Stable isotope ratios as indicators of black-throated sparrow habitat use in the northern Chihuahuan Desert, New Mexico

Abstract

As humans accelerate their alteration of the landscape, it is increasingly important to understand relationships between organisms and their environment. Stable isotope analysis is an increasingly popular tool for identifying ecological processes and can be a valuable asset for understanding how landscape alteration influences species and communities. In the Chihuahuan Desert, shrub encroachment due primarily to historic overgrazing and fire suppression and exacerbated by drought is a primary manifestation of indirect human landscape alteration and has consequences for endemic biodiversity.

Shrub encroachment changes many habitat components for consumers, of which and physical structure potential energy source (i.e., diet) are paramount and lack of ability to discriminate between structural and dietary components of habitat suitability has confounded general understanding of the impacts of aridland desertification on faunal communities; however, stable isotopes have the potential to delineate components of overall habitat composition that comprise consumer diet. The research detailed in my dissertation uses black-throated sparrows (Amphispiza bilineata) along with grasshopper species Trimerotropis pallidipennis, Opeia obscura, and Melanoplus occidentalis to broaden understanding of consumer use of changing environments and to further the application of stable isotopes to analysis of ecological processes.

My first chapter contributes to advancing stable isotope analysis as an accurate tool in avian ecology. Using feathers from Japanese quail (Coturnix japonica) obtained from the George M. Sutton Avian Research center, I explored the question, does the method by which a feather is cleaned prior to stable isotope analysis affect its resulting stable isotope values? I applied a paired cleaning design to test the isotopic influence of the two most common cleaning agents, 2:1 chloroform:methanol solvent and dilute detergent, and not cleaning feathers. I found that different cleaning methods resulted in highly variable stable isotope ratios for hydrogen and nitrogen, but stable carbon isotope ratios were not affected. However, I further found that employing a two-step cleaning procedure, cleaning first with dilute detergent and second with 2:1 chloroform:methanol, greatly increased the precision and predictability for stable hydrogen and nitrogen values. This experiment underscores the importance of delineating standardized procedures for tissue preparation for stable isotope analyses, increases reliability of data and improves the potential for data comparison between studies.

In my second chapter, I explore the role of variation in isotopic sources in influencing consumer stable isotope ratios and how this variation affects data application toward two popular philosophies, 1) isotopic niche width, and 2) degree of individual isotopic specialization. Using stable carbon isotope data from plants, grasshoppers, and black-throated sparrows collected at the Jornada Experimental Range in Las Cruces, New Mexico, I compared means and variances of consumers to two plant isosources--C3 and C4 plant photosynthetic types. I found that consumers of C3 plants had higher variation in stable carbon isotopes, primarily due to higher variation in the C3 plant isosource. I further compared my data to simulations in which consumers used different isosources. I concluded that variance in consumer stable carbon isotope ratios can be more clearly applied to inferences of individual specialization than to measures of a population's niche width.

In my third chapter, I use stable carbon isotope ratios to see how consumers analyzed in chapter two are using different facets of their carbonaceous habitat in the face of landscape change. My study area is located in the northern Chihuahuan Desert, an area formerly dominated by semi-arid grasslands, but now, due to alteration of grazing and fire regimes, has become dominated by shrubs. This landscape change inevitably affects the wildlife living there, so my objective was to see how shrubland habitats are used by species relative to grassland habitats, given current knowledge of species habitat use. The results of this study indicate that all species analyzed occupy habitats representative of the overall available range of habitat types from grassland to shrubland. Each grasshopper species employs a different foraging pattern: T. pallidipennis consumes primarily C3 components of its habitat for food and O. obscura consumes primarily C4 components, despite both occupying areas ranging from nearly 100% C4 grass to 100% C3 shrubs. M. occidentalis uses C3 and C4 habitat components for food proportional to their availability. Black-throated sparrows, despite frequent classification as a shrubsteppe or shrub-preferring species and frequent nesting in shrubs, consume primarily or exclusively C4 grass-based food. This study illustrates that stable isotopes can help clarify how different components of habitats can be used for different purposes, that different species can have different consumption patterns across ecotones, and how classification of species like black-throated sparrows into particular guilds may obscure use of other habitat components.

My fourth chapter details an observed correlation between stable nitrogen and stable carbon isotope ratios in black-throated sparrows and suggests possible explanations. Stable nitrogen isotopes are typically used to indicate trophic level or nutritional stress in consumers. In this system, for all three grasshopper species analyzed in chapter 3 and both hatch-year and adult black-throated sparrows, no relationship was seen between a consumer's stable nitrogen isotope ratio and the site at which the consumer was sampled, suggesting that the type of habitat which a consumer occupies does not influence stable nitrogen isotope ratios. However, for black-throated sparrows, particularly adults, there are higher stable nitrogen isotope ratios for individuals consuming a higher proportion of C3 carbon. Considering previous studies on stable nitrogen enrichment, trophic level, and nutritional and water stresses, the most feasible explanation for this pattern is that some black-throated sparrows are consuming higher trophic level food items containing C3-based carbon. Further studies of gut contents and foraging behavior would help clarify this explanation. Additionally, the low trophic enrichment values seen (+2.75 / for grasshoppers to a maximum of +3.37 / for adult black-throated sparrows) relative to the average accepted trophic nitrogen enrichment of +3.4 / (range 3-5 /) indicate that although this Chihuahuan Desert system is water limited, it does not exhibit overall water or nutritional-stress. This suggests that dietary differences may explain differential stable nitrogen enrichment in black-throated sparrows.