Aims
We studied diversity, patterns of endemism and turnover of vegetation
composition in dry inter-Andean valleys (DIAVs) where little
is known about the influence of the abiotic drivers controlling
plant species composition and occurrences, and the life forms that
contribute most to α- and β-diversity, respectively. This study was
focused on DIAVs located in the highlands (800–2800 m) around
the Equator (1°N–5°S). The following questions were addressed: (i)
what differences exist between endemic and non-endemic species
in terms of species number, frequency and abundance? (ii) are patterns
of α- and β-diversity correlated with latitude? (iii) what are the
major environmental drivers controlling spatial patterns in species
composition and occurrence?
Methods
We established 63 transects of 5 × 100 m in areas with DIAV
vegetation, impacted as little as possible by human activities. In
each transect, all mature trees and shrubs were identified and
counted to determine their density. The coverage of terrestrial
herbs was estimated. Generalized additive models were used to
quantify the relationship between α- and β-diversity with latitude.
To record α-diversity, we used the exponential Shannon index.
The Sørensen index was used to measure β-diversity or species
turnover. We used canonical correspondence analysis to determine
species composition and generalized linear mixed models
to quantify simultaneously the determinants of species occurrence
across species and sites. The models were evaluated using
the Akaike information criterion. All analyses were run separately
for trees, herbs and shrubs.
Important Findings
Overall, we found that shrubs was the most diverse life form,
constituting 43% (12% endemic) of the 313 species, followed by
herbs (36%; 11.5% endemic) and trees (20%; 5% endemic). Trees
comprise mostly species with low abundance (e.g. <3 individuals
ha−1), which was also true for herbs, whereas shrubs showed a
larger number of abundant species (>30 individuals ha−1), including
four endemic species. Most of the endemic species were
locally rare, and most of them were restricted to southern valleys,
suggesting low migration rates due to local barriers. We identified
seven species distribution patterns that gave rise to high β-diversity
in the dry inter-Andean valleys. The β-diversity of trees was the
lowest. Herbs had the highest β-diversity, which increased steadily
with geographic distance. Our results further highlight the influence
of disturbance, water availability and low temperature on
plant species composition and occurrence. We also found significant,
contrasting patterns in responses to environmental drivers,
when analyzing our data separately by life form. Our results show
that analyzing diversity patterns separately by life form can shed
new light on the mechanisms that have generated present-day patterns
of plant diversity in DIAVs.
