Species and habitat distribution
This study focuses on the native vascular flora of the Azorean
archipelago, which includes 149 indigenous species, 59 of them being
endemic (Schaefer et al., 2011). The Azores is an oceanic archipelago
located close to the mid-Atlantic ridge in the North Atlantic Ocean,
composed of volcanic islands of recent origin. It is one of the most
remote in the world, being separated by 1376 km from the closest shore
in the Iberian Peninsula (Florencio et al., 2021). Its islands are
located along a NW - SE axis, with the oldest island, Santa Maria, being
located in the southeast, whereas the youngest, Pico, is located in the
central region of the Azores (see fig. 1). The climate is temperate and
wet with mild summers following the Köppen Climate classification
(Borges et al. 2019).
Species occurrences were extracted from the “ATLANTIS initiative in the
Azores” GBIF dataset (GBIF.org). It comprises a total of 1,338,102
georeferenced records at a 500 x 500 m cell resolution (Borges et al.,
2018), including native and non-native species. Taxonomic and temporal
data cleaning procedure is detailed in Leo et al. (2021), resulting in
135,014 suitable records of native vascular plants.
We considered three types of habitats representing the dominant, less
disturbed onesin most Azorean islands: native forests, characterized by
a dense tree and shrub cover of small stature and currently restricted
to high elevations in the islands (detailed description of the current
types of native forest can be seen at Elias et al. 2016); naturalized
vegetation patches mostly composed by native plants and located outside
the native forest; and seminatural pastures, which are mid- and high-
elevation pastures that maintain some native plants and are subject to
short-term cattle grazing activity and low inputs of fertilizers (fig.
1). The first two categories were already delimited in the shapefiles
developed by Picanço et al. (2017), and the latter corresponds to the
areas of pastures located above 400 m. Altitudinal data were extracted
from a digital elevation model with a 25 m spatial resolution of the
islands (EU-DEM v1.1, 2021). Then, we calculated the area and percentage
of each habitat type for each 500 x 500 m cell of the occurrences grid;
in the case of seminatural pastures this was done by intersecting the
altitudinal and pasture shapefiles. These analyses were performed using
the QGIS Development Team software (2020). To evaluate if there were any
differences in species richness between habitats, we built species
accumulation curves for each habitat considering only cells covered with
at least 50, 75 or 99% of the focal habitat (Supplementary Material 1).
Since the curves at 50 and 75% stabilized at larger richness than the
99%, we used this latter threshold to assign a cell to a specific
habitat. We set the threshold at 99% cover instead of 100% to consider
a 1% of disagreement due to projection and processing variation of
areas. Thus, cells with less than 99% of a specific unique habitat were
discarded from the analyses. Due to the absence of cells matching this
criterion in Santa Maria and Graciosa, and also the low number of cells
identified in Corvo for at least one habitat, we excluded these three
islands from this study (fig. 1). The number of cells retrieved from
each island and for each habitat within islands is given in
Supplementary Material 1.