A Century of Avian Community Change in the Desert Southwest
Species extinctions and population declines have accelerated over recent decades due to habitat destruction, overexploitation, and invasive species, with cascading effects on ecosystem functions and services as well as human well-being. Climate change has emerged as another powerful driver of species decline, one whose effects are beginning to intensify. It should lead to shifts in species distributions and rearranged communities, unless climatic disruption acts as a systemic threat leading to a community collapse. Desert birds comprise a species-rich, easily detectable assemblage, and are closely coupled to their physical environment, which makes them suitable indicators of climatic change. I assessed how desert bird populations have been impacted over the past century by resurveying 106 sites throughout the Mojave and Great Basin deserts that were originally surveyed for avian diversity in the early 20th century by Joseph Grinnell and colleagues from the Museum of Vertebrate Zoology at University of California, Berkeley.
Multispecies occupancy models were employed throughout this research to capture the dynamics of the entire avian community. In my first chapter, I review the impacts of imperfect detection on the estimation of community diversity and how the application of multispecies occupancy models to estimate these measures can alleviate this source of error. The hierarchical structure of the model allows data from the entire sample to inform the estimation of occupancy, colonization, survival, and detection probabilities, despite encounter histories being stratified by historic and modern surveys, species, site, and visit. Data from all species’ informs the estimation of community-level values, and the structure of the model itself facilitates the modeling of all species, including rare ones. Correcting for detection is particularly important when using historic data, because differences in methodology and changes in technology that can alter the detectability of species through time can also influence the conclusions drawn from the comparison.
Deserts, already defined by climatic extremes, have warmed and dried more than other regions in the contiguous United States due to climate change. In my second chapter, I assessed how climate change and habitat disturbance have impacted bird populations of the Mojave Deserts. The resurveys of sites originally visited in the early 20th century found Mojave Desert birds strongly declined in occupancy and sites lost nearly half of their species. Declines were associated with climate change, particularly decreased precipitation. The magnitude of the decline in the avian community and the absence of species that were local climatological “winners” is exceptional. Our results provide evidence that bird communities in the Mojave Desert have collapsed to a new, lower baseline. Declines could accelerate with future climate change, as this region is predicted to become drier and hotter by the end of the century.
Where the Mojave and Great Basin meet is a juncture of two distinct avifauna. My third chapter uses a dynamic multispecies occupancy model to evaluate the cumulative effects of the redistribution of all 162 observed breeding species across space, time, and biomes. Cross-system comparisons can verify that trends are more than just regional in nature, which legitimizes their application to the development of broad-scale predictive models or management recommendations. The Mojave, a warm desert, and the Great Basin, a cold desert, are two very different systems with distinct vegetation and animal assemblages. The contiguity of the deserts creates an ideal place to study how 20th century climate change is differentially impacting communities across biomes. A transition zone can persist through a changing climate if biotic factors lag behind climate, such as the leading and trailing-edge range disequilibria in vegetation response, and inherent abiotic factors, such as topography and edaphic factors that contribute to the Mojave-Great Basin transition. Barriers to northward expansion for birds could result in range collapse if the southern limits of warm desert species are also contracting. I evaluated whether species of the warmer Mojave, which continues to warm, are expanding into the colder Great Basin Desert, which has incurred less warming, or whether the transition zone behaves as a barrier to northward expansion. Individual species experienced occupancy changes in consistent directions in both deserts. A substantial proportion of species were in decline in one or both deserts (39.5%), while relatively few species were increasing (6.2%). Most species (n = 80) shifted one or both of their latitudinal range limits. Range shifts were highly idiosyncratic in nature, causing the avifaunas of the two deserts to be less strongly structured than they were in the past. The redistribution of species is driving the genesis of novel communities which may have ecological consequences that are yet to be realized.
This dissertation presents a detailed picture of how desert avifauna has change over the past century. The unique ecology of the deserts means that can serve as bellwethers of climate change. Community collapse of the Mojave avifauna and the redistribution of species across both deserts would have been overlooked without the original faunal surveys of Joseph Grinnell and colleagues from the early 20th century. Although similar changes may be occurring in other ecoregions that lack comparable historical data, the harsh nature of desert environments makes them more likely to become less suitable for life and offers a prescient warning for biodiversity loss as future climates are pushed further toward extremes.