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

Questions: Small islands are outstanding model systems to study community assembly. Due to harsher environmental conditions on smaller islands compared to larger ones, environmental filtering may preclude some species, potentially resulting in island size-dependent species pools. We tested whether the species pool size follows a similar species–area relationship as the observed richness. This can provide new insight into community assembly processes and the elusive small-island effect (SIE), which states that species richness on smaller islands is less dependent on area than on larger islands. 

Location: Raja Ampat Archipelago, Indonesia. 

Methods: We studied the woody vegetation on sixty small islands ranging from 3 m² to 11,806 m². For each recorded species, we estimated its area requirements and compared them against random colonization models. We developed a novel method to calculate probabilistic species pools for each island. We compared different species–area models for observed species richness and our index of species pool size to test whether the SIE results from differences in species pool size. 

Results: We found that most species were restricted to islands significantly larger than expected from random colonization. The occurrence probability of all species increased with island size, indicating a lack of species that are specialized to the conditions on small islands. We found a SIE in observed species richness, but not in species pool size. 

Conclusion: Woody plants in the studied island system have specific requirements that are linked to island area and determine island-specific species pools. Lower community completeness on smaller islands compared to larger ones indicated that the SIE is shaped by local limiting processes that have no impact on the species pool, but control how much of it is realized on an island. Together, these results clearly indicate non-random plant community assembly on small islands.