Questions: Habitat islands are often characterized by the presence of more or less sharp boundaries to adjacent matrix habitats. However, knowledge on boundaries of natural habitat islands is scarce, especially regarding patterns of beta diversity and its two underlying components: species turnover and nestedness. We therefore aim to quantify the effects of fine-scaled and sharp boundaries of quartz islands (quartz-gravel covered soils) on the different components of plant beta diversity and how they are linked to different soil environmental drivers.

Location: Knersvlakte, Western Cape, South Africa.

Methods: We sampled plant species richness in 56 fine-scale transects of 6 × 1 m² plots across eight different boundary types (four quartz island – matrix, four between habitats on quartz islands). Soil depth and chemistry (pH, electrical conductivity) were analyzed for each 1 m² plot. Differences in the two beta diversity components (turnover and nestedness) for each boundary type were tested by t-tests. We used linear models to test relationships between species and environmental dissimilarity.

Results: All boundary types showed high beta diversity. Species turnover was the prevailing component for six boundary types, the nestedness component was only important for two boundary types. We found a significant linear increase of species dissimilarity with increasing dissimilarity in soil pH and distinct plant communities for the habitat types, but no significant increase for electrical conductivity or soil depth.

Conclusions: The spatial distinctiveness of the quartz islands leads to sharp boundaries, which result in high beta diversity, mainly through species turnover. This reflects the high levels of diversification and adaptation of the local plant communities. Nestedness occurred at two boundaries to the matrix, indicating that the latter does not necessarily represent an impermeable boundary for all species of the respective ecosystem. Studying diversity patterns across boundaries contributes to the question of applicability of island biogeography theory to habitat islands.

Eibes_etal_JVS_RA_05693_environmental data:

ID: Plot ID, combining "transect_no", "boundary_type" and "transect_quadrat" of the respective 1m² plot

transect_no: Transect number, 56 transects in total

electrical_conductivity: Electrical conductivity of mixed soil sample per quadrat (mS/cm)

pH: soil pH of mixed soil sample per quadrat

soil depth: mean soil depth per quadrat (in cm) transect_quadrat: Position of 1m² plot (=quadrat) along the transect (see schematic figure)

boundary_type: The boundary/transition type, in total 8 different types, see habitat legend

habitat_A: Habitat type of site A of the respective transect

habitat_B: Habitat type of site B of the respective transect

habitat: Habitat type of the quadrat, 6 different habitat types

FS Argyroderma fissum - Cephalophyllum spissum (quartz habitat)

D Dicrocaulon brevifolium (quartz habitat)

R Ruschia burtoniae (quartz habitat)

S Salt pan (quartz habitat)

NQF Non-quartz field (matrix habitat)

H Heuweltjies (matrix habitat)

Eibes_etal_JVS_RA_05693_vegetation data: ID: Plot ID, combining "transect_no", "boundary_type" and "transect_quadrat" of the respective 1m² plot

Rownames: Scientific (or preliminary) species names of the 101 present plant species