Background/Question/Methods The spider genus
Tetragnatha (commonly known as the long-jawed orb-weaver, family Tetragnathidae) occurs globally. Throughout most of its range, this genus is remarkably homogeneous in morphology and life history, characterized by an elongate body; long, curved chelicerae (jaws); and the use of a horizontal orb web situated over or near water. However, in the Hawaiian islands,
Tetragnatha has undergone a spectacular adaptive radiation, with several dozen species displaying a striking diversity of appearances, ecologies, and behaviors. Most of these species are not constrained to riparian or lake habitats and have adopted orb web designs of many different shapes, sizes, and orientations in space. Our study seeks to define the ecological niches occupied by five sympatric
Tetragnatha species in Waikamoi Preserve in East Maui, using data on both the physical placement (site choice) and the geometry (architecture) of the spiders’ webs. We use field measurements and digital photography to quantify interspecific variation in both site choice and web architecture, as well as to measure the extent to which various biotic (spiders’ local species richness and density) and abiotic (temperature, humidity, elevation, and time) factors affect these aspects of the niche.
Results/Conclusions While we found no significant effect of either biotic or abiotic environmental variables, there was a significant species effect on both site choice and architecture, particularly in the density (length per unit area) of sticky capture silk in the webs. Differences in architecture, including silk density, have been previously demonstrated to influence the taxonomic assemblage of prey captured in a web. Hence, our results suggest niche partitioning – differences in foraging strategy as well as microhabitat – among the five spider species. Niche partitioning may have played a role in the divergence of these closely related species (currently thought to comprise a clade) from their last common ancestor. Though the exact circumstances of the spiders’ divergence are not yet fully understood, sympatric ecological speciation has been suggested for this group. By measuring the ecological and behavioral niches presently occupied by these species, we can gain crucial insights into the processes that have shaped their evolutionary history.
