60 years of avian community change in the Willamette Valley, Oregon

Graduation date: 2015Birds perform valuable ecological functions and are useful environmental indicators. Avian distributions and diversity are predicted to change over the next 50 years. Little information exists on the role of local and regional conditions in fluctuations of avian communities over time. Historic datasets present a legacy of information that helps to understand changes in avian community composition. This study utilizes a rare, highly detailed avian survey of sites in the Willamette Valley, Oregon from 1952 (Eddy 1953). Because they include counts for all species detected on each survey, these data are uniquely valuable to understanding how avian species assemblages in the Willamette Valley changed. I resurveyed sites in 2013 to quantify the nature and extent of avian community compositional change. I used aerial photographs and satellite imagery to quantify changes in land use and habitat cover between periods. I compared metrics of avian alpha, beta, and gamma diversity between survey periods on multiple spatial extents. I also tested for shifts in categorical species abundances. Data was ordinated along environmental gradients to determine important axes of change between historic and modern species assemblages. Nonparametric procedures identified indicator species for each survey era. Observed differences between historic and modern communities suggested substantial turnover; nearly 50% of species were replaced over 60 years. Species richness increased on both local and regional spatial extents. Modern avian communities tended towards bimodal, less even distributions of abundances. Ordination of survey sites in species space confirmed modern avian communities were different from their historic counterparts. However, sites underwent relatively little change in vegetation and land use cover between survey eras. Indicator species characterized changes in regional abundances as well as differences in survey methodologies between eras. I took additional steps to confirm discrepancies in site location, survey effort, and detection probabilities did not influence analyses. These results suggest avian species assemblages are naturally dynamic. Complex processes outside of local habitat characteristics may influence community composition. Additional factors, such as vegetation structure and composition should be considered when modeling future shifts in species assemblage. Despite uncertainties in methodology, the historic data in this research provided unique, valuable insight into long-term changes in avian communities