Results
Our original models suggested that the elevational distributions of 19
of the 32 species shifted (Tables S1-S4). Simulations indicated high
confidence that the shifts of 12 species were not an artifact of
stochasticity (Table S5 and S6). Further discussion focuses on these 12
species.
In the western Great Basin, the elevational distributions of four
species shifted. The distributions of three species shifted downslope
along the full elevational gradient: House Wren (Troglodytes
aedon ) by 99 m, Black-headed Grosbeak (Pheucticus
melanocephalus ) by 65 m, and Lazuli Bunting (Passerina amoena )
by 194 m (Table 1). At the lower edge of the elevational gradient, the
distributions of three species, including two that shifted along the
full elevational gradient, changed. Warbling Vireo (Vireo gilvus )
shifted upslope, whereas House Wren and Lazuli Bunting shifted downslope
(Table 1, Figure 2). At the upper edge of the elevational gradient, the
distribution of House Wren shifted downslope.
In the central Great Basin, no species shifted along the full
elevational gradient. However, movement at the edges of the elevational
ranges was relatively common. At the lower edge of the elevational
gradient, five species’ distributions changed. Brewer’s Sparrow
(Spizella breweri ) and Lazuli Bunting shifted upslope, whereas
Broad-tailed Hummingbird (Selasphorus platycercus ), Mountain
Chickadee (Poecile gambeli ), and Northern Flicker (Colaptes
auratus ) shifted downslope (Table 2, Figure 3). Lazuli Bunting was the
only species that shifted within the lower edge of the elevational
gradient in both the western and central Great Basin. However, the
species shifted downslope in the western Great Basin and upslope in the
central Great Basin. At the upper edge of the elevational gradient, the
elevational distributions of five species changed. Mountain Chickadee,
Rock Wren (Salpinctes obsoletus ), and Vesper Sparrow
(Pooecetes gramineus ) shifted upslope, whereas Mountain Bluebird
(Sialia currucoides ) and Spotted Towhee (Pipilo maculatus )
shifted downslope (Table 2, Figure 3).
The average distance moved was smaller at elevational edges than along
the full gradient. In the western Great Basin, the absolute value of the
average elevational shift across the full elevational gradient was 119
m, compared to 50 m at the lower edge and 59 m at the upper edge (Table
1). In the central Great Basin, the absolute value of the average
elevational shift was 33 m at the lower edge and 48 m at the upper edge
(Table 2).
Irrespective of distributional shifts, many of the associations between
bird occupancy and temperature or precipitation were statistically
significant, and this information is relevant to understanding species’
responses to climate variables. However, given that the effect of
elevation on temperature or precipitation did not change through time
(e.g., higher elevations do not seem to be warming faster than lower
elevations, or receiving more or less precipitation through time), these
associations should not necessarily be interpreted as drivers of the
observed distributional shifts. Occupancy of 10 of the 12 species with
elevational distributions that shifted was significantly associated with
winter or spring precipitation (Tables 1 and 2). Spring temperature was
associated with shifts of 2 of the 12 species, and NDVI was associated
with shifts of 5 of those species. Only occupancy of Spotted Towhee in
the central Great Basin and Lazuli Bunting in the western Great Basin
was not significantly associated with any of those four variables. Both
associations with spring temperature were positive. In all but two cases
(House Wren in the western Great Basin and Lazuli Bunting in the central
Great Basin), the association with precipitation was negative. NDVI was
significantly related to occupancy of House Wren in the western Great
Basin both across the full elevational gradient and within the lower
edge (positive association; Table 1), and with occupancy of four species
in the central Great Basin (two positively and two negatively; Table 2).
Over the study period, spring temperature, winter precipitation, and
spring precipitation increased significantly in both the western and
central Great Basin (Figure 4). NDVI also changed, but not in a uniform
manner. In the western Great Basin, NDVI was negatively associated with
the interaction of year and elevation, whereas in the central Great
Basin, the association was the opposite (Figure 4); both effects were
small. Over time, NDVI decreased as elevation increased in the western
Great Basin, and slightly increased with elevation in the central Great
Basin.
Discussion
Our results add to a growing body of evidence that many temperate bird
species are not consistently shifting upslope as climate changes.
Elevational distributions of bird species in the western and central
Great Basin shifted in a variety of ways, with a greater number of
distributional shifts occurring at the edges of the elevational gradient
than along the full gradient. All three distributional shifts along the
full elevational gradient were downslope. Our results also illustrate
the importance of assessing population variability in conjunction with
range shifts. In 7 of 19 cases, what initially appeared to be a
deterministic elevational shift is likely attributable to stochasticity.
Although bird occupancy was strongly associated with climate variables,
there was little consistency between an elevational shift and temporal
changes in temperature, precipitation, or primary productivity. The
duration of our time-series data was relatively short, but comparable to
other studies of elevational shifts (Campos-Cerqueira et al. 2017,
DeLuca and King 2017), and indicated considerable plasticity in
elevational distributions.
Our simulations explicitly tested whether the linear effect of year was
driving significant interactions between year and elevation, which we
interpreted as evidence of elevational shifts. Although we did not
collect demographic data, we found that observed shifts of seven species
could reflect stochastic changes in temporal and elevational occupancy.
There are two main reasons why a significant interaction term in the
original model might not be consistent with simulation results. First,
mean occupancy at high or low elevations might differ between the latest
and earliest years, in effect driving a linear trend. Second, annual
mean occupancy of some species is highly variable, and the apparent
linear trend may have been coincidental. Annual variability in abundance
or occupancy of passerines can be caused by factors including conditions
at overwintering grounds, cyclic weather events, or variable migration
mortality. Population dynamics are rarely considered in studies of
distributional shifts, but likely influence the accuracy of detected
shifts (McCain et al. 2016).
Among the environmental variables we examined, only NDVI changed over
both time and elevation. This may be due to the resolution at which the
climate variables and NDVI were measured. Because the resolution of the
climate variables was 4 km, some of our survey points within a canyon
fell within the same pixel. By contrast, because the resolution of the
NDVI data was 250 m, each survey point had a unique value, allowing for
finer-resolution changes in NDVI to be captured in our analysis.
Additionally, spatial variation in temperature and moisture availability
in montane environments is much greater than in lowlands (Suggitt et al.
2011). For example, some narrow montane canyons are prone to temperature
inversions (Curtis et al. 2014, Rupp et al. 2020). As a result,
microclimate in areas with complex topography may be unpredictable, and
short-distance movements may be sufficient for birds to access
microclimates favorable for feeding, mating, or nesting.
The majority of observed elevational shifts occurred at range edges.
Distributional shifts may be more common at elevational edges than along
the full elevational gradient due to higher rates of climate change at
higher elevations and reduced competition at lower elevations (Alexander
et al. 2015). In our study system, plant phenology at higher elevations
can be 21 days later than at lower elevations (Zillig, unpublished
manuscript). As climate change results in increasing temperatures and
earlier snowmelt, birds may shift upslope to take advantage of habitat
that is becoming available earlier in the breeding season, resulting in
changes in species-level occupancy. For example, Rock Wren and Vesper
Sparrow, both of which nest on the ground, moved upslope at the upper
edge of their elevational ranges. Individuals may have dispersed into
nesting habitat that previously was covered in snow or did not green up
until late in the breeding season.
At the lower range edges, elevational movement may be driven by
downslope expansion of riparian vegetation. About 60–70% of the
vertebrate species native to the Great Basin, including most of the
birds that breed in the region, are associated with riparian areas
(Brussard et al. 1998, Poff et al. 2011). NDVI significantly increased
during the time span of our study, with NDVI at higher elevations
increasing faster in the central Great Basin and NDVI at lower
elevations increasing faster in the western Great Basin (Figure 4). The
primary productivity and extent of riparian areas may be expanding in
some parts of the Great Basin in response to greater water-use
efficiency as concentrations of carbon dioxide increase, especially
where the intensity of livestock grazing is decreasing, as it is in our
study system (Dwire et al. 2018, Fesenmyer et al. 2018, Albano et al.
2020). In the western Great Basin, species that nest in riparian
vegetation, such as Lazuli Bunting and House Wren, may be moving
downslope in response to expansion of that nesting habitat at the lower
edge of their elevational ranges.
In all but two of the 13 instances in which winter or spring
precipitation was significantly associated with occupancy, the
association was negative (Tables 1 and 2). In general, one would expect
bird communities in arid ecosystems to respond positively to increased
precipitation, as many species may be limited by water availability
(Bolger et al. 2005, Riddell et al. 2019). Our counterintuitive result
may be explained by climate change-driven changes in precipitation
across the Great Basin. Increased winter and spring precipitation across
much of the western United States, including the Great Basin (Chambers
2008), is driven in part by increasingly severe storms (Xue et al.
2017), which could affect birds adversely. Increases in precipitation
also may have delayed the breeding season or decreased survival or
reproduction, leading to a decrease in occupancy (Kozlovsky et al. 2018,
Zuckerberg et al. 2018). Moreover, the proportion of precipitation that
falls as rain rather than as snow is increasing, resulting in decreases
in snow depth, earlier snowmelt, and water inputs to the soil becoming
earlier and more sporadic (Abatzoglou and Kolden 2011, Petersky and
Harpold 2018).
Our inferences might be biased if the elevational gradient we surveyed
did not encompass species’ full elevational distributions in our study
regions. However, our point-count locations appeared to capture the
upper limits of each species’ elevational distribution, and the lower
elevational limits of most species (Figures S1 and S2). Of the 32 bird
species we examined, the elevational ranges of 12 appeared to shift.
With the exception of a 194 m downslope shift by Lazuli Bunting in the
western Great Basin, the shifts were less than 100 m. The breeding
ranges of all species examined extend beyond our study system. We
acknowledge that the bird populations we examined may be responding to
climate change differently than populations in wetter, less
topographically diverse systems, and do not suggest that
population-level responses we observed are necessarily indicative of
species-level responses.
Although temperature and precipitation changed considerably even over
the 10-20 years of our study, few elevational shifts were significantly
related to temperature. Diel temperature in our study canyons during the
breeding season is highly variable: day and night differ by as much as
19°C (M. Zillig unpublished data). We suspect that Great Basin bird
populations have relatively broad thermal tolerances, consistent with
higher tolerances of temperate than tropical bird species to high and
low temperatures after controlling for body mass and experiential
humidity (Pollock et al. 2020). The elevational shifts that we observed
were relatively rapid. The lack of consistent associations between
elevational shifts and temperature or precipitation suggest that birds
may be responding to elements of habitat that are indirectly associated
with our measured variables or with those that we did not measure, such
as competition or quantity and quality of food.
We are aware of no other studies that examine elevational range shifts
in arid bird populations. Our results reinforce that not only are
responses to climate species-specific, but birds respond to numerous and
compounded types of environmental change. Great Basin bird populations
may be responding to climate change through shifts within the edges of
the elevational gradient, yet the lack of a strong overall
climate-response signal suggests that these populations may be
relatively resilient to climate change.