Birds track their Grinnellian niche through a century of climate change

Abstract

In the face of environmental change, species can evolve new physiological tolerances to cope with altered climatic conditions or move spatially to maintain existing physiological associations with particular climates that define each species' climatic niche. When environmental change occurs over short temporal and large spatial scales, vagile species are expected to move geographically by tracking their climatic niches through time. Here, we test for evidence of niche tracking in bird species of the Sierra Nevada mountains of California, focusing on 53 species resurveyed nearly a century apart at 82 sites on four elevational transects. Changes in climate and bird distributions resulted in focal species shifting their average climatological range over time. By comparing the directions of these shifts relative to the centroids of species' range-wide climatic niches, we found that 48 species (90.6%) tracked their climatic niche. Analysis of niche sensitivity on an independent set of occurrence data significantly predicted the temperature and precipitation gradients tracked by species. Furthermore, in 50 species (94.3%), site-specific occupancy models showed that the position of each site relative to the climatic niche centroid explained colonization and extinction probabilities better than a null model with constant probabilities. Combined, our results indicate that the factors limiting a bird species' range in the Sierra Nevada in the early 20th century also tended to drive changes in distribution over time, suggesting that climatic models derived from niche theory might be used successfully to forecast where and how to conserve species in the face of climate change.

Fig. 1.

Locations of 82 bird survey sites in both geographical and climatic space. (A) Geographical locations of cross-sectional resurvey transects through the Sierra Nevada superimposed onto topography of California (higher elevations in lighter gray). Locations of neighboring survey sites (red circles) have been aggregated to provide visual clarity. The number of sites per transect from south to north are: Southern Sierra, 25; Yosemite, 24; Interstate 80, 3; and Lassen, 30. (B) Locations of resurvey sites in climate space, with arrows pointing from historical breeding season climate to modern breeding season historical climate. Color codes correspond to transect: Southern Sierra in blue, Yosemite in orange, Interstate 80 in purple, and Lassen in green.

Fig. 2.

Colonization-extinction dynamics as mediated by shifting climates lead to changes in position of occupied range centroids relative to the climatic niche. (A) Four sites (circles labeled 1–4) within a geographic area (black ellipse) experience shifts in climate over time (dotted arrows), moving sites from a prior climatic position (blue circles) to a current climatic position (red circles). For a hypothetical species with a certain climatic niche defined by temperature and precipitation (light blue ellipse), a site can be unoccupied in both time periods if it remains outside the climatic niche (site 1), go extinct if the site shifts out of the climatic niche (site 2), stay occupied in both time periods if it remains inside the climatic niche (site 3), or be colonized if the site enters the climatic niche (site 4). (B) The centroids of the observed occupied ranges for a species in each time period (asterisks: blue for historic and red for current) can provide evidence of niche tracking when compared with the centroid of a species' climatic niche (gray cross). If the temperature or precipitation components of the vector from the historic range centroid to the climatic niche centroid (H_T and H_P, respectively) agree in sign with the corresponding climatic components of the vector from the historic range centroid to the current range centroid (R_T and R_P), then there is evidence for tracking for that component. (C) Individual sites can be defined by vector components describing the position of a site (e.g., site 4) either historically (h_T and h_P) or currently (m_T and m_P) relative to the climatic niche centroid. These site-specific vectors are used in combinations as covariates of colonization and extinction in occupancy models. Examples of movements of range centroids for three species show different levels of climatic niche tracking (for further details and other species, see Table S1). (D) Lazuli Bunting showed niche tracking of both temperature and precipitation, shifting to a cooler and wetter occupied range. The light blue circle is a 95% density ellipse around the full range of historic specimens that defined the climatic niche centroid. (E) Townsend's Solitaire showed niche tracking of temperature, but not precipitation. (F) Nuttall's Woodpecker tracked neither temperature nor precipitation components of the climatic niche.

Fig. 3.

Models of site-specific change in occupancy in relation to the climatic niche. Sites are observed to be either occupied (black circles) or unoccupied (white circles), and between time periods they can either go extinct (red circles) or be colonized (green circles). (A) The null model estimates a constant probability of colonization and extinction; consequently, sites are equally likely to change occupancy status regardless of their proximity to the climatic niche centroid (gray cross). (B) The static model estimates turnover probabilities as a function of the distance from a site to the niche centroid, leading to changes in occupancy at the periphery of the climatic range. (C) The dynamic model (see Methods for six formulations) incorporates the degree to which sites have shifted in climate space over time (arrows). Thus, turnover probabilities are a function of the distance from a site to the niche centroid at present, relative to where it was in the past. This leads to a directional pattern in occupancy turnover.