Niche ecology across time in an adaptive radiation of Hawaiian spiders

The Hawaiian Tetragnatha spiders are a spectacular example of adaptive radiation, comprising over fifty species with strikingly diverse morphologies and behaviors. Many species within this radiation retain the ancestral web-building behavior, but a large clade (the Spiny Leg Clade) has abandoned web-building and adopted a cursorial hunting strategy. We seek to understand the role that ecology has played in the diversification of the Hawaiian Tetragnatha. Hawaii’s age-structured arrangement of islands from oldest (Kauai) in the northwest to youngest (Hawai’i) in the southeast presents the ideal system to study evolutionary processes over time. Using web structure, microhabitat preference, and carbon and nitrogen stable isotope signatures, we measured niches of Tetragnatha species in four communities of different substrate ages. Our results indicate that within each site, co-occurring species partition the niche space by using species-specific web structures and selecting unique assemblages of host plants. The web structures used by each species may be particularly suited to capturing different assemblages of prey taxa; and interspecific differences in spiders' stable isotope signatures lend support to this hypothesis. Along the age gradient, dramatic differences in δ¹⁵N were found across all taxa, with a clear stepwise pattern of increasing δ¹⁵N with increasing substrate age from ca. 250 to 700,000 years old. Measurements of δ¹³C showed no signal of substrate age, but did show predictable patterns of differences between taxa at all sites. Intriguingly, within each site, δ¹⁵N was consistently higher in web-building Tetragnatha than in their cursorial Spiny Leg congeners. This suggests a significant difference in trophic level among the two groups, with web-building Tetragnatha occupying a higher trophic position than the actively hunting Spiny Leg spiders. Our next step is to identify the taxonomic assemblage of spiders' diets using gut content metabarcoding.