Diverging responses of taxonomic, functional, and phylogenetic plant diversity to biogeographic parameters in arid habitat islands of southern Africa
Data files
Feb 12, 2026 version files 39.06 KB
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island_indices.csv
2.47 KB
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README.md
6.94 KB
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species_x_islands.csv
18.76 KB
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traits_x_species.csv
10.89 KB
Abstract
Habitat islands conceptually bridge true islands and habitat patches, adding new perspectives to island biogeographic principles that shape community assembly processes. Despite growing interest, empirical studies are scarce, and the drivers of biodiversity in these systems, such as different aspects of island shape, isolation, and habitat diversity, remain unclear. Our aim was to assess how biogeographic parameters influence taxonomic, functional, and phylogenetic plant diversity on habitat islands using quartz islands in South Africa as our model system. We conducted floristic surveys on 20 quartz islands in the harsh and arid summer-rainfall region around Pofadder (Nama–Karoo, South Africa) to quantify species richness, functional dispersion, and phylogenetic mean pairwise distance. Via remote sensing, we derived island metrics and evaluated their effects on the diversity metrics, using multivariate regression analysis. We fitted negative binomial generalized linear models for species richness and linear models for functional and phylogenetic diversity. Island area emerged as the strongest predictor of species richness, consistent with island biogeographic theory. Connectivity was a significant predictor for functional dispersion and explained most of the variation. The mean pairwise distance was significantly influenced solely by the area of the nearest neighbour island, although its explanatory power was only moderate. Our findings highlight that different facets of diversity respond distinctively to biogeographic parameters. Integrating taxonomic, functional, and phylogenetic perspectives is therefore essential for a comprehensive understanding of biodiversity patterns on habitat islands and for providing valuable insights for conservation.
Dataset DOI: 10.5061/dryad.15dv41p8x
Description of the data and file structure
Summary:
The data were collected during field surveys (February and March 2024) on 20 quartz islands in the Pofadder region (Northern Cape, South Africa) to investigate vascular plant species composition, plant functional traits, and island biogeographic characteristics. Species occurrence was recorded as presence–absence per island, and functional traits of perennial plant species were measured on multiple individuals to derive species-level mean trait values. In addition, island shape, isolation, and habitat diversity indices were calculated on the basis of island polygons derived from a quartz island classification using satellite imagery and elevation-based landform classifications.
The data consist of following files which were used to do our analysis:
island_indices.csv: Calulated island biogeographic indices
species_x_islands.csv: Vascular plant species occurence on quartz islands
traits_x_species.csv: Functional trait data measured for perennial plants occurring on quartz islands
Files and variables
File: island_indices.csv
Description: This file contains eleven island parameters for the surveyed quartz islands of the Pofadder region. Island polygons and landform data retrieved from our quartz island classification were used to calculate parameters describing island shape, isolation and habitat diversity indices.
Variables
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Insel ID: Surveyed quartz island.
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Area [m²]: Area of the target island.
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Perimeter [m]: Perimeter of the target island
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Shape Index: Describes island shape complexity (Schrader et al., 2019) based on (Patton, 1975).
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Nearest Neighbour Distance [m]: Edge-to-edge distance from target island to nearest neighboring island.
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Five Nearest Neighbours Mean Distance [m]: Mean edge-to-edge distance from target island to its 5 nearest neighboring islands.
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Nearest Neighbour Area [m²]: Area of the nearest neighboring island.
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Five Nearest Neighbours Mean Area [m²]: Mean area of the 5 nearest neighboring islands.
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Neighbour Index: Quantifies the influence of neighboring islands by summing their areas divided by the squared edge-to-edge distance to the target island (Kalmar & Currie, 2006). Compared to the original reference a buffer (1000 m) was introduced to make it comparable to the proximity index and between sites.
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Proximity Index [m⁻¹ ]: Sum of the areas of all islands within a buffer around the target island, divided by the distance to each island’s nearest neighbouring island (Gustafson & Parker, 1994). Buffer was set to 1000 m.
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Habitat Richness: Simple count of distinct habitat types based on topographical analysis. We classified 16 distinct landform classes, including peaks, ridges, valleys, slopes, further subdivided by texture and convexity
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Habitat Shannon Diversity: Quantifies diversity of habitat types using relative area proportions (Shannon, 1948).
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Habitat Diversity Index: Similar to the shape index, but calculated with the total perimeter, including both the outer boundary and internal habitat edges (Patton, 1975).
Literature:
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Eibes PM, Schmiedel U, Oldeland J, Irl SDH (2025) Specific island biogeographic and landscape features shape plant diversity and habitat specialism on edaphic quartz islands in an arid ocean. Scientific Reports 15: 9982. https://doi.org/10.1038/s41598-025-94562-5
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Gustafson EJ, Parker GR (1994) Using an index of habitat patch proximity for landscape design. Landscape and Urban Planning 29: 117–130. https://doi.org/10.1016/0169-2046(94)90022-1
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Kalmar A, Currie DJ (2006) A global model of island biogeography. Global Ecology and Biogeography 15: 72–81. https://doi.org/10.1111/j.1466-822x.2006.00205.x
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Patton DR (1975) A diversity index for quantifying habitat edge. Wildlife Society Bulletin 3: 171–173.
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Schrader J, Moeljono S, Keppel G, Kreft H (2019) Plants on small islands revisited: the effects of spatial scale and habitat quality on the species–area relationship. Ecography 42: 1405–1414. https://doi.org/10.1111/ecog.04512
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Shannon CE (1948) A mathematical theory of communication. Bell System Technical Journal 27: 379–423. https://doi.org/10.1002/j.1538-7305.1948.tb01338.x
File: species_x_islands.csv
Description: This file contains vascular plant species occurrence data for all surveyed islands. Geographic coordinates are not provided because the dataset includes sensitive species that are subject to serious poaching threats.
Variables
- Species: Scientific species name (species)
- Family: Plant family (familia)
- Order: Plant order (ordo)
- island.X: Presence on quartz island X (1 = present, 0 = absent)
File: traits_x_species.csv
Description: This file contains functional trait data for 134 perennial plant species occurring on quartz islands. Continuous traits are calculated as the mean across multiple individuals per species. Qualitative traits were assigned based on the majority expression among sampled individuals.
Variables
- Species: Scientific species name
- Describing Author: Name of the author who originally described the species.
- Number Of Sampled Individuals: Number of distinct individuals per species from different quartz islands whose functional traits were measured and included in the calculation of the mean trait value.
- Plant Height [cm]: Plant height measured in situ from ground level to the highest point of the photosynthetic tissue, excluding the inflorescence.
- Trichomes: Presence of trichomes on the leaf epidermis (1 = present, 0 = absent).
- Prickles: Presence of prickles on the leaf epidermis (1 = present, 0 = absent).
- Waxes: Presence of a distinct wax layer on the leaf epidermis (1 = present, 0 = absent).
- Glands: Presence of glands on the leaf epidermis (1 = present, 0 = absent).
- Stem Succulence: Degree of stem succulence (2 = distinct stem succulence, 1 = slight stem succulence, 0 = no stem succulence).
- SLA [mm²/mg]: Specific leaf area (SLA), defined as leaf area (mm²) divided by dry mass (mg).
- Relative Leaf Water Content [%]: Calculated as: [1 − (dry mass / fresh mass)] * 100.
- Relative Leaf Phosphorous Content [%]: Relative leaf phosphorus content measured using dried leaf material.