Summary Mycoheterotrophic plants depend entirely on fungal associations for organic nutrients. While most mycoheterotrophic plants are associated with the mycorrhizal partners of surrounding green plants, some mycoheterotrophs are believed to obtain carbon from decaying litter or dead wood by parasitising saprotrophic fungi, based on culture experiments and 13C and 15N isotopic signatures. The carbon age (the time since carbon was fixed from atmospheric CO2 by photosynthesis) can be estimated by measuring the concentration of 14C arising from the bomb tests of the 1950s and 1960s. Given that mycorrhizal fungi obtain photosynthate from their plant partners, and saprotrophic wood‐decaying fungi obtain carbon from older sources, radiocarbon could represent a new and powerful tool to investigate carbon sources of mycoheterotrophic plants. We showed that the Δ14C values of mycoheterotrophs exploiting ectomycorrhizal fungi were close to 0‰, similar to those of autotrophic plants. By contrast, the Δ14C values of mycoheterotrophs exploiting saprotrophic fungi ranged from 110.7‰ to 324.8‰, due to the 14C‐enriched bomb carbon from dead wood via saprotrophic fungi. Our study provides evidence supporting that some mycoheterotrophic orchids depend on forest woody debris. Our study also indicates that radiocarbon could be used to predict the trophic strategies of mycoheterotroph‐associated fungal symbionts.

The long-distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (d 15 N Base ), which can be estimated by using compound-specific isotope analysis. d 15 N Base exclusively reflects the d 15 N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, d 15 N Base allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a d 15 N Base isoscape in the northern North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective d 15 N Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth by using a state-space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a new migration route to the Bering Sea Shelf during a later growth stage.

Human activities have had the strongest impacts on natural ecosystems since the last glacial period, including the alteration of interspecific relationships such as food webs. In this paper, we present a historical record of major alterations of trophic structure by revealing millennium-scale dietary shifts of brown bears (Ursus arctos) on the Hokkaido islands, Japan, using carbon, nitrogen, and sulfur stable isotope analysis. Dietary analysis of brown bears revealed that salmon consumption by bears in the eastern region of Hokkaido significantly decreased from 19% to 8%. In addition, consumption of terrestrial animals decreased from 56% to 5% in western region, and 64% to 8% in eastern region. These dietary shifts are likely to have occurred in the last approximately 100–200 years, which coincides with the beginning of modernisation in this region. Our results suggest that human activities have caused an alteration in the trophic structure of brown bears in the Hokkaido islands. This alteration includes a major decline in the marine-terrestrial linkage in eastern region, and a loss of indirect-interactions between bears and wolves, because the interactions potentially enhanced deer predation by brown bears.