The Effect of Alien Predatory Ants (Hymenoptera: Formicidae) on Hawaiian Endemic Spiders (Araneae: Tetragnathidae)

Abstract

The fauna of the Hawaiian Islands is characterized by spectacular species radiations with high levels of endemism, which is coupled with an extreme vulnerability to invasion by alien species. Of all alien invertebrate predators, ants are most notorious in their effect on native Hawaiian biota. This study examined distribution of ants in mesic and wet forests throughout the Hawaiian Islands and the extent to which they overlap the range of representatives of a lineage of endemic Hawaiian invertebrates, the genus Tetragnatha (Araneae: Tetragnathidae). Two species, Pheidole megacephala (F.) and Anoplolepis longipes (Jerdon), were implicated in the exclusion of native spiders from native and disturbed forest. One species, Solenopsis papuana Emery, showed extensive overlap in its range with that of the native spiders. However, we found a significant inverse relationship between the abundance of S. papuana in an area and the diversity of the indigenous Tetragnatha. Interactions between the spiders and the two species of ants, P. megacephala and A. longipes, were conducted in the laboratory and indicated that the spiders were very vulnerable to attack by these ants. Alien spiders appear to tolerate the presence of ants because they have either a strong exoskeleton, can appendotomize their legs, or else are capable of wrapping the ant in silk. Spiders that normally coexist with ants appear to use one or more of these methods for defense. The riparian existence of the genus Tetragnatha outside Hawaii may protect it from predation by ants. In Hawaii, where their habitat preference is no longer restricted to riparian sites, they may be extremely vulnerable to these alien predators.