Negative effects of ant foraging on spiders in Douglas-fir canopies |
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Authors: | J Halaj D W Ross A R Moldenke |
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Institution: | (1) Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA, US;(2) Department of Entomology, Cordley Hall 2046, Oregon State University, Corvallis, OR 97331, USA fax: (541) 737-3643; e-mail: halajj@ccmail.orst.edu, US |
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Abstract: | Spiders and ants are potential competitors and mutual predators. Indirect evidence from previous research has suggested that
ant foraging may significantly lower the abundance of arboreal spiders in young Douglas-fir plantations in western Oregon.
This study tested the effect of foraging by ants, dominated by Camponotus spp., on spider assemblages in Douglas-fir canopies in a 5-month ant-exclusion experiment. The biomass of potential prey organisms
on foliage, dominated by Psocoptera, increased significantly by 1.9- to 2.4-fold following ant exclusion. The removal of ants
did not affect the abundance of flying arthropods in the vicinity of tree canopies as indicated by sticky trap catches. The
abundance of hunting spiders, the majority being Salticidae, increased significantly by 1.5- to 1.8-fold in trees without
ants in the late summer; neither the abundance of web-building spiders nor the average body size of hunting and web-building
spiders were significantly affected by ant removal. Spider diversity and community structure did not differ significantly
between control and ant-removal trees. The majority of prey captured by ants were Aphidoidea (48.1%) and Psocoptera (12.5%);
spiders represented only 1.4% of the ants’ diet. About 40% of observed ants were tending Cinara spp. aphids. Our observations suggest that the lower abundance of hunting spiders in control canopies with ants may be due
to interference competition with ants resulting from ant foraging and aphid-tending activities. Direct predation of spiders
by ants appeared to be of minor importance in this study system. This study did not provide sufficient evidence for exploitative
competition for prey between ants and spiders.
Received: 21 February 1996 / Accepted: 14 August 1996 |
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Keywords: | Competition Predation Spiders Ants Douglas-fir canopy |
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