Research
Geographic patterns of upward shifts in treeline vegetation across western North America, 1984–2017
Previous research has shown that (1) treelines are shifting upward in elevation on high mountain peaks worldwide, and (2) the rate of the upward shift appears to have increased markedly in recent decades, at least in a few cases that have been studied in detail. Because treeline elevational shift is a process manifested over broad scales of space and time, a particular challenge has been that of obtaining a broad enough view of patterns of treeline shift to permit inferences about geographic and environmental patterns. What is more, intensive studies of treelines have been concentrated in north temperate regions such that little information is available about treeline shift patterns at lower latitudes. We attempted to address this challenge by analyzing long time series of vegetation indices derived from Landsat imagery obtained and prepared via Google Earth Engine from the 1980s to the present. We sampled vegetation indices at points spaced every 100 m along 100 km transects radiating out in eight directions from 115 high peaks across western North America (Canada to Central America), which means that we are sampling approximately every second or third pixel in the corresponding Landsat images. Considerable data preparation was necessary, including ending transects <2 km into closed forest, identifying current treelines via reference to Google Earth imagery, and consideration only of up to <1 km above the treeline. Patterns that emerged were – as is well known – that treelines are generally higher at lower latitudes but – previously unknown – that the magnitude of treeline shifts is nonrandomly distributed with respect to latitude, longitude, and their interaction. This analysis, via a broad-scale view of treeline shifts over almost 40 years and a geographic span of more than 40° of latitude, demonstrates that climate change effects and consequent treeline shifts are most dramatic in tropical regions where few or no detailed treeline studies have been or are being conducted.
Master's Project: Iridescent plumage variation on an urban-rural gradient
Urbanization has brought grave consequences that lead to declines in many species; however, some species thrive in urban ecosystems. This allows us the opportunity to study evolution in real time, as species adapt and change in a new setting. Birds, specifically songbirds, have been successful in colonizing urban areas though it remains unclear if urban birds have different phenotypes than rural ones. Iridescent plumage is understudied in general, and little is understood about how environmental changes shape iridescent plumage. For example, urban bird populations have more feather bacteria than rural species, which can increase feather wear. As a result, iridescent feathers—which lack dense melanin pigments to protect feathers—may be relatively more costly for urban birds. Many birds produce anti-microbial preen oil that they may use to combat elevated pressure from feather degrading bacteria, but preen oil is expensive to produce, and preening itself is time consuming. This introduces a potential trade-off between bacterial load and preening. To further complicate matters, preen oil is known to reduce reflectiveness in many species. Therefore, if preening is increased, the birds are duller, and if preening is the same between urban and rural populations, then urban birds should have more microbial abundance and still be duller. We sampled adult starlings on an urban rural gradient to assess relationships between microbial density, feather reflectiveness, preening gland volume, and reproductive success. My research will test whether urban and rural birds differ in their reflection parameters in relation to these urban-associated trade-offs.
Undergraduate Work: Experimental evidence of the function of nest orientation: a behavioral strategy to control microclimate?
As global climate change and local weather become less predictable, understanding the extent to which animals can mitigate those changes is of growing interest. Birds exhibit a variety of behavioral strategies to cope with variable environmental conditions during reproduction, including altering nest construction. In species building enclosed nests, the microclimate within nests is influenced by not only its structure and the surrounding vegetation, but also by the orientation of the nest opening. Many grassland-dependent birds build dome-shaped nests with clear directionality of openings. We studied two such species in northeastern Kansas that typically orient their nests east to northeast in this region. However, in a drought year, both Grasshopper Sparrows (Ammodramus savannarum) and Eastern Meadowlarks (Sturnella magna) shifted orientations southward toward prevailing winds. We hypothesized that this shift reduced deleterious effects of heat stress on parents and developing young by reducing morning solar radiation and increasing cooling due to the southerly prevailing winds. To test this hypothesis, we measured temperature, humidity, and wind speed at pairs of field-collected sparrow and meadowlark nests experimentally manipulated to face south or east (control) in a non-drought year. Nest orientation affected the daily patterns of microclimate with south-facing nests warming later in the day relative to east-facing nests. The differences in temperature depended upon both wind speed and humidity with south-facing nests being relatively cooler under more humid and calm conditions. This study provides the first experimental evidence of the putative fitness benefits of plasticity of nest construction under challenging thermoregulatory conditions in ground-nesting birds.
Environmental and geographic characteristics promoting the evolution of iridescence
The signaling environment is a crucial consideration in understanding the evolution and maintenance of animal color. For iridescent birds, the light environment has particular importance, as the qualities of incoming light dramatically affect signals. The light environment is complex to quantify, but associated, coarse-scale landscape characteristics can be used as proxies. In this study, we tested how habitat characteristics relate to prevalence of North American bird iridescence, hypothesizing that iridescence evolves and is maintained under specific external selective pressures, which in turn suggests that iridescent birds should be most diverse and iridescence most strongly expressed in specific environments. In eBird, we filtered for hotspots of avian diversity. For each of a series of factors potentially associated with iridescence, we selected pairs of geographically proximate sites that differ in that factor (e.g., elevation, forest cover, climate characteristics, and urbanization). We then created a list of breeding birds in the US and Canada using eBird data. To establish which birds are iridescent and to what degree, we evaluated specimens in the University of Kansas Natural History Museum collection, consulting other sources as necessary. We assigned each species as expressing iridescence or not and estimated the extent of species iridescence as a percentage, assessing how much of the body is iridescent, how much of their life cycles are they iridescent, and whether both sexes are iridescent. We tested effects of each habitat dichotomy on frequency and extent of iridescence using linear mixed models, taking advantage of the paired nature of the sites chosen for each analysis. Our findings on geographic and environmental biases leading to iridescent evolution show that this plumage type is more likely to evolve under certain conditions and can be advantageous in particular habitats, which we explore in detail in this presentation.
