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Title:The influence of spatiotemporal variation in ambient temperature on the ecology and physiology of birds
Author(s):Pollock, Henry S
Director of Research:Brawn, Jeffrey D; Cheviron, Zachary A
Doctoral Committee Chair(s):Brawn, Jeffrey D; Cheviron, Zachary A
Doctoral Committee Member(s):Fuller, Rebecca; Suski, Cory; Dalling, James
Department / Program:School of Integrative Biology
Discipline:Ecol, Evol, Conservation Biol
Degree Granting Institution:University of Illinois at Urbana-Champaign
Thermal physiology
Ambient temperature
Thermoneutral zone
Heat tolerance
Phenotypic flexibility
Microclimate selectivity
Abstract:Ambient temperature has profound effects on the ecology, physiology and distribution of organisms. Ambient temperature regimes vary across spatial and temporal scales and understanding how organisms cope with this variation is a primary goal of ecological and evolutionary physiology. An emerging framework for understanding how geographic variation in ambient temperature is related to thermal physiology is the climatic variability hypothesis (CVH), which predicts that thermal tolerance breadth should increase with increasing levels of temperature variability. Studies documenting latitudinal increases in the thermal tolerance breadth of ectotherms and thermoneutral zone (TNZ –a proxy for thermal tolerance) breadth in endotherms have provided broad empirical support for the CVH. However, most previous tests of the CVH have focused on large-scale patterns of variation in thermal tolerance breadth across latitudinal temperature gradients, and little is known about how the CVH applies to small spatial scales (i.e. within a geographic locality) and also to temporal (seasonal) patterns of temperature variability. Furthermore, understanding how ambient temperature regime influences heat tolerances is becoming increasingly important due to climate warming, yet patterns of geographic variation in heat tolerances of endotherms have not been characterized. Finally, few studies have investigated the potential ecological consequences of variation in thermal physiology, particularly in endotherms. My dissertation chapters address each of these knowledge gaps in turn to provide a more complete investigation of the CVH and a clearer picture of how spatiotemporal variation in ambient temperature has influenced the physiology and ecology of birds. Chapter 2 explores the influence of ambient temperature regime on variation in TNZ breadth at multiple spatial scales. I captured ~140 bird species at three geographic localities (Illinois and South Carolina, U.S.A. and the Republic of Panama) and used flow-through respirometry to measure TNZ breadth. In phylogenetically informed analyses, I found that temperature variability rather than latitude per se was the best predictor of both interspecific and intraspecific geographic variation in TNZ breadth at large spatial scales (across geographic localities). I also found that migratory tendency was an important predictor of interspecific variation in TNZ breadth, with migratory species having TNZ breadths intermediate between temperate and tropical resident species. In contrast, at a smaller spatial scale (within a geographic locality), ecological traits associated with ambient temperature regime (vertical niche and habitat association) were not significant predictors of variation in TNZ breadth. My results indicate that temperature variability at the micro- or macro-habitat level is not associated with TNZ breadth in birds, and suggest that the CVH may not apply to smaller spatial scales in endotherms. Chapter 3 extends the CVH from spatial to temporal (seasonal) patterns of temperature variability and tests a secondary prediction of the CVH – that flexibility of thermoregulatory traits should increase with temperature seasonality. To test this prediction, I used flow-through respirometry to measure seasonal flexibility in five thermoregulatory traits [body mass (Mb), mass-adjusted basal metabolic rate (BMR), LCT, UCT and TNZ breadth] in suites of temperate (high seasonality) and tropical (low seasonality) birds. In phylogenetically-controlled analyses, I found that temperate species exhibited greater seasonal flexibility in LCT, UCT and TNZ breadth than did tropical species. Winter-acclimatized individuals of temperate species exhibited large reductions (up to ~9 °C) in LCT and modest (~2-3 °C) reductions in UCT relative to summer-acclimatized individuals, resulting in an increased winter TNZ breadth. Although some tropical species exhibited flexibility in LCT, UCT and TNZ breadth, patterns of seasonal flexibility were idiosyncratic and the mean amount of seasonal change was close to 0. In contrast, seasonal flexibility in Mb and mass-adjusted BMR did not differ between temperate and tropical species. My analysis suggests that patterns of geographic flexibility in thermoregulatory traits directly linked to thermal tolerance (e.g. LCT, UCT, TNZ breadth) conform to the CVH, whereas other traits (e.g. BMR, Mb) do not. Furthermore, the patterns I documented contribute to a growing body of evidence suggesting that species from environments with low temperature variability (e.g. lowland tropical species) may have reduced physiological flexibility relative to their temperate counterparts and may be more sensitive to climate change. Chapter 4 examines patterns of heat tolerance among tropical and temperate birds in an effort to provide the first analysis of geographic variation in avian heat tolerance. I then coupled heat tolerance data with ambient temperature data to test the prediction that tropical species will be more vulnerable than temperate species to future climate warming, as has been documented in numerous ectothermic taxa. I used flow-through respirometry coupled with temperature-sensitive passive integrated transponder (PIT) tags to measure three traits [UCT, heat-strain coefficient (HScoeff – the slope of the relationship between Tb and Ta above the UCT) and critical thermal maximum (CTmax – the Ta at which birds become hyperthermic and lose the ability to regulate Tb)] that comprise heat tolerance. Contrary to previous studies of ectotherms, I found limited evidence of reduced heat tolerances in tropical species. In phylogenetically-controlled analyses, although temperate species had slightly higher (~2 °C) CTmax, HScoeff and UCT did not differ between temperate and tropical birds. Importantly, these subtle differences in heat tolerance do not seem to translate into systematic differences in vulnerability to climate warming. Thermal safety margins did not differ significantly between temperate and tropical species, and seem to be large enough to allow for physiological tolerance of projected warming rates. Overall, my data do not indicate that tropical birds will be any more physiologically vulnerable to climate warming than temperate species, in contrast with previous patterns described in ectotherms. Chapter 5 focuses on the potential ecological consequences of variation in thermal physiology. Understory insectivorous birds of Neotropical forests are a guild that is disproportionately sensitive to disturbance, experiencing population declines and local extirpation in response to forest fragmentation. One hypothesis for their declines is the microclimate hypothesis, which posits that the novel abiotic conditions (i.e. increased temperature and solar radiation, decreased humidity) that habitat fragmentation introduces may physiologically challenge understory insectivores and contribute to their population declines. An important assumption of the microclimate hypothesis is that understory insectivores have narrow physiological tolerances because they inhabit the forest understory, which is an incredibly stable and buffered environment. To test the microclimate hypothesis, I radio-tagged individuals of nine understory insectivore species at three sites along a precipitation gradient in central Panama and compared the microclimates (ambient temperature, relative humidity, solar radiation) that they selected with randomly chosen microclimates to test for the preferential use of particular microclimates (i.e. microclimate selectivity). I found no evidence of selectivity for any of the nine species I sampled on a seasonal or spatial basis. Microclimate variation was minimal in the forest understory at all sites, particularly in the wet season. Understory insectivores did not use microhabitats characterized by high light intensity, and may be sensitive to light, though the mechanism remains unclear. The lack of microclimate variation in the understory of tropical forests may have serious fitness consequences for understory insectivores due to climate warming coupled with a lack of thermal refugia.
Issue Date:2016-12-02
Rights Information:Copyright 2016 Henry Pollock
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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