Scale-dependent neighborhood effects: shared doom and associational refuge |
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Authors: | Sara E Emerson Joel S Brown Christopher J Whelan Kenneth A Schmidt |
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Institution: | (1) Biology Department, Lawrence University, 711 E. Boldt Way SPC 24, Appleton, WI 54911, USA;(2) Illinois Natural History Survey, c/o Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor St. (M/C 066), Chicago, IL 60607, USA;(3) Department of Biological Sciences, Texas Tech University, Box 43143, Lubbock, TX 79409-3131, USA |
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Abstract: | A resource’s susceptibility to predation may be influenced by its own palatability and the palatability of its neighbors.
We tested for effects of plant chemical defenses on seed survival by manipulating the frequency of palatable and less palatable
sunflower seeds in food patches subject to harvest by fox squirrels (Sciurus niger) and gray squirrels (Sciurus carolinensis). We varied resource distributions at three scales: among stations (aggregates of patches ca. 50 m apart), among patches
immediately adjacent to each other, and within patches. When food patches were segregated into high-palatability and low-palatability
stations (Experiment 1), seeds suffered greater mortality at stations with high levels of palatable seeds. In the same experiment,
within patches, squirrels selected strongly for palatable seeds over less palatable seeds. When high- and low-palatability
food patches were placed together at the same stations (Experiment 2), increasing densities of co-occurring palatable seeds
amplified the mortality of less palatable seeds, indicating “shared doom.” When palatable and less palatable seeds were partitioned
into micropatches (Experiment 3), associational effects disappeared, as predicted. Furthermore, selectivity in less palatable
patches increased as the initial densities of palatable seeds increased, and selectivity in palatable patches decreased as
the initial densities of less palatable seeds increased. Foraging theory predicts associational effects among prey that vary
in palatability. Our results show how the type and magnitude of associational effects emerge from the interplay among the
spatial scale of prey heterogeneity, the diet selection strategy, and the scale-dependent foraging responses of the consumer. |
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