Competitive mechanisms in a food-limited species: relative importance of interference and exploitative interactions among labyrinth spiders (araneae: Araneidae).

Abstract

Field experiments were conducted to clarify the relationship between the extent of food limitation and the amount and nature of intraspecific competition in an orb weaver, the labyrinth spider Metepeira labyrinthea. In one experiment agonistic encounters between marked adult females were induced at natural web sites. In other experiments populations were established by adding spiders to standardized, open experimental units in the spider's natural habitat. Replicate low and high density populations of marked adult females were established in one study. The other density manipulation was with immature spiders and incorporated a third treatment in which prey availability was increased over ambient levels.The larger female gained control of the web in 76% of the induced encounters. Resident status of the spider did not significantly influence the outcome of a contest. Apparently labyrinth spiders will spontaneously invade webs, since some females in the density-manipulation experiment appeared in recently occupied webs of other spiders. Despite this evidence of interference competition and indirect evidence that prey scarcity limited fecundity, neither survival nor egg production was lower at higher spider densities. The experiment with juvenile spiders provided direct experimental evidence that prey were scarce for M. labyrinthea. Spiders that received supplemental food were significantly larger at the end of the experiment than those that fed at natural prey levels only (24±3 mg versus 15±2 mg, respectively). However, density of immature spiders did not affect their growth or survival.The labyrinth spider appears to be a food-limited species in which exploitative competition for food is weak or absent. Most spiders never build webs close enough together to interfere indirectly with each other's prey capture. Occurrence of aggressive interactions between some females suggests that territorial behavior may lead to spacing patterns that eliminate exploitative competition. This hypothesis is evaluated and it is concluded that other factors better explain the absence of exploitative competition among most M. labyrinthea. Aggressive encounters occur between some labyrinth spiders and clearly constitute interference competition, but such interactions do not contribute significantly to density-dependent regulation of this population.