Figure 1 . Conceptual illustration of project workflow and niche
characterization, using Astragalus alpinus L. (‘alpine
milkvetch’; photo in (a )) as an example. (a ) We aggregated
georeferenced specimen records from GBIF and iDigBio. Here, GBIF records
for A. alpinus are shown in blue. North American mountain ranges
are highlighted in grey. These records were then integrated with climate
data from WorldClim to build species distribution models (SDMs) using
MaxEnt; (b ) SDMs provided a probabilistic view of where we
expected species to occur based on their known distributions. In this
example, warmer (cooler) colors represent higher (lower) probability ofA. alpinus occurring at that site (this species primarily occurs
in and around higher elevation habitat in the Northern Hemisphere). The
SDMs were then parsed and analyzed in two different ways, as follows:
(c ) We assigned each species a presence/absence rating to each
site based on its SDM (requiring at least 5% across at least 25% of
the area covered by that site). These presence/ absence matrices allowed
us to estimate expected species richness of species in our dataset
across different mountain ranges (Fig. 2). (d ) Separately from
(c ), we also parsed each SDM by three abiotic
variables—temperature, precipitation, and elevation— to build a
quantitative, continuously-valued description of each species’ occupied
niche space. In this case, the SDM of A. alpinus describes a
range centered within montane habitat bounded between -17 and +10
degrees C° and generally below 1000 mm of precipitation. For temperature
and precipitation, the horizontal axes in each plot indicate values for
the climatic variable and the vertical axis indicates the fraction of
the SDM attributable to those values. For elevation, axis orientation is
switched; vertical axis represents elevational categories (alpine areas
in blues, montane in purples, and foothills or lowland in greens) while
the horizontal axis indicates the SDM fraction falling into each
category. Thus, the climatic niche of each species was described by
three vectors, which jointly describe the weighted proportion of each
species range falling within different abiotic conditions.