Plants on small islands revisited: the effects of spatial scale and habitat quality on the species–area relationship

Abstract

Understanding how species diversity is related to sampling area and spatial scale is central to ecology and biogeography. Small islands and small sampling units support fewer species than larger ones. However, the factors influencing species richness may not be consistent across scales. Richness at local scales is primarily affected by small‐scale environmental factors, stochasticity and the richness at the island scale. Richness at whole‐island scale, however, is usually strongly related to island area, isolation and habitat diversity. Despite these contrasting drivers at local and island scales, island species–area relationships (SARs) are often constructed based on richness sampled at the local scale. Whether local scale samples adequately predict richness at the island scale and how local scale samples influence the island SAR remains poorly understood. We investigated the effects of different sampling scales on the SAR of trees on 60 small islands in the Raja Ampat archipelago (Indonesia) using standardised transects and a hierarchically nested sampling design. We compared species richness at different grain sizes ranging from single (sub)transects to whole islands and tested whether the shape of the SAR changed with sampling scale. We then determined the importance of island area, isolation, shape and habitat quality at each scale on species richness. We found strong support for scale dependency of the SAR. The SAR changed from exponential shape at local sampling scales to sigmoidal shape at the island scale indicating variation of species richness independent of area for small islands and hence the presence of a small‐island effect. Island area was the most important variable explaining species richness at all scales, but habitat quality was also important at local scales. We conclude that the SAR and drivers of species richness are influenced by sampling scale, and that the sampling design for assessing the island SARs therefore requires careful consideration.

Requirements of plant species are linked to area and determine species pool and richness on small islands

“…Moreover, the estimated and observed richness increased at a similar rate with island area, and both were best described by a sigmoidal SAR (Appendix ). That led us to conclude that the observed richness was a good indicator for the expected total species richness on the islands (see also Schrader et al, ).…”

“…Due to their larger edge‐to‐interior ratio, the studied small islands are more strongly affected by disturbances such as sea spray, solar radiation, or storms (see also Morrison, ; Niering, ; Whittaker, ). Moreover, we observed that certain habitat features, such as the presence of an organic soil layer, were mostly lacking on small islands (Schrader et al, ; Appendix ). These effects may act as strong environmental filters, preventing successful colonization for many species.…”

“…The sampled area per island was roughly proportional to island area. We inventoried all woody plants rooted within the transect and with a diameter at breast height ≥2 cm and recorded species identities (Schrader, Moeljono, Keppel, & Kreft, ). A diameter at breast high ≥2 cm is commonly used as threshold in tropical transects (Brambach, Leuschner, Tjoa, & Culmsee, ; Molino & Sabatier, ), and we expected species ≥2 cm to be well established in the community and to not reflect spontaneous shifts in species composition.…”

“…We considered the power and linear models because they are considered as the best‐fitting SAR for convex and linear shapes (Matthews et al., ; Triantis et al., ) and do not indicate a SIE. We included the Lomolino model as it can be interpreted as a SIE model due the flat slope for small island sizes (Lomolino, ; Schrader et al, ), indicating varying species richness or lesser increase of species richness with area for small islands. The slope then increases rapidly for larger islands, which is typical for a diminishing SIE.…”

On piecewise models and species–area patterns

“…Third, sometimes a surveyed location is involved in a larger‐scale system, for instance, Bahamas, where Morrison () studied the SIE of plants, is actually a portion of the whole West Indies region. The regional SAR patterns could give insight into the causes of local patterns (Schrader, Moeljono, Keppel, & Kreft, ), so islands collected from a larger spatial scale are also suggested.…”