronds was obtained through continuous washing and sieving. The sampling period was characterized by being atypical, presenting high temperatures throughout the year and a summer with low rainfall. This fact strongly influenced the seasonal variation of S. filipendula and associated epiphytes, causing high values of dry weight between spring and summer. The results of the comparisons of the measures analyzed showed that a single parameter is not representative of the structural complexity of the algae, since each measure presented differences in relation to the abundance and diversity of the fauna. These two indicators also showed a positive correlation with all parameters of substrate complexity. There was a significant difference between the beaches, and the Lambert fronds were structurally more complex, bearing the greatest abundance. However, it was the fronds of Fortaleza that exhibited the highest values of group richness. We discuss the use of one more of a quantitative measure to measure the structural complexity of the habitat.
6

Dimensão fractal e métodos quantitativos aplicados ao estudo de comunidades do macrobentos marinhos / Fractal Dimension and quantitative methods applied to the study of marine communities

Carina Waiteman Rodrigues 19 October 2017 (has links)
A macroalga Sargassum C. Agardh é de reconhecida importância ecológica nos ecossistemas costeiros, particularmente nas comunidades de costões rochosos de regiões tropicais e temperadas quentes. Está amplamente distribuída na costa sudeste brasileira, sendo frequente em costões rochosos de locais moderados ou protegidos do embate de ondas. Nesses ambientes pode formar bancos densos e extensos, estruturalmente complexos, capaz de prover microhabitats variados para uma grande diversidade de organismos. Os bancos de Sargassum são suscetíveis a mudanças sazonais na sua biomassa e/ou estado fisiológico relacionados a fatores abióticos e bióticos, sendo que essas variações podem influenciar drasticamente a distribuição e densidade dos organismos associados às algas. A complexidade desse substrato tem sido avaliada por meio de várias medidas, sendo quantitativas, como peso, as mais utilizadas. O presente trabalho tem como objetivo avaliar o desempenho de diferentes medidas quantitativas da complexidade estrutural de bancos de Sargassum filipendula i) no desenvolvimento temporal da complexidade estrutural da alga em dois ambientes hidrodinamicamente diferentes e ii) na composição e distribuição dos organismos epifaunais. Foram coletadas 15 frondes de S. filipendula por mês, durante 13 meses, nas praias da Fortaleza e do Lamberto, Ubatuba, SP. A fauna presente nessas frondes foi obtida através de lavagem e peneiramento contínuo. O período de amostragem caracterizou-se por ser atípico, apresentando altas temperaturas ao longo de todo o ano e um verão com baixa pluviometria. Este fato influenciou fortemente a variação sazonal do S. filipendula e epífitas associadas, ocasionando elevados valores de peso seco entre a primavera e verão. Os resultados das comparações das medidas analisadas mostraram que um único parâmetro não é representativo da complexidade estrutural da alga, uma vez que cada medida apresentou diferenças em relação à abundância e diversidade da fauna. Estes dois indicadores também mostraram correlação positiva com todos os parâmetros de complexidade do substrato. Houve diferença significativa entre as praias, e as frondes do Lamberto foram estruturalmente mais complexas, suportando a maior abundância. Contudo, foram as frondes do Fortaleza que exibiram os maiores valores de riqueza de grupos. Discute-se o emprego de mais uma de uma medida quantitativa para mensurar a complexidade estrutural do habitat. / The Sargassum C. Agardh macroalgae is of recognized ecological importance in coastal ecosystems, particularly in the rocky coastal communities of tropical and warm temperate regions. It is widely distributed on the southeast coast of Brazil, being frequent in rocky shores of moderate locations or protected from the impacts of waves. In these environments can form dense and extensive banks, structurally complex, capable of providing microhabitats varied for a great diversity of organisms. The Sargassum banks are susceptible to seasonal changes in their biomass and/or physiological status related to abiotic and biotic factors, and these variations can drastically influence the distribution and density of organisms associated with algae. The complexity of this substrate has been evaluated by means of several measures, being quantitative, as dry weight, the most used. The present work aims to evaluate the performance of different quantitative measures of the structural complexity of Sargassum filipendula banks i) in the temporal development of algae structural complexity in two hydrodynamically different environments and ii) in the composition and distribution of epifaunal organisms. Fifteen fronds of S. filipendula were collected per month, during 13 months, on the beaches of Fortaleza and Lamberto, Ubatuba, SP. The fauna present in these fronds was obtained through continuous washing and sieving. The sampling period was characterized by being atypical, presenting high temperatures throughout the year and a summer with low rainfall. This fact strongly influenced the seasonal variation of S. filipendula and associated epiphytes, causing high values of dry weight between spring and summer. The results of the comparisons of the measures analyzed showed that a single parameter is not representative of the structural complexity of the algae, since each measure presented differences in relation to the abundance and diversity of the fauna. These two indicators also showed a positive correlation with all parameters of substrate complexity. There was a significant difference between the beaches, and the Lambert fronds were structurally more complex, bearing the greatest abundance. However, it was the fronds of Fortaleza that exhibited the highest values of group richness. We discuss the use of one more of a quantitative measure to measure the structural complexity of the habitat.
7

The acquisition of contrast : a longitudinal investigation of initial s+plosive cluster development in Swedish children

Karlsson, Fredrik January 2006 (has links)
<p>This Thesis explores the development of word-initial s+plosive consonant clusters in the speech of Swedish children between the ages of 1;6 and 4;6. Development in the word-initial consonant clusters is viewed as being determined by 1) the children’s ability to articulate the target sequence of consonants, 2) the level of understanding of which acoustic features in the adult model production are significant for the signalling of the intended distinction, and 3) the children’s ability to apply established production patterns only to productions where the acquired feature agrees with the adult target, to achieve a contrast between rival output forms. This Thesis employs a method where output forms are contrasted with attempted productions of potential homonym target words. Thus, development is quantified as an increase in the manifestations of phonetic features where it agrees with the adult norm, coupled by a decrease in the same feature in output forms where it is inappropriate according to the specifications of the phonological system of the ambient language. Acoustic investigations of cues of voicing, aspiration, place of articulation and syllable onset complexity, and auditory investigations of place, manner and syllable onset complexity were conducted. The Thesis has four outcomes. One, a description of the perceptual quality of the productions in terms of place, manner, voicing and syllable onset complexity is presented. Two, a developmental sequence of stable acquisition of these features is proposed; manner is shown to be acquired first, followed by syllable onset complexity and place of articulation. Evidence is provided that the voiced/aspirated distinction is still being acquired at the end of the investigated age period. Three, the developmental use of acoustic cues of place and voicing are described. Voice Onset Time and Spectral Skewness are shown to be used by children in order to increase the likeness to the adult target in terms of voicing and place of articulation. Aspiration Amplitude is shown to be used as an auxiliary cue to Voice Onset Time. The place cues Spectral Tilt Change, F2, Spectral Mean and Spectral Variance were shown to be used in order to refine already produced consonants rather than approach the adult target model. Four, the Thesis provides evidence of periods of confusions in the output of children. With the reductions of these patterns of confusion, evidence is provided of children’s re-organisation of their internal representation of the consonant to be produced.</p> / <p>Denna avhandling undersöker utvecklingen av ordinitiala konsonantkluster av formen s+klusil i talet hos svenska barn mellan åldrarna 1;6 och 4;6. Utvecklingen av de ordinitiala klustren betraktas som bestämd av 1) barnets förmåga att artikulera den konsonantsekvens som utgör målet, 2) barnets förståelse för vilka akustiska särdrag i den vuxna målproduktionen som är signifikanta för att signalera en viss distinktion och 3) barnets förmåga att tillämpa ett etablerat produktionsmönster endast på de produktioner där det tillägnade draget överensstämmer med den vuxna målproduktionen, så att en kontrast uppnås mellan konkurrerande utformer. Avhandlingen tillämpar en metod där producerade utformer kontrasteras med produktioner av målord som utgör potentiella homonymer till dessa. Utvecklingen kan då kvantifieras som en ökning av antalet förekomster av ett fonetiskt drag som överensstämmer med den vuxna normen för den relevanta kontexten, kopplad till en minskning av antalet förekomster av samma drag i kontext där draget är inkorrekt givet det fonologiska systemet i det språk som tillägnas. De drag som undersöktes var de akustiska korrelaten till stämbandston, aspiration, artikulationsställe och komplexitet i stavelseansatsen, och vidare de auditiva korrelaten till artikulationsställe, artikulationssätt och komplexitet i stavelseansatsen. Fyra resultat redovisas. För det första presenteras en beskrivning av den perceptuella kvaliteten hos barnens produktioner i termer av artikulationsställe, artikulationssätt och komplexitet. För det andra föreslås en utvecklingssekvens för stabilt tillägnande av dessa drag: artikulationssätt tillägnas först, följt av komplexitet hos stavelseansatsen och artikulationsställe. Sist tillägnas distinktionen mellan tonande och aspirerad klusil, där data visar att draget inte tillägnats till fullo vid slutet av det undersökta åldersintervallet. För det tredje ger avhandlingen en beskrivning av hur barnen utvecklar sitt bruk av akustiska korrelat till artikulationsställe och ton. Voice Onset Time och snedhet i spektrum används för att närma sig det vuxna målet gällande aspiration och artikulationsställe. Aspirationsamplitud används som ett hjälpkorrelat till VOT vad gäller fonetisk aspiration. Andra korrelat för artikulationsställe, som förändring i spektral lutning, F2, spektralt medelvärde och spektral varians, används för att förfina de egna produktionerna snarare än för att få produktionen att närma sig det vuxna målet. För det fjärde ges i avhandlingen evidens för perioder då barnens produktion uppvisar förväxlingar vad avser distributionen hos vissa fonetiska drag. Minskade förekomster av sådana förväxlingar visar på en omstrukturering av barnens underliggande representation för den konsonant som ska produceras.</p>
8

The acquisition of contrast : a longitudinal investigation of initial s+plosive cluster development in Swedish children

Karlsson, Fredrik January 2006 (has links)
This Thesis explores the development of word-initial s+plosive consonant clusters in the speech of Swedish children between the ages of 1;6 and 4;6. Development in the word-initial consonant clusters is viewed as being determined by 1) the children’s ability to articulate the target sequence of consonants, 2) the level of understanding of which acoustic features in the adult model production are significant for the signalling of the intended distinction, and 3) the children’s ability to apply established production patterns only to productions where the acquired feature agrees with the adult target, to achieve a contrast between rival output forms. This Thesis employs a method where output forms are contrasted with attempted productions of potential homonym target words. Thus, development is quantified as an increase in the manifestations of phonetic features where it agrees with the adult norm, coupled by a decrease in the same feature in output forms where it is inappropriate according to the specifications of the phonological system of the ambient language. Acoustic investigations of cues of voicing, aspiration, place of articulation and syllable onset complexity, and auditory investigations of place, manner and syllable onset complexity were conducted. The Thesis has four outcomes. One, a description of the perceptual quality of the productions in terms of place, manner, voicing and syllable onset complexity is presented. Two, a developmental sequence of stable acquisition of these features is proposed; manner is shown to be acquired first, followed by syllable onset complexity and place of articulation. Evidence is provided that the voiced/aspirated distinction is still being acquired at the end of the investigated age period. Three, the developmental use of acoustic cues of place and voicing are described. Voice Onset Time and Spectral Skewness are shown to be used by children in order to increase the likeness to the adult target in terms of voicing and place of articulation. Aspiration Amplitude is shown to be used as an auxiliary cue to Voice Onset Time. The place cues Spectral Tilt Change, F2, Spectral Mean and Spectral Variance were shown to be used in order to refine already produced consonants rather than approach the adult target model. Four, the Thesis provides evidence of periods of confusions in the output of children. With the reductions of these patterns of confusion, evidence is provided of children’s re-organisation of their internal representation of the consonant to be produced. / Denna avhandling undersöker utvecklingen av ordinitiala konsonantkluster av formen s+klusil i talet hos svenska barn mellan åldrarna 1;6 och 4;6. Utvecklingen av de ordinitiala klustren betraktas som bestämd av 1) barnets förmåga att artikulera den konsonantsekvens som utgör målet, 2) barnets förståelse för vilka akustiska särdrag i den vuxna målproduktionen som är signifikanta för att signalera en viss distinktion och 3) barnets förmåga att tillämpa ett etablerat produktionsmönster endast på de produktioner där det tillägnade draget överensstämmer med den vuxna målproduktionen, så att en kontrast uppnås mellan konkurrerande utformer. Avhandlingen tillämpar en metod där producerade utformer kontrasteras med produktioner av målord som utgör potentiella homonymer till dessa. Utvecklingen kan då kvantifieras som en ökning av antalet förekomster av ett fonetiskt drag som överensstämmer med den vuxna normen för den relevanta kontexten, kopplad till en minskning av antalet förekomster av samma drag i kontext där draget är inkorrekt givet det fonologiska systemet i det språk som tillägnas. De drag som undersöktes var de akustiska korrelaten till stämbandston, aspiration, artikulationsställe och komplexitet i stavelseansatsen, och vidare de auditiva korrelaten till artikulationsställe, artikulationssätt och komplexitet i stavelseansatsen. Fyra resultat redovisas. För det första presenteras en beskrivning av den perceptuella kvaliteten hos barnens produktioner i termer av artikulationsställe, artikulationssätt och komplexitet. För det andra föreslås en utvecklingssekvens för stabilt tillägnande av dessa drag: artikulationssätt tillägnas först, följt av komplexitet hos stavelseansatsen och artikulationsställe. Sist tillägnas distinktionen mellan tonande och aspirerad klusil, där data visar att draget inte tillägnats till fullo vid slutet av det undersökta åldersintervallet. För det tredje ger avhandlingen en beskrivning av hur barnen utvecklar sitt bruk av akustiska korrelat till artikulationsställe och ton. Voice Onset Time och snedhet i spektrum används för att närma sig det vuxna målet gällande aspiration och artikulationsställe. Aspirationsamplitud används som ett hjälpkorrelat till VOT vad gäller fonetisk aspiration. Andra korrelat för artikulationsställe, som förändring i spektral lutning, F2, spektralt medelvärde och spektral varians, används för att förfina de egna produktionerna snarare än för att få produktionen att närma sig det vuxna målet. För det fjärde ges i avhandlingen evidens för perioder då barnens produktion uppvisar förväxlingar vad avser distributionen hos vissa fonetiska drag. Minskade förekomster av sådana förväxlingar visar på en omstrukturering av barnens underliggande representation för den konsonant som ska produceras.
9

Amesurement Framework For Component Oriented Software Systems

Salman, Nael 01 November 2006 (has links) (PDF)
A measurement framework is presented for component oriented (CO) software systems. Fundamental concepts in component orientation are defined. The factors that influence CO systems&rsquo / structural complexity are identified. Metrics quantifying and characterizing these factors are defined. A set of properties that a CO complexity metric must satisfy are defined. Metrics are evaluated first using the set of properties defined in this thesis and also using the set of properties defined by Tian and Zelkowitz in [84]. Evaluation results revealed that metrics satisfy all properties in both sets. Empirical validation of metrics is performed using data collected from graduate students&rsquo / projects. Validation results revealed that CO complexity metrics can be used as predictors of development effort, Design effort, integration effort (characterizing system integrabiltiy), correction effort (characterizing system maintainability), function points count (characterizing system functionality), and programmer productivity. An Automated metrics collection tool is implemented and integrated with a dedicated CO modeling tool. The metrics collection tool automatically collects complexity metrics from system models and performs prediction estimations accordingly.
10

Effects of heat stress and local human disturbance on the structure of coral reef ecosystems at multiple scales of biological organization

Magel, Jennifer 04 January 2019 (has links)
The world’s coral reefs are being impacted by myriad disturbances, from localized overfishing and nutrient pollution to global climate change-induced temperature increases and ocean acidification. Conservation of coral reefs in the face of increasing variability and uncertainty requires an understanding of the interacting effects of multiple stressors on the diverse components of these vital ecosystems. In this thesis, I use data from reefs around Kiritimati atoll (Republic of Kiribati) in the central equatorial Pacific Ocean to examine the effects of a severe pulse heat stress event and local human disturbance on two important components of the coral reef ecosystem – three-dimensional (3D) structural complexity and reef fish assemblages. Using 3D reef models constructed through structure-from-motion photogrammetry, I examined changes in reef structural complexity in the year following the 2015-2016 El Niño and mass coral bleaching event. I found that exposure to prolonged thermal stress and subsequent coral mortality resulted in declines in reef structural complexity, particularly reef surface rugosity and terrain ruggedness. Baseline levels of structural complexity were also negatively influenced by local human disturbance, while complexity was positively related to the densities of branching and massive coral growth forms. These findings have important implications for the maintenance of healthy reef ecosystems, as high levels of structural complexity are important for supporting diverse reef-associated fish assemblages. Next, using underwater visual censuses of reef fish assemblages, I quantified fish abundance, biomass, species richness, and assemblage structure before, during, and after the 2015-2016 El Niño. Total reef fish abundance, biomass, and species richness declined during the El Niño, suggesting that pulse heat stress events may have short-term, negative consequences for reef fish. Although these metrics did not vary substantially across the local human disturbance gradient, recovery of assemblages following the heat stress event was impeded by higher levels of local human disturbance. Reef fish assemblage structure was influenced by a more diverse array of factors, showing significant shifts in response to heat stress, human disturbance, and net primary productivity. Given the many important roles that fish play on coral reefs, declines such as those observed here may impair the ecological functioning of these ecosystems. Together, my results highlight the negative impacts of heat stress and local human disturbance on coral reefs, demonstrating ways in which these stressors may interact to limit reef resilience in the face of increasing anthropogenic pressures. / Graduate / 2019-12-07

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