Table 1.
Glossary of niche terminology, adapted from Peterson et al. [4], with reference to the present study; the terms collectively describe three niche classifications that are ordered hierarchically based on their size: fundamental ≥ existing ≥ occupied.
Table 2.
Data sources used in the contemporary (C) and temporal (T) niche analyses, including years of coverage and raw sample sizes.
Figure 1.
Contemporary niche space for the North American house sparrow, calculated with respect to minimum temperature of the coldest month and maximum temperature of the warmest month.
Physiological limits define the boundaries of the triangular fundamental thermal niche for different mortality thresholds (LD100 [light blue], LD50 [medium blue], and TNZ [dark blue]), projected onto the corresponding contemporary realized environmental space for North America (black dots, circumscribed by black minimum convex polygon [MCP]) and the occupied environmental space of the house sparrow (red dots, circumscribed by red MCP). As per definition (Table 1), the intersection of the environmental space with the fundamental niche establishes the existing niche (for a given fitness threshold). Letters identify locations of niche centroids.
Table 3.
Ability of the occupied niche to describe properties of the physiological niche, comparing across three niche property estimators (centroid, area, MCP), both fundamental and existing niches, and three fitness thresholds (TNZ, LD50, LD100).
Figure 2.
Time series of the occupied and existing LD50 niche centroid locations in climate space: (A) minimum temperature of the coldest month; and (B) maximum temperature of the warmest month.
Light orange polygons show the 95% confidence intervals, and dark orange lines the means, of occupied niche centroids evaluated for 100 bootstrap samples (controlling for sample size over time, see Materials and Methods). Green lines show the existing LD50 niche centroids. Plots of the annual location of the occupied niche centroid versus the existing LD50 niche centroid, 1930–2004, for: (C) minimum temperature of the coldest month; and (D) maximum temperature of the warmest month. Solid lines show slope of observed Pearson’s correlations.
Figure 3.
Directional changes in temperature for North America, 1930–2004.
(A) The contemporary environmental space showing grid cells that changed in a directional fashion with respect to minimum temperature of the coldest month or maximum temperature of the warmest month (colored grid cells, with directionality symbolized by arrows, matching legend in panel C); grid cells that did not experience directional change are not shown. Labeled regions delineate environmental spaces with respect to different thermal niche fitness thresholds for the house sparrow: (1) the full environmental space; (2) the fundamental LD100 niche; (3) the fundamental LD50 niche; and (4) the fundamental TNZ niche. (B) Pie charts show the proportions of grid cells undergoing each class of directional change within each of the four spaces. (C) Geographic projection of grid cells that experienced directional changes in minimum temperature of the coldest month or maximum temperature of the warmest month. Positive (+) = increasing temperature, negative (–) = decreasing temperature.
Figure 4.
Directional changes in temperature in North America, 1930–2004, with respect to the existing LD50 niche.
(A) The contemporary environmental space showing grid cells that moved either toward or away from the existing LD50 niche centroid due to directional changes in minimum temperature of the coldest month or maximum temperature of the warmest month (colored grid cells, legend in panel C); grid cells that did not experience directional change are not shown. Labeled regions delineate four environmental spaces with respect to different thermal niche fitness thresholds for the house sparrow (see Fig. 3). (B) Pie charts show the proportions of grid cells moving either toward or away from the existing LD50 niche centroid within each of the four spaces. (C) Geographic projection of grid cells that moved either toward or away from the existing LD50 niche centroid of the house sparrow. Positive (+) = moving toward the niche centroid, negative (–) = moving away from the niche centroid.