ESTIMATES AND TESTS OF PER-CAPITA INTERACTION STRENGTH - DIET, ABUNDANCE, AND IMPACT OF INTERTIDALLY FORAGING BIRDS

Predicting the dynamics of natural food webs requires estimates of the strength of interactions among species. The ability to estimate per c apita interaction strength from observational data is desirable becaus e of the logistical difficulty of using experimental manipulations to obtain such measures for all species within complex natural communitie s. In this paper, I derive observational measures of per capita intera ction strength having units matching those of dynamic food web models (per capita consumption and assimilation rates). I also highlight the difference between per capita interaction strength (a parameter used i n theoretical models) and species impact (empirical measures of total species effect). I then use behavioral observations and population cen suses in a rocky intertidal community to estimate both per capita inte raction strengths and species impacts on invertebrate prey of Glaucous -winged Gulls (Larus glaucescens), American Black Oystercatchers (Haem atopus bachmani), and Northwestern Crows (Corvus caurinus). Estimated per capita interaction strengths exhibited a skewed distribution with many weak interactions and few strong interactions: mean +/- 1 SD of l og(10)(interaction strength) = -1.95 +/- 1.40 (bird-day/m of shore)(-1 ). Per capita interaction strength correlated poorly (r(2) = 0.152-0.1 57) and nonlinearly with both consumption rates and percentage contrib ution of a prey species to the diet. Using my observational estimates of per capita interaction strengths, I predicted the species impact of bird predation on different prey taxa. Predictions included strong ef fects of birds on goose barnacles (Pollicipes polymerus), limpets (Lot tia and Tectura spp.), sea urchins (Strongylocentrotus spp.), and larg e starfish (Pycnopodia helianthoides and Solaster stimpsoni), but litt le effect on mussels (Mytilus californianus and M. trossulus), dogwhel k snails (Nucella spp.), and acorn barnacles (Semibalanus cariosus). I compared nine of the predictions with 126 results of experimental man ipulations of birds. The predictions agreed both qualitatively and qua ntitatively with the experimental results. These findings suggest that observational measures of interaction strength that have units matchi ng those of dynamical food web models may be reasonable to use in esti mating those found in natural communities.