Dataset considerations.
Our distribution model (SDM) approach, rather than the exclusive use of
point occurrence records, helped to overcome issues of incomplete
sampling (Barthlott et al., 2007; Meyer et al., 2015; Brummitt et al.,
2021), and provided broader, macroecological insight into how species in
lowland, montane, and alpine regions differ in terms of their occupied
climatic niche space. In general, while the quality and extent of
species sampling (Fig. 2) was good, we were unable to generate SDMs for
every American seed plant. With 68,241 species matched to our phylogeny,
we covered only ~18% of estimated global seed plant
lineages (Stevens, 2001), and less than ~48% of the
total estimated American floral diversity (Ullola Ullola et al., 2017).
Further, the threshold minimum of twelve records to build reliable SDMs
certainly excluded numerous range-limited endemics, which is likely to
disproportionately impact representation of Andean alpine lineages,
where the degree of alpine endemism is substantial (Hughes and Eastwood,
2006). With these limitations in mind, we have attempted to draw broad,
macroecological conclusions that are unlikely to be significantly
impacted by loss of models for range-limited endemics. Nevertheless,
species excluded as a result of the conservative nature of our data
cleaning protocol highlight the need for increased sampling of
range-limited endemics to enhance the broader understanding of how such
species impact macroecological patterns and conclusions.
Further, although we have grouped communities by elevational categories,
important differences between Northern and Southern Hemisphere alpine
habitats, such as the degree of seasonality, growing season length, and
extent and duration of snow cover, should be taken into account when
forming conclusions from this macroecological dataset. For instance, in
our dataset, we did not find a correlation between species richness and
elevation in alpine-only communities (data not shown), and it may be
that there are many avenues by which a region or local community can
achieve a high level of biodiversity (Hughes, 2017). Future studies
should investigate how more local-scale processes might be influencing
the observed macroecological patterns we find here.
Contrasts across elevation belts and specialization .
A species pool defines a collection of organisms in the region. However,
another aspect of a species pool is the subset of that potential
community species pool that is able to colonize and survive in the local
setting (Emerson and Gillespie, 2008). Although myriad factors determine
which plant species are able to persist in montane and alpine regions
(Körner, 2003), here we investigated a portion of the macroscale abiotic
niche space occupied by American seed plants to provide insight into how
factors such as temperature and precipitation might impact species
distributions (Moles et al., 2014). Compared to non-alpine species,
those inhabiting alpine habitats, regardless of whether such species had
ranges centered in the alpine belt or elsewhere, showed unique richness
patterns (Fig. 2c), occupied a drastically reduced climatic niche space
(Fig. 3d), and showed broad temperature but narrow precipitation niche
breadth (Fig. 4d). Elsewhere, we have also shown that these alpine
species showed distinct patterns of phylogenetic diversity (Figueroa et
al., pers. comm. ). In contrast, species in the montane belt were
relatively similar to both the species pool overall and lowland taxa in
their observed richness patterns, occupied niche space, niche breadth
(Fig. 2-4), and phylogenetic diversity (Figueroa et al., pers.
comm. ). These findings underscore the unique confluence of factors that
define plant diversity at the extreme elevational limits of the alpine
belt (Körner, 2003) and reiterate the complexity of defining an
appropriate regional pool for such an ecosystem.
These results showed a notable separation between the occupied climatic
niche space of alpine and non-alpine communities. That there was not a
similarly clear separation between montane and lowland communities
raises the question of what distinguishes montane communities from
adjacent assemblies. One distinguishing factor is certainly their
greater plant diversity (Grytnes and Vetaas, 2002; Grytnes, 2003;
Cardelús, Colwell, Watkins, 2006). Additionally, montane environments at
tropical latitudes display greater phylogenetic affinity with temperate
lowlands (González-Caro et al., 2020), suggesting cooler-climate
corridors have been used to track amenable climate, allowing montane
species to colonize lowlands (Donoghue, 2008).
In this study, we found that niche breadth tended to be maximal for
montane generalists (Fig. 6), indicating the possibility of montane
regions representing a transitional zone, in which the greatest variety
of forms from other elevations could coexist. This is conceptually
analogous to the often-observed mid-elevation peak in species richness
(Grytnes and Vetaas, 2002; Grytnes, 2003; González-Caro et al., 2020).
However, our results differ subtly but importantly from such
observations in two ways. First, our elevational categories are based on
habitat type (Körner et al., 2011) and not absolute elevation per
se . Second, we demonstrate that montane communities occupy a broad
climatic niche space and have greater niche breadth than alpine
communities. These niche characteristics are not identical to species
richness, though they could be influenced by it.
Concomitantly, montane communities could also be distinguished from
alpine ones by factors other than those examined here, such as soil and
mineral conditions (Egli and Poulenard, 2016), pest and pathogen
distributions (Rasmann et al., 2014), and/or the distributions of
dispersal agents. Additionally, although at the regional scale montane
communities overlapped in climatic niche space with lowland sites, this
did not mean montane community composition matched that of lowland areas
occurring at the same latitude. At the same time, the greatly reduced
alpine diversity in Central America might suggest a greater role for
dispersal limitation for alpine lineages compared to montane ones
(Figueroa et al., pers. comm. ), which could also contribute to
explaining differences in distributional patterns between alpine and
montane communities in this region.
In addition to contrasting the climatic niches of species occurring
within different elevation belts, our SDM approach allowed us to
incorporate some biological variation in the climate species experience
across their ranges, while also quantifying the fraction of each
species’ range occurring in different climatic conditions. We could thus
distinguish between species with ranges centered in alpine or montane
habitat (i.e., high-elevation ‘specialists’) from those whose ranges
extended into higher elevations but were centered in different habitats
(i.e., high-elevation ‘generalists’). These distinctions influence what
constitutes an alpine species (Körner, 2003) and could reflect different
adaptive responses and tolerances. Under our classifications, we found
that alpine generalists differed significantly inBTEMP from both alpine specialists and non-alpine
species (regardless of elevation).
These results might suggest different strategies are needed by alpine
generalists and specialists with respect to temperature responses, as
the generalist species may encounter a wider range of temperatures than
the specialists. At the same time, in this dataset, genera with alpinespecialists had a greater fraction of both alpinegeneralists and montane specialists (Supplemental Fig. S2;
Figueroa et al., pers. comm. ). This could indicate that alpine
specialists derive mainly from alpine generalists and montane
specialists (i.e., diversification has occurred as species encountered
novel, higher-elevation habitat), consistent with a ‘montane speciation
model’ (sensu Roy, 1997; but see Dagallier et al., 2020). It
could also indicate that alpine specialists tend to arise within
lineages having greater evolutionary potential to adapt to
high-elevation conditions, even if the strategies involved differ among
these lineages (Folk et al., 2020; Martínez-Padilla, Estrada, Early,
García-Gonzalez, 2017). However, we also acknowledge that our climate
analyses incorporated only macroscopic conditions and did not address
ways in which species at high elevations find and create microclimatic
conditions to enhance survival (Körner, 2003; Ohler, Lechleitner,
Junker, 2020).