Document: ADR-3-51-27

Temporal variability in the role of prey availability in the population dynamics of an orb-weaving spider, Tetragnatha versicolor.

SMYTH, A.P.*

University of California, Berkeley, CA 94720 USA ¹

Abstract:
The dynamics of most natural populations are determined by a variety of interacting factors. Distinguishing among the influences of these different factors is often difficult, partly because the relative importance of each may vary due to temporal variability in the environment. Here, I present the results of a 1999 experiment investigating the relative influences of prey availability and web-building structure on the abundance, reproduction and behavior, of Tetragnatha versicolor, an orb-weaving spider common along the banks of many temperate streams. The experiment was conducted along two streams in northern California, the South Fork Eel River and its less productive tributary, Elder Creek. Prey was augmented by moving floating mats of the alga Cladophora to the base of sedge tussocks used as web-building structure by tetragnthids. Web-building structure was manipulated by clipping tussocks to 50% of their original stem density. Two weeks after prey augmentation, half of the tussocks receiving extra prey were clipped, producing a treatment combining both prey and structure manipulations. Prey augmentation led to significant increases in spider density and reproduction as compared with controls along both streams. Reducing structure in non-augmented tussocks led to a significant reduction in spider densities, but reproduction and behavior did not change significantly. In contrast, reducing structure in augmented tussocks led to significant increases in aggressive interactions as compared with any other tussocks in the study. These results indicate that in 1999, prey availability was more important than structure on both study streams in determining spider abundance, behavior, and reproduction. These results contrast with those obtained in a similar 1997 experiment, when spiders on the South Fork Eel responded much more strongly to a structure manipulation than to prey augmentation, while those on Elder Creek responded much the same as in 1999. Insect productivity was higher along the South Fork Eel in 1997 than in 1999, so other factors, such as web-building structure, might have been more critical during that year. These results show that while prey availability appears to be a critical determinant of the population dynamics of tetragnathids at this site, its importance may vary temporally.

Keywords: population dynamics, temporal variability, spider ecology, Tetragnatha versicolor

This abstract is being presented at: 11:30 AM in session:
Oral Session #44: Terrestrial Invertebrates: Foodwebs and Plant Responses.