Predatory behavior and top-down effects in marine ecosystems are well-described, however, intraguild interactions among co-occurring marine top predators remain less understood, but can have far reaching ecological implications. Killer whales and white sharks are prominent upper trophic level predators with highly-overlapping niches, yet their ecological interactions and subsequent effects have remained obscure. Using long-term electronic tagging and survey data we reveal rare and cryptic interactions between these predators at a shared foraging site, Southeast Farallon Island (SEFI). In multiple instances, brief visits from killer whales displaced white sharks from SEFI, disrupting shark feeding behavior for extended periods at this aggregation site. As a result, annual predations of pinnipeds by white sharks at SEFI were negatively correlated with close encounters with killer whales. Tagged white sharks relocated to other aggregation sites, creating detectable increases in white shark density at Ano Nuevo Island. This work highlights the importance of risk effects and intraguild relationships among top ocean predators and the value of long-term data sets revealing these consequential, albeit infrequent, ecological interactions.

Revising the convergence hypothesis of avian use of honeydew: evidence from Dominican subtropical dry forest. -Oikos 93: 250-259.A honeydew-producing scale insect (family Margarodidae, tribe Xylococcini, genus Stigmacoccus) was found associated with the tree Bursera simaruba in subtropical dry forests at elevations of 100 -400 m on the south side of the Sierra de Bahoruco, Pedernales Province, Dominican Republic. At two study sites, 91% of Bursera trees supported locally dense populations of Margarodidae. Fifteen species of birds were observed foraging on the honeydew, but most observations were of the winter resident Cape May warbler (Dendroica tigrina) and black-throated blue warbler (D. caerulescens), and the permanent resident bananaquit (Coereba fla6eola) and blackcrowned palm tanager (Phaenicophilus palmarum). The Cape May warbler actively defended the honeydew resource but frequency of use of honeydew was influenced by the close presence of flowering agave and scale insect density. Data suggest that honeydew may be a critical component of the diet of this species especially during the late winter dry-season. Hymenopteran insects also were observed feeding on honeydew, but rates of consumption did not approach that of avian species. The occurrence of this phenomenon in Dominican dry forest is discussed in light of the convergence hypothesis of bird use and defense of homopteran honeydew in which it is proposed that birds are able to maintain relationships with scale insects in moist, warm temperate forests because it is in these climates where ant abundance is low. We suggest that our observation of a well-developed bird -homopteran system in classic subtropical dry forest supports the proposed mechanism of reduced competition with ants allowing bird use of honeydew, but we suggest that a broader array of especially insular habitats which may be relatively depauperate in terms of ants also can be expected to support bird -homopteran systems.

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Birds respond rapidly to changes in both habitat and climate conditions and thus are good indicators of the ecological effects of a changing climate, which may include warmer temperatures, changing habitat conditions, and increased frequency and magnitude of extreme events like drought. We investigated how a widespread tree mortality event concurrent with a severe drought influenced the avian community of the Sierra Nevada mountain range in California. We assessed and compared the separate effects of climate stresses and altered habitat conditions on the avian community and used this information to evaluate the changes that are likely to occur in the near future. We built tree mortality maps from freely available Landsat imagery with Google Earth Engine. We analyzed avian point counts from 2010 to 2016 in the southern Sierra Nevada, to model temperature, water deficit, and tree mortality effects on the abundances of 45 bird species, and then used these models to project abundances into the future based on three climate projections. A large portion of the avian community, 47%, had a positive relationship with temperature increase, compared to 20% that responded negatively. More species (36%) declined with drier conditions than increased (29%). More species declined in response to high tree mortality (36%) than increased (9%). A preponderance of species adapted to colder temperatures (higher elevation) had negative responses to high tree mortality and water deficit, but positive responses to increasing temperature. We projected the highest total bird abundances in the future under the warmest climate scenario that we considered, but habitat modification (e.g., tree mortality) and water deficit could offset the positive influence of temperature for many species. As other studies have shown, climate warming may lead to substantial but idiosyncratic effects on wildlife species that could result in community composition shifts. We conclude that future climate conditions may not have a universally negative effect on biodiversity in the Sierra Nevada, but probable vegetation changes and increased likelihood of extreme events such as drought should be incorporated into climate‐smart forest and wildlife management decisions.

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