Introduction
Changes in elevational distributions of diverse taxonomic groups are
occurring globally, with the potential to reshape ecological
communities, alter ecosystem function, and affect climate (Pecl et al.
2017). Given that temperature generally decreases by 0.6°C per 100 m
increase in elevation (Barry 2008), and the assumption that species
distributions are limited by temperature, a dominant paradigm suggests
that montane animals and plants will move upslope in response to climate
change (Martin 2001, McNab 2003). Theories of upslope elevational shifts
in multiple taxonomic groups are often supported by field data (Chen et
al. 2011, Freeman et al. 2018). However, in many instances, populations
and species are not moving in synchrony with warming temperatures and
either have not shifted or shifted downslope (Tingley et al. 2012,
Campos-Cerqueira et al. 2017, DeLuca and King 2017, Freeman et al.
2018). The interacting effects of temperature, precipitation, and other
climatic variables could lead to large variation in interspecific or
intraspecific responses to climate change (Tingley et al. 2012). The
direct or indirect effects of land use may further lead to unpredictable
shifts in species’ distributions (Fleishman and Murphy 2012).
Understanding mechanisms of elevational range shifts is complicated by
the fact that climate variables are not changing uniformly across
elevational gradients. For example, in some cases, high elevations are
warming more rapidly than low elevations (Pepin et al. 2015).
Precipitation rates also typically are greater at high elevations, and
climate change is projected to strengthen this relation (Barry 2008, Van
Tatenhove et al. 2019). Theoretically, differences in rates of climate
change along elevational gradients may result in larger distributional
shifts at the upper edges than at the lower edges of a species’
elevational distribution. Additionally, biotic interactions, especially
competition, may be stronger at lower distributional limits, stabilizing
distributions at the lower edge (Alexander et al. 2015). Conversely,
high-elevation taxa often have greater thermal tolerance than
low-elevation taxa, proportional to the magnitude of seasonal and diel
thermal variation at high elevations (Janzen 1967, Deutsch et al. 2008).
High-elevation species’ physiology may therefore result in relatively
small elevational range shifts.
Birds, especially long-distance migrants, are highly vagile, and can
track microhabitats within and across years (Greenwood and Harvey 1982,
Cline et al. 2013, Gow and Stutchbury 2013). Elevational range shifts of
birds have primarily been examined in tropical regions, as species
richness and endemism of birds often is concentrated in tropical
mountains. Tropical species are generally more physiologically sensitive
to temperature than temperate species, and the extent of temperature
tracking may be stronger in tropical than in temperate bird species
(Pollock et al. 2020, Freeman et al. 2021). However, temperate regions
are warming at a faster rate than tropical regions (Friedman et al.
2013), and research on the effects of climate change on non-tropical
montane species is essential to understanding how elevational shifts may
affect global responses to climate change. Recent research on bird
species in North America has identified a variety of elevational shifts.
The distributions of 84% of avian species in the Sierra Nevada that
were documented by Grinnell in the early 1900s shifted over the past 100
years, with 51% of species moving upslope and 49% moving downslope
(Tingley et al. 2012). Over 16 years, 9 of 16 low-elevation passerine
species in the northern Appalachian Mountains shifted an average of 99 m
upslope, whereas 9 of 11 high-elevation species shifted an average of 19
m downslope (DeLuca and King 2017). In the Adirondack Mountains, repeat
surveys found that abundance-weighted mean elevational distributions of
42 species shifted upslope by 83 m over 40 years, with shifts observed
at the both the upper and lower elevational range edges (Kirchman & Van
Keuren 2017). Although elevational shifts in temperate bird communities
appear to be more variable than those in tropical communities (Freeman
et al. 2021), sizable yet unexplained variation among species is
apparent in both temperate and tropical regions.
Mechanisms of elevational range shifts in birds are difficult to
investigate, in part due to data or sampling constraints. Particularly
among resurveys of historical sampling locations, variation in
distribution or abundance between two time points can impede strong
inferences (Sparks and Tryjanowski 2005, McCain et al. 2016). In
general, upslope shifts are attributed to the direct and indirect
effects of climate change, such as changes in the composition of plant
species, plant phenology, or primary productivity (Morison and Morecroft
2006, Lenior et al. 2008, Amano et al. 2010). Downslope shifts in bird
populations are often attributed to changes in the predator community or
other shifts in interspecific competition (Lenoir et al. 2010).
Theorized mechanisms of stable population distributions include temporal
lags in species’ responses to climate and land-use changes, stochastic
fluctuations in population size, and small magnitudes of climate change
(Parmesan et al. 2005, Tingley and Bessinger 2009, McCain et al. 2016).
However, if variability in population size is high, upslope or downslope
changes in occupancy may reflect stochastic fluctuations. For example,
stochastic increases in population size may lead individuals to colonize
unoccupied locations, whereas population declines may result in vacant
lower-quality habitat (Thomas and Lennon 1999). Annual variability in
abundance may be especially high at the edges of species’ elevational
distributions (McCain et al. 2016). Therefore, accounting for population
fluctuations is necessary to detect a deterministic distributional
shift. Population fluctuations can be identified through long-term data,
which can capture annual oscillations that often are undetectable in
studies comparing two time points, and by inclusion of tests for
population variability in statistical analysis.
We examined whether the elevational distributions of two communities of
birds in the Great Basin are shifting. The Great Basin is a cold desert
characterized by extensive sagebrush shrubsteppe and variable
topography. We are aware of few studies that examined shifts in the
elevational distributions of birds in arid ecosystems (Iknayan and
Bessinger 2020), where species may be at the edges of both their thermal
and xeric tolerances. We examined data from long-term, nearly continuous
avian point-count surveys in two regions of the Great Basin to explore
potential mechanisms of the shifts. These data span a considerably
larger area (>100 km) and greater number of elevational
transects (35), and characterize annual variability in occupancy more
rigorously, than most resurveys of birds and other taxonomic groups
(e.g., Moritz et al. 2008, Tingley et al. 2012). We used single-species
occupancy models of 32 species to examine elevational movement at three
spatial extents: the full elevational gradient and the lowest and
highest 25% of the elevational gradient. Additionally, we examined the
effects of temperature, precipitation, and primary productivity on
single-species occupancy and elevational movement.