RESULTS
Dataset construction and description of abiotic niches .
Our data cleaning and filtering methods produced a dataset of 50,002,722
georeferenced occurrence records spanning 72,372 seed plant species, of
which 68,241 could be matched back to our phylogeny (totaling 5397
genera or ~18% of total estimated seed plant lineages;
Stevens, 2001; Smith and Brown, 2018), with broad geographic coverage of
the Americas, including all mountains (Fig. 2a). There were notable
geographic areas of poor sampling, including the Amazon basin and
extreme southern and northern latitudes. From these occurrence records,
we built species distribution models (SDMs) to characterize mean annual
temperature (MAT), precipitation (MAP), and elevation niches.
Importantly, these niches were characterized as continuously valued
distributions and not as scalar values. Figure 1 illustrates an example
of this niche characterization for a single species, Astragalus
alpinus L. (‘alpine milkvetch’), whose composite niche is centered
within montane habitat (i.e., >60% of the modelled range
falls within montane area) and is bounded between approximately -17° and
+10° C, and generally below 1000 mm of precipitation. This type of niche
characterization allowed us to define the fraction of each species’
range occurring within different combinations of abiotic conditions.
Species richness in different elevational categories .
Comparing species richness across different elevational belts revealed
contrasting patterns of diversity for species occupying lowland
(Nlowland=36,420), montane
(Nmontane=33,015), and alpine
(Nalpine=2937) habitats. Across all seed plants, we
observed a traditional latitudinal diversity gradient, with richness
peaking near the equator (Fig. 2a). This pattern was nearly identical
when considering solely montane communities (i.e., species with at least
5% of their range in the montane belt; Fig. 2b). For alpine
communities, however, species richness was latitudinally bimodal,
peaking in both the western North American cordillera and the central
Andes (Fig. 2c).
Climatic niche space of lowland, montane, and alpine communities .
The temperature and precipitation niche space occupied across all
species in our dataset (Fig. 3a) generally mirrored that defining the
climate space of the regional biomes, indicating that our modelled
distributions recapitulated the likely climatic niche space of the
Americas overall. Lowland and montane species occupied similar regions
of climatic space as seed plants generally (Fig. 3a-c), though lowland
taxa exhibited a greater density at higher precipitation values, and
very few lowland species had ranges centered in habitats with mean
annual temperature below freezing (though certainly many had ranges
extending into these regions). Indeed, montane species seemed to occupy
a greater total niche volume than lowland taxa here. In contrast, the
climatic niche space of alpine species (Fig. 3d) was drastically reduced
and shifted toward colder and drier habitats. Alpine communities were
more uniform in their occupied precipitation niche space, but somewhat
bimodal in the occupied temperature space, with a small, dense cluster
of species having ranges centered on areas with -10° C MAT.
Climatic niche breadth across elevational gradients .
The distributions of average, standardized, niche breadths (B )
across all American seed plants (Fig. 4a) were generally narrow
(BTEMP = 0.130 ± 0.08; BPREC = 0.099 ± 0.08), but with long tails, indicating much greater niche
breadth for only some species as relatively few species had both largeBTEMP and BPREC . The niche
breadths of montane species were generally similar to those of seed
plants overall (Fig. 4c), though montane species had increasedBTEMP . Alpine species, however, occupied a
reduced total niche breadth space, characterized by broadBTEMP and narrow BPREC (Fig. 4d).
Generalist species within each elevational category had maximalBTEMP (Fig. 5, top row ), with an abrupt
increase in BTEMP observed for alpine
generalists. BPREC , in contrast, was generally
narrow regardless of elevation or specialization (Fig. 5, bottom
row ). Across our dataset, genera with alpine specialists had a
greater fraction of both alpine generalists and montanespecialists (Supplemental Fig. S2; Figueroa et al., pers. comm. ).
Spatial variation in community-averaged niche breadth (Supplemental Fig.
S3) indicated higher BTEMP at greater absolute
latitudes (particularly in the Northern Hemisphere) and in mountains
compared to surrounding lowlands. In contrast,BPREC was largest near the equator, regardless of
elevation. Most mountains showed lower BPREC than
nearby lowlands, exceptions being some Central American mountains, the
Northern Andes, and the southernmost Andes. As a result, the Northern
Andes are notable here for having greater community-averaged niche
breadth for both temperature and precipitation.
Climatically Similar Non-Alpine (CSNA) species .
Climatically Similar Non-Alpine (CSNA) species, defined here as
non-alpine species whose ranges otherwise overlapped with the climatic
conditions of alpine taxa (Fig. 2d and 6) tended to cluster
geographically in the southwestern US and Mexico (Fig. 2d, 6e), and
therefore also departed from a traditional latitudinal diversity
gradient as their greatest richness occurred at
~20° N. Although regions with higher CSNA
richness harbored the lowest alpine diversity (Fig. 2d), richness
between these groups at these areas was strongly and positively
correlated (R2=0.68, p<0.001; Fig. 6e). CSNA
species ranges (Fig. 6d) contained a significantly greater fraction of
frost-exposed mountain foothills (p<0.01) and lower fraction
of frost-free lowland (p<0.01), compared to other non-alpine
species. To a lesser extent, they also had a greater proportion of their
ranges in the lower montane belt than other non-alpine taxa. At the
phylogenetic scale of the American seed plant flora, there was no clear
separation between alpine and CSNA species (Fig. 6a). However, there
were substantial differences in the taxonomic composition of these
groups at finer phylogenetic scales (Table 1 and online supplement),
with several orders containing CSNA but not alpine species, and only
~28% overlap in genera between CSNA and alpines
(Nalpine_genera=717;
NCSNA_genera=1421;
Nshared_genera=395).