Keitaro Fukushima

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts

Nematomorph parasites manipulate crickets to enter streams where the parasites reproduce. These manipulated crickets become a substantial food subsidy for stream fishes. We used a field experiment to investigate how this subsidy affects the stream community and ecosystem function. When crickets were available, predatory fish ate fewer benthic invertebrates. The resulting release of the benthic invertebrate community from fish predation indirectly decreased the biomass of benthic algae and slightly increased leaf break-down rate. This is the first experimental demonstration that host manipulation by a parasite can reorganise a community and alter ecosystem function. Nematomorphs are common, and many other parasites have dramatic effects on host phenotypes, suggesting that similar effects of parasites on ecosystems might be widespread.

Contribution of atmospheric nitrate to stream-water nitrate in Japanese coniferous forests revealed by the oxygen isotope ratio of nitrate.

Evaluation of the openness of the nitrogen (N) cycle in forest ecosystems is important in efforts to improve forest management because the N supply often limits primary production. The use of the oxygen isotope ratio (delta(18)O) of nitrate is a promising approach to determine how effectively atmospheric nitrate can be retained in a forest ecosystem. We investigated the delta(18)O of nitrate in stream water in order to estimate the contribution of atmospheric NO(3) (-) in stream-water NO(3) (-) (f(atm)) from 26 watersheds with different stand ages (1-87 years) in Japan. The stream-water nitrate concentrations were high in young forests whereas, in contrast, old forests discharged low-nitrate stream water. These results implied a low f(atm) and a closed N cycle in older forests. However, the delta(18)O values of nitrate in stream water revealed that f(atm) values were higher in older forests than in younger forests. These results indicated that even in old forests, where the discharged N loss was small, atmospheric nitrate was not retained effectively. The steep slopes of the studied watersheds (>40 degrees ) which hinder the capturing of atmospheric nitrate by plants and microbes might be responsible for the inefficient utilization of atmospheric nitrate. Moreover, the unprocessed fraction of atmospheric nitrate in the stream-water nitrate in the forest (f(unprocessed)) was high in the young forest (78%), although f(unprocessed) was stable and low for other forests (5-13%). This high f(unprocessed) of the young forest indicated that the young forest retained neither atmospheric NO(3) (-) nor soil NO(3) (-) effectively, engendering high stream-water NO(3) (-) concentrations.

Seasonal changes in nitrate use by three woody species: the importance of the leaf-expansion period

Seasonal changes in plant NO3−-N use were investigated by measuring leaf nitrate reductase activity (NRA), leaf N concentration, and leaf expansion in one evergreen woody species (Quercus glauca Thunb.) and two deciduous woody species [Acer palmatum Thunb. and Zelkova serrata (Thunb.) Makino]. Leaf N concentration was highest at the beginning of leaf expansion and decreased during the expansion process to a steady state at the point of full leaf expansion in all species. The leaf NRA of all species was very low at the beginning of leaf expansion, followed by a rapid increase and subsequent decrease. The highest leaf NRA was observed in the middle of the leaf-expansion period, and the lowest leaf NRA occurred in summer for all species. Significant positive correlations were detected between leaf NRA and leaf expansion rates, while leaf N concentrations were negatively correlated with leaf area. In the evergreen Q. glauca, the N concentration in current buds increased before leaves opened; concurrently, the N concentration in 1-year-old leaves decreased by 25%. Our results show that the leaf-expansion period is the most important period for NO3−-N assimilation by broadleaf tree species, and that decreases in leaf N concentration through the leaf-expansion period are at least partly compensated for by newly assimilated NO3−-N in current leaves.

Soil Nitrogen Dynamics in Larch Ecosystem

About one-third of the stored soil carbon in the world is contained in large organic pools in northern taiga and tundra systems (Oechel and Billings 1992). In these ecosystems, at least 95% of the nutrients were incorporated in the soil (Marion et al. 1982). Therefore, plant growth or net primary production (NPP) is severely constrained by nutrient availability in high-latitude ecosystems, since the cold, wet conditions of arctic soils act to slow the release of nutrients (particularly inorganic N and P) from organic matter and the oxidation of organic C to CO2 (Hobbie et al. 2002). These processes have led to the critical nutrient limitation on plant growth in these ecosystems (Shaver et al. 1992).

Among-year variation in deer population density index estimated from road count surveys

A method for obtaining a relative deer population density index with low cost and effort is urgently needed in wildlife protection areas that need their own deer management guidelines. We recorded the number of deer sighted during our daily trips on forest roads by car in Ashiu Forest at Kyoto University, Japan, beginning in 2006. We used generalized additive mixed models (GAMMs) to estimate among-year trends in the number of deer sighted. We applied models for the total number of deer (TND), number of adults (NA), and number of fawns (NF) sighted, which included both current-year and 1-year-old fawns. Full models included the terms of year (2007, 2008, 2009, and 2010), weather (fine, cloudy, and rain/snow), and nonlinear effects of season (date) and time (time). The optimal GAMMs for TND, NA, and NF did not include the effect of weather but included those of time, date, and year. The detected among-year trends in deer population may be influenced by differences in snow environments among the years. The modeling of road count data using GAMM quantitatively determined among-year variation in the number of deer sighted. This trend was similar to that of the population density estimated using a block count survey conducted in Ashiu Forest.

Responses of macroinvertebrate communities to 4 years of deer exclusion in first- and second-order streams

Abstract. We compared hydrological environments and macroinvertebrate communities in 1st- and 2nd-order streams between a deer-excluded catchment (EC) and a control catchment (CC) to test effects of deer-induced hillslope soil erosion and sedimentation on macroinvertebrates. Overland flow contribution to the streams was greater in CC than in EC, and substrate in 1st-order streams contained more fine sediment in CC than in EC, whereas fine sediment in substrate in 2nd-order streams was similar between catchments. Macroinvertebrate community structure in 2nd-order streams was similar between catchments, but community structure in 1st-order streams differed between catchments. In 2nd-order streams, grazer and predator taxa predominated in both catchments, whereas in 1st-order streams, a clinger taxon predominated in EC and a burrower taxon predominated in CC. Diversity of macroinvertebrates in 1st-order streams was 1.14× higher in EC than in CC. We suggest that effects of deer on macroinvertebrates were less apparent in 2nd- than in 1st-order streams because fine sediments did not accumulate in 2nd-order streams exposed to deer browsing. Our results suggest that effects of sediment addition caused by deer browsing depends on the hydrogeomorphic properties of headwater streams.

Maintenance of soil ecotypes of Solidago virgaurea in close parapatry via divergent flowering time and selection against immigrants

The often patchy distribution of serpentine geology can lead to abrupt changes in soil and microclimates. Thus, serpentine areas provide an ideal natural setting to understand how divergent selection drives the process of local adaptation in edaphically specialized plants. When the serpentine ecotype is surrounded by a related nonserpentine ecotype, a balance of natural selection and potential gene flow should maintain the different ecotypes over very short distances. We aimed to reveal the mechanisms allowing soil ecotypes of a goldenrod species to co‐occur sympatrically in Japan. We performed field surveys to characterize microenvironments and flowering phenology of each ecotype, common garden and reciprocal transplant experiments and artificial crossing, and population genetic analysis to investigate the levels of genetic differentiation between ecotypes. Growth chamber experiments show that serpentine plants showed lower specific leaf area (SLA) and greater resource allocation to their root systems than did their nonserpentine counterparts, a potential adaptation to drier and less fertile soil condition in serpentine habitats. Reciprocal transplant studies demonstrated a clear pattern of local adaptation in the plant growth rate. Importantly, serpentine populations completed flowering by midsummer versus late summer in nonserpentine plants. This pattern is consistent with the hypothesis that early flowering ensures reproductive success, before the microclimatic conditions becomes severe in open habitats. Although prezygotic isolation was a strong barrier to gene flow, genetic differentiation was very low, indicating a recent origin for the serpentine ecotypes and/or gene flow at low frequencies. Synthesis. The findings indicate that the early flowering times of serpentine ecotypes, which would have been selected for by microclimates in serpentine areas, can play roles in local adaptation, but also population isolation via a by‐product of diverged reproductive timings. This study contributes to general understanding of the initial stages of plant ecological speciation under potential gene flow in very small geographic scales.

Development and characterization of EST-SSR markers for Carex angustisquama (Cyperaceae), an extremophyte in solfatara fields

Premise of the Study Expressed sequence tag–simple sequence repeat (EST‐SSR) markers were developed for Carex angustisquama (Cyperaceae) to investigate the evolutionary history of this plant that is endemic to solfatara fields in northern Japan. Methods and Results Using RNA‐Seq data generated by the Illumina HiSeq 2000, 20 EST‐SSR markers were developed. Polymorphisms were assessed in C. angustisquama and the closely related species C. doenitzii and C. podogyna. In C. angustisquama, many loci were monomorphic within populations; the average number of alleles ranged from one to five, and levels of expected heterozygosity ranged from 0.000 to 0.580, while all markers were polymorphic in a population of C. doenitzii. This indicates that low genetic polymorphism of C. angustisquama is likely due to the species’ population dynamics, rather than to null alleles at the developed markers. Conclusions These markers will be used to assess genetic diversity and structure and to investigate evolutionary history in future studies of C. angustisquama and related species.