Abstract
We assessed how assemblages of spiders were structured in small Hawaiian tropical forest fragments (Hawaiian, kipuka) within a matrix of previous lava flows, over both space (sampling kipuka of different sizes) and time (comparison with a similar study from 1998). Standardized hand-collection by night was carried out in May 2016. In total, 702 spiders were collected, representing 6 families and 25 (morpho-)species. We found that the number of individuals, but not species richness, was highly correlated with the area of sampled forest fragments, suggesting that kipuka act as separate habitat islands for these predatory arthropods. Species richness was significantly lower in the lava matrix outside the kipuka compared to the kipuka habitats, although there was no statistical difference in species composition between the two habitats, largely because of similarity of non-native species in both habitats. Over the last 20 years, the abundance of non-native spider species substantially increased in both kipuka and lava habitats, in marked contrast to the vegetation that has remained more intact. With endemicity of terrestrial arthropods reaching over 95% in native forests, non-native predatory species present a critical challenge to the endemic fauna.
Highlights
Kipuka are small tropical forest fragments surrounded by recent lava flows on Hawaiian islands
Species richness of spiders was significantly higher in kipuka habitats compared to the surrounding lava (Mean±SE: 10.8 ± 0.6 vs. 6.8 ± 0.6 respectively; Paired T-test, t = 6.34, 9 df, p < 0.001), there was no statistical difference in species composition between the two habitat types (Permanova: F1,8 = 0.06, p = 0.349, Fig. 1)
The total number of individuals across all spider species was strongly associated with the area of sampled forests, suggesting that kipuka act, at least partly, as separate habitat islands for the spiders
Summary
Kipuka are small tropical forest fragments surrounded by recent lava flows on Hawaiian islands. Many of these kipuka are rich ecosystems that have been previously investigated for their importance for native passerine birds, plants (trees and ferns) and picture-winged flies Muir and Price 2008) Due to their spatial arrangement in a volcanic matrix, kipuka can be considered as islands within islands (Tielens et al 2019), and important, and original, to test classical assumptions regarding the combined effects of area and isolation on species assemblages. To the best of our knowledge, no study has yet assessed how non-native spiders colonized kipuka, they represent a major group of predators in this habitat, potentially reducing native populations
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