Abstract. Interactions between grazers and autotrophs are complex, including both topdown consumptive and bottom-up facilitative effects of grazers. Thus, in addition to consuming autotrophs, herbivores can also enhance autotroph biomass by recycling limiting nutrients, thereby increasing nutrient availability. Here, we evaluated these consumptive and facilitative interactions between snails (Littorina littorea) and seaweeds (Fucus vesiculosus and Ulva lactuca) on a rocky shore. We partitioned herbivores' total effects on seaweeds into their consumptive and facilitative effects and evaluated how community context (the presence of another seaweed species) modified the effects of Littorina on a focal seaweed species. Ulva, the more palatable species, enhanced the facilitative effects of Littorina on Fucus. Ulva did not modify the consumptive effect of Littorina on Fucus. Taken together, the consumptive and facilitative effects of snails on Fucus in the presence of Ulva balanced each other, resulting in no net effect of Littorina on Fucus. In contrast, the only effect of Fucus on Ulva was to enhance consumptive effects of Littorina on Ulva. Our results highlight the necessity of considering both consumptive and facilitative effects of herbivores on multiple autotroph species in order to gain a mechanistic understanding of grazers' top-down and bottom-up roles in structuring communities.

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Terrestrial insect predators and parasitoids commonly use volatile plant compounds released during grazing to find living herbivorous prey (i.e. tritrophic signaling). Because few studies have tested for tritrophic signaling in marine organisms, it is unclear if marine predators commonly use these fluid-borne cues to find living herbivore prey, especially under realistic settings. Such interactions might occur in the ocean because marine predators often use chemosensory-based foraging, and herbivore-induced production of defensive chemistry is common in some algal taxa. To examine the ability of predators to find live herbivores using waterborne chemical cues from grazed algae, we tested the responses of marine predators to several baits using in situ trapping and video experiments. We tested the attractiveness of chemical cues released by (1) Macrocystis pyrifera and Ulva sp. actively grazed by herbivores for short-or longterm periods, (2) crushed herbivores, (3) positive controls containing fish carrion, or (4) negative controls. Most predators trapped and filmed were spiny lobsters Panulirus interruptus. Under realistic field conditions, chemical cues from actively grazed algae with live herbivores failed to attract predators, but chemical cues from crushed herbivores and fish carrion were highly attractive. Thus, while marine predators like lobsters may utilize fluid-borne chemical cues to find dead or injured prey, such cues appear less important for finding live herbivore prey. Our literature review of 142 terrestrial studies revealed that tritrophic cues were unrelated to diet breadth of predators or herbivores and that most examples of terrestrial tritrophic signaling contained parasitoid predators. We additionally found that similar field studies in natural, non-agricultural settings such as ours are lacking even in better-studied terrestrial food webs, where they represent only 3% of manipulative experiments.

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The sensitivity of long-spined sea urchins (Diadema savignyi) collected from Guam (Northern Marianas Islands), USA, to nickel and copper in seawater was explored using 48-h embryo-larval development toxicity tests. The median effective concentrations (EC50) averaged 94 µg L(-1) for nickel, and 19 µg L(-1) from a single exposure to copper, and suggest relatively high sensitivity of this species to nickel compared with other sea urchin genera, but similar sensitivity to copper. Ambient nickel and copper concentrations concurrently sampled from 16 near-shore locations around Guam were one to two orders of magnitude lower than those that would be expected to result in adverse effects to D. savignyi embryos. Although nationally recommended chronic ambient water quality criteria, currently 8.2 and 3.1 µg L(-1) for nickel and copper, respectively, were not exceeded, recently derived qualifying toxicity data should be considered for updating these criteria to ensure protectiveness of sensitive tropical species.