Trophic dynamics of two sympatric species of riparian spider (Araneae: Tetragnathidae)

Abstract

Four species of Tetragnatha were found along the banks of Duffin Creek, Ontario: T. versicolor Walckenaer, T. elongata Walckenaer, T. laboriosa Hentz, and T. straminea Emerton. However, only the first two species were common; together they represented 91% of all species of spiders observed. Highest densities of T. elongata occurred in July, a time when numbers of T. versicolor were at their lowest. Growth rates differed between the two species. Both species were more common (2–3 times) on the east bank of the river than on the west. The locations (in three dimensions) of individual spiders along the banks were similar for both species, although T. elongata frequented shrubs overhanging the river more than T. versicolor, which was found farther away from the water's edge. The mean number of prey caught by T elongata was significantly higher than that caught by T. versicolor. Webs caught most prey when located 2–4 m from the water's edge, both in annual vegetation (grass level to a height of about 0.5 m) and in tall shrubs (1.5–2 m). Large numbers of prey were caught also in webs spun in shrubs and tree branches that hung over the water surface at a height of 1–2 m. The number of prey caught was not related to web diameter. Maximum prey capture by T. elongata coincided with the time of maximum total insect emergence in the river. Although the insect taxa found in the webs reflected those that were emerging in greatest numbers, typically chironomids and mayflies, other commonly emerging taxa (e.g., caddisflies and stoneflies) were conspicuously absent. The minimum estimate of the proportion of total insect emergence from this river that is captured by these two spider species is 0.2%.