Contrasting assembly mechanisms between lowland and highland spider communities of Sulawesi, Indonesia

The relative importance of in-situ diversification versus colonization by pre-adapted lineages in montane community assembly remains poorly understood. Here, we leverage the geologically complex island of Sulawesi, Indonesia as a natural experiment to test competing hypotheses of community assembly. Using DNA barcode sequences from 2,357 spider specimens collected along elevational gradients on three mountains, we found that both elevation and mountain identity independently explained significant variation in community composition. Lowland-highland comparisons showed near-maximum phylogenetic dissimilarity regardless of mountain comparison, indicating a fundamental elevational split. Within the same elevation zone, however, communities were more similar within mountains than between them, revealing a hierarchical structure. This pattern was stronger for high-elevation communities, with evidence of clade diversification and increased nestedness. Phylogenetic dissimilarity increased steeply with elevational distance within mountains, then flattened and converged with between-mountain patterns. Widespread phylogenetic overdispersion across all communities suggests historical priority effects rather than contemporary competition shape assembly. Our results support a split-community model of assembly for Sulawesi's spiders and are consistent with a scenario in which lowland radiations on paleo-islands generated mountain-specific endemism, while recent highland communities assembled from pre-adapted lineages that diversified within elevational bands. This hierarchical assembly demonstrates how historical isolation and ecological opportunity interact to structure biodiversity on complex islands over evolutionary time.