DOI: 10.5061/dryad.rxwdbrvd4
title: Concordant and opposing effects of climate and land-use change on avian assemblages in California’s most transformed landscapes

description: Climate and land-use change could exhibit concordant effects that favor or
 disfavor the same species, which would amplify their impacts, or species
 may respond to each threat in a divergent manner, causing opposing effects
 that moderate their impacts in isolation. We used early 20th-century
 surveys of birds conducted by Joseph Grinnell paired with modern resurveys
 and land-use change reconstructed from historic maps to examine avian
 change in Los Angeles and California’s Central Valley (and their
 surrounding foothills). Occupancy and species richness declined greatly in
 Los Angeles from urbanization, strong warming (+1.8°C) and drying (-77.2
 mm), but remained stable in the Central Valley, despite large-scale
 agricultural development, average warming (+0.9°C), and increased
 precipitation (+11.2 mm). While climate was the main driver of species
 distributions a century ago, the combined impacts of land-use and climate
 change drove temporal changes in occupancy, with similar numbers of
 species experiencing concordant and opposing effects.
description: We reconstructed an early-20th century ecological baseline using
 unique historical resources: (i) systematic bird surveys conducted by
 Joseph Grinnell from 1895–1904 at 71 sites around the Greater Los Angeles
 metropolitan area where he grew up and in the surrounding foothills
 (hereafter “Los Angeles”), and in California’s Central Valley and
 surrounding foothills (hereafter “Central Valley”) with colleagues in the
 early 1900’s, and (ii) hand-digitized land-use maps from the same period.
 These were matched with contemporary bird resurveys and measures of
 land-use and climate change at the same sites. To
 obtain data on historic localities and bird species occurrences, we
 reviewed original field notebooks written by Joseph Grinnell and several
 of his colleagues, which are curated by the Museum of Vertebrate Zoology
 at UC Berkeley. These field notebooks provide detailed descriptions and
 maps of survey routes, as well as systematic lists of bird species
 observed each day. We identified 71 sites (43 Central Valley, 28 Los
 Angeles) with historic surveys of bird diversity that sampled
 representative habitats and climates throughout the range of each study
 region. Bird surveys by Grinnell and colleagues were carefully documented
 in the form of field notebooks, museum specimens, photographs of sampling
 sites, and annotated topographic maps. They were occasionally in the form
 of standardized abundance surveys that were precursors to modern line
 transects, more often as lists of species encountered each day that
 provide detection/non-detection data, and rarely as a daily list that
 identified only species that had not been detected previously.
 We conducted modern resurveys during the breeding seasons (April
 through July) of 2015–2017, matching the following characteristics of the
 historic surveys: geographical location and extent of survey sites,
 elevational range covered, habitats surveyed within sites, and timing of
 the survey during the breeding season. We used standardized
 variable-distance point counts along a 2.25 km transect, with 10 points
 placed 250 m apart, corresponding as closely as possible to the area and
 habitats noted by the historic surveyors and indicated by specimen
 collecting locations. Surveys began at dawn and lasted 2–3 hours. At each
 point along the transect, we recorded all birds seen or heard during a
 seven-minute period. Each site was surveyed daily over three consecutive
 days. Bird counts from modern surveys were collated for each day across
 all 10 points surveyed along a transect and reduced to
 detection/nondetection data per day per site for occupancy
 modeling. We used a dynamic multispecies occupancy
 model (described in the paper) to estimate the probabilities of occupancy,
 local colonization, and local persistence between the historic and modern
 survey periods. R scripts are provided. To characterize
 climate (long-term average weather conditions often presented as 30-year
 climate normals), data were obtained from 800 m resolution interpolated
 maps produced by the PRISM climate group and averaged over 30-year periods
 corresponding to the historic (1900–1929) and modern (1988–2017)
 surveys. To quantify land-use change (modern –
 historic), we created maps of historic land use within 1 km of our bird
 survey transects by hand‐digitizing historic maps using ArcMap for
 comparison with modern land-use data obtained from the National Land Cover
 Database (NLCD) at 100m, 200m, 500m and 1km distances from modern survey
 points. Water and urban area were hand-digitized from historic USGS
 topographic maps (ca. 1906–1932). Water bodies were directly outlined as
 polygons. Urban area was mapped as buildings (area of the building icon on
 the topographic map plus a buffer of 50 m) and roads (digitized as line
 features from the topographic map and given a width of 30 m). Historic
 agriculture was delineated using a series of three maps of irrigated lands
 in California in 1920.
description: Excel and R