Agriculture induces isotopic shifts and niche contraction in Horned Larks (Eremophila alpestris) of the Colorado Desert

Abstract

Listen

Translate article

Agriculture can dramatically alter community composition, food webs, and resource availability, yet the impacts of agriculture on many ecosystems and their constituents remain unstudied. Here, we examined the effect of agriculture on the isotopic ecology of Horned Larks (Eremophila alpestris) in the Colorado Desert. Over the last century, the Imperial Valley in southeastern California has transformed from desert scrub into agricultural land, while breeding Horned Larks have persisted in the area. We compared the isotopic composition of nitrogen (δ15N), carbon (δ13C), and hydrogen (δ2H) from feathers of lark specimens from two time periods in the Imperial Valley: a historical series collected between 1916 and 1923 and more recent vouchers collected between 1984 and 2014. We also quantified δ15N and δ13C values from plants and soil samples in contemporary agricultural land and adjacent desert scrub to estimate isotopic baseline changes associated with agricultural intensification. Contemporary larks had lower δ15N and δ13C ratios compared to historical larks, while plant and soil δ15N ratios were higher at agricultural sites compared to adjacent desert sites. These opposing trends suggest temporal change in isotope values that cannot be explained by shifting baselines alone. Furthermore, isotopic niche breadth (‰2; δ15N and δ13C) was lower among contemporary larks. Thus, contemporary larks in the Imperial Valley may capitalize on seeds and phytophagous insects that are abundant among crops, possibly representing an opportunistic change in diet over time in response to agriculture. Finally, δ2H values did not differ between historical and contemporary larks, but contemporary larks had δ2H values that differed from historical ground water, suggesting intake of water introduced by irrigational canals. Collectively, these findings highlight the power of stable isotope analyses combined with natural history collections to examine ecological change in the Anthropocene.