Graduate Thesis Or Dissertation
 

The role of avian predators in an Oregon rocky intertidal community

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gh93h180k

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  • Birds affected the community structure of an Oregon rocky shore by preying upon mussels (Mytilus spp.) and limpets (Collisella spp.). The impact of such predation is potentially great, as mussels are the competitively dominant mid-intertidal space-occupiers, and limpets are important herbivores in this community. Prey selection by birds reflects differences in bill morphology and foraging tactics. For example, Surfbird (Aphriza virgata) uses its stout bill to tug upright, firmly attached prey (e.g. mussels and gooseneck barnacles [Pollicipes polymerus]) from the substrate. The Black Turnstone (Arenaria melanocephala), with its chisel-shaped bill, uses a hammering tactic to eat firmly attached prey that are 1) compressed in shape and can be dislodged, or 2) have protective shells that can be broken by a turnstone bill. In addition, the Black Turnstone employs a push behavior to feed in clumps of algae containing mobile arthropods. Bird exclusion cages tested the effects of bird predation on 1) rates of mussel recolonization in patches (50 x 50 cm clearings), and 2) densities of small-sized limpets (< 10 mm in length) on upper intertidal mudstone benches. Four of six exclusion experiments showed that birds had a significant effect on mussel recruitment. These experiments suggested that the impact of avian predators had a significant effect on mussel densities when 1) the substrate was relatively smooth, 2) other mortality agents were insignificant, and 3) mussels were of intermediate size (11-30 mm long). Another series of exclusion experiments demonstrated that birds decreased densities of limpets 5-10 mm long, but not densities of smaller sized limpets. Experiments in which limpets were added to protected and unprotected plots indicated that bird predation varied seasonally; and that emigration, in addition to predation, may be responsible for the general absence of larger limpets on high intertidal mudstone benches.
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