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Canopy-forming plants and algae, in addition to their role as food, can positively affect higher trophic levels by providing protection from predation or stressful abiotic conditions. In the low rocky intertidal zone of the Pacific Northwest, the chiton Katharina tunicata is closely associated with the canopy-forming alga Hedophyllum sessile. In a 27-month field experiment, I used experimental devices to mimic characteristics of the Hedophyllum canopy and monitored the abundance and physiology of Katharina to determine the mechanism behind this association. The data show that this association is due to selection of shaded microhabitats by Katharina, and that neither predation nor Hedophyllum per se influenced the distribution of the chiton. Exclusion of visual predators had no effect on chiton abundance, and tests of the effects of shade independent of Hedophyllum presence indicated that the shade produced by the alga was critical to the chiton. Seasonal fluctuations in levels of 70 kDa (1.16 × 10−19 g) heat shock proteins (Hsp70) indicate that Katharina experience sublethal physiological stress during the spring and summer. The corresponding use of shaded areas during spring and summer low tides suggests that the chiton's distribution is limited during these seasons by the availability of refugia from sublethal abiotic stresses. Subtle variations in environmental conditions are also important to this association; the strength of the positive interaction varied annually, becoming more positive in warm summers (1998 and 2000) and neutral in a cooler summer (1999). On a finer scale, physiological measurements indicate that the response of chitons to summertime temperatures depends on both absolute temperature change and the animal's recent thermal history. In particular, the seasonal onset of daytime low tides in the spring is associated with increased Hsp70 levels, indicating that this is a significant physiological event for Katharina. The importance of facultative positive interactions that allow consumers to escape from sublethal abiotic stresses will likely be variable in time but can have large impacts on communities both by affecting the small-scale distribution of consumers over the short term and potentially by affecting the persistence of organisms over the long term.

Foraging strategies of Glaucous-winged Gulls (Larus glaucescens) were studied in rocky intertidal habitats of the western Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles (Balanus glandula) and mussels (Mytilus edulis) comprised most of the gull's diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins (Strongylocentrotus polyacanthus), chitons (Katharina tunicata), and limpets (Collisella pelta and Notoacmaea scutum). Sea urchins, chitons, and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters (Enhydra lutris). With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Preference experiments were conducted in the field, in which the common species and sizes of prey were made equally available to foraging gulls, thus eliminating search and capture times. In comparison with natural food choice, where sea urchins were most preferred, chitons became most preferred. We suggest that chitons are infrequent in natural diets because they adhere more strongly to the substratum than do sea urchins. Benefits of selective foraging were determined by comparing the net rate of energy gain of simulated random foragers with energy gained by selection of intertidal zones, prey species, and prey sizes. Observed selection patterns provided increased energy as gulls became more selective, and averaged 155% more than that obtained by the simulated random foragers. Results of the study support the two main predictions of optimal foraging theory in that (1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and (2) gulls became more selective foragers when energetically more profitable prey were more available.

The effects of consumers on prey populations are traditionally expressed as a function of the intensity or frequency of consumption. Interactions among the intertidal alga Iridaea cordata and five common herbivores of the Oregon coast emphasize that the variety of consumers can be as important as the magnitude of feeding. The five herbivores in this system divide into two functional groups with respect to feeding on Iridaea. (1) Katharina tunicata (chiton), Strongylocentrotus purpuratus (sea urchin), and Collisella pelta (limpet) readily feed on Iridaea in all reproductive conditions. In contrast, (2) Lacuna marmorata (coiled snail) and Idotea wosnesenskii (isopod) feed on reproductively mature plants, but avoid nonfertile individuals. Idotea is deterred from eating immature plants by the alga's cuticle. Lacuna, while impartial to the presence of the cuticle, avoids nonfertile plants for an unidentified reason. At several locations along the Oregon coast these herbivores are abundant and spatially segregated. In these areas Iridaea has a restricted distribution (isolated on vertical walls) that correlates with the qualitative feeding differences among the five herbivores. Iridaea persists with Idotea and Lacuna (although reproductively mature blades are heavily damaged), but is rare or absent when either Katharina or Strongylocentrotus is common. When Katharina is removed from isolated rocky benches, Iridaea establishes in areas where it was previously absent. Qualitative variation in herbivory as a result of feeding by different species enhances between—habitat algal diversity by restricting the distribution of Iridaea. When the effects of herbivores are experimentally reduced, Iridacea shows a more uniform distribution.