Shun Takagi

<p>We recorded a Japanese marten biting the neck of a wild boar piglet, using an infrared-triggered camera installed in the forest in Gifu Prefecture, Japan. It is unclear whether Japanese martens actively capture wild boar piglets as a food resource, but we propose that Japanese marten as a potential predator of wild boar piglets.</p>

Trapa spp. dominate many shallow eutrophic lakes in Japan, which must affect the nutrient dynamics in lakes. Trapa spp. are utilized by several animals, in particular the leaf beetle, Galerucella. To quantify the trophic links and material transfers between Trapa spp. and G. nipponensis, their seasonal dynamics, the amount of leaves being consumed, and carbon and nitrogen stable isotope ratios (C-13 and N-15) in Lake Inba were tracked. Trapa spp. began to increase in May, reached their highest biomass in September, and disappeared in October. G. nipponensis exhibited continuous reproduction from May until October. Grazing by G. nipponensis did not seem to affect the abundance of Trapa, as the average percentage of Trapa leaf consumed was 3%. Rather, G. nipponensis was influenced by Trapa, as C-13 and N-15 of Trapa spp. and G. nipponensis showed very similar seasonal fluctuations. The difference in N-15 was 3-4 parts per thousand, which indicates that Trapa leaves were a main food source for G. nipponensis and a trophic linkage exists between them. The estimated nitrogen and phosphorus amounts in Trapa corresponded to 1% of nitrogen and 18% of phosphorus in the entire lake. If G. nipponensis migrate and die in a terrestrial environment, G. nipponensis can transport certain nutrients from lake to terrestrial environments. The nutrients from Trapa spp. are mostly regenerated, except for that biomass transported to terrestrial ecosystems by humans.

Water chestnuts (Trapa spp.) dominate many shallow eutrophic lakes, and impact aquatic fauna. Use of Trapa beds by animals may vary with the growth stage of Trapa, but little is known regarding when and how animals use Trapa and nearby, non-Trapa habitat. To clarify seasonal habitat use by the Trapa-feeding chrysomelid beetle (Galerucella nipponensis), we examined seasonal changes in the density of G. nipponensis from Trapa beds and from adjacent emergent vegetation. Furthermore, to determine whether habitat use by beetles is associated with their physiological state, we measured their oviposition and foraging activities. The density of G. nipponensis in each life-stage changed seasonally, with a high density of adults inhabiting emergent vegetation in July, after beetle densities in Trapa beds had declined. Adults collected from Trapa beds showed active egg-laying and foraging activities, while beetles collected from emergent vegetation were in reproductive diapause. This is the first report of G. nipponensis leaving its host plant to initiate summer diapauses in another habitat type.

<p> Recently, the floating-leaved plants, especially Trapa spp., have been increased in many shallow and eutrophic lakes in Japan. In this study, we investigated the seasonal changes in water qualities in relation to the presence / absence of Trapa natans in the eutrophic Lake Inba. In the Trapa growing seasons (from July to September), the percentage of dissolved oxygen and turbidity were lower in the Trapa beds than the open water without Trapa. Dissolved oxygen significantly decreased near the bottom especially at night. Turbidity was significantly lower both at the surface and bottom layer in the Trapa beds only in the Trapa growing seasons. Low oxygen and turbidity in the beds might be due to the reduced water currents and the suppression of photosynthesis by phytoplankton under the low light conditions. There were no significant differences in nutrient concentrations between Trapa beds and the open water, though high concentrations of NH₄-N in the Trapa beds were observed in August and September. Decay and decomposition of Trapa and anoxic condition near the sediment might have caused the release of inorganic nitrogen into water. While in the non-growing seasons, there were no significant differences on water qualities between the stations. Differences in water qualities were more remarkable between seasons that total phosphorus concentration was high from July to September, while NO₃-N and NO₂-N were high in other months. Since those seasonal changes in nutrient concentrations were common in Lake Inba, the effects of Trapa spp. on nutrient concentrations were much smaller than the other factors affecting seasonal variations.</p>

<p>We investigated the spatial distribution of macrobenthos in salt marshes along the coast of Tokyo Bay, with environmental factors that influence their distribution. The field survey was conducted at 31 sites along the entire coast of the bay, in which presence or absence of the salt marsh animals and environmental factors (salinity, ORP, median grain size, elevation, vegetation area) of the habitats were examined. Most of common species of mollusks and crustaceans were distributed on entire coast of the bay, but some of endangered species showed restricted distribution in the coast. The restricted distribution of endangered salt marsh crabs were explained by several environmental factors; higher salinity and lower elevation of the habitat were suitable for Helicana japonica and Parasesarma affines, which were distributed in eastern coast in the bay, and Sesarmops intermedium, that was distributed in western coast and inner part, tended to inhabit higher elevation sites. Since local populations of these salt marsh species in the bay may be consisted of bay scale metapopulation that are formed by connectivity between small local populations through larval dispersal, distributional restriction of the salt marsh crabs in the bay may indicate that these endangered species are regarded as being difficult to persist in the bay. New local populations of the salt marsh benthos animals could be restored by creation of salt marsh habitats along the bay, to increase of the connectivity between local populations in the bay.</p>

Invasions of some areas of Japan by the exotic redback spider Latrodectus hasseltii Thorell (Araneae: Theridiidae) have been reported. While most of these invasions have occurred in urban areas, anthropogenic habitat modifications may provide an opportunity for L. hasseltii to invade semi-natural ecosystems, but the ecological impacts of L. hasseltii have only rarely been studied. We therefore examined the distribution of L. hasseltii in sand dune ecosystems and its potential impacts on other animals. In addition, we surveyed the occurrence of spiders on the exotic yucca Yucca gloriosa L. (Asparagaceae), another invader of sand dune ecosystems. Latrodectus hasseltii was observed in six of 18 sand dunes in the Chita Peninsula, central Japan, and was the dominant web-building spider at one site. The web contents of L. hasseltii consisted of various arthropod species, including the threatened ground beetle Scarites sulcatus Olivier (Carabidae). In all, 24 of 172 patches of exotic yucca were occupied by L. hasseltii, suggesting that colonization by exotic plants may facilitate the invasion of L. hasseltii into sand dunes. This is the first report of the invasion of L. hasseltii into semi-natural habitats in Japan, and these results suggest that L. hasseltii poses a threat to the conservation of coastal insects inhabiting human-modified sand dune ecosystems.

Despite recent attempts to quantify the relative strength of density- and trait-mediated indirect effects, rarely has the issue been properly addressed at the population level. Most research is based on short-term small-scale experiments in which behavioural and/or physiological responses prevail. Here, we estimated the time-scales during which density- and trait-mediated effects manifest, as well as the strength of these effects, using an interaction chain with three organisms (deer-plant-butterfly). A hierarchical Bayesian model was performed by using a long-term data set of deer density in the Boso Peninsula, central Japan (where local densities differ spatially and temporally) as well as densities of the swallowtail butterfly Byasa alcinous and its host plant Aristolochia kaempferi. The time-scale effect of deer on plant quantity and quality was estimated according to the degree of carry-over effects. The negative influence on leaf density showed a temporal saturation pattern over the long term, while the positive influence on leaf quality due to resprouting of leaves after deer browsing showed no clear temporal trend. The net indirect effect changed from positive to negative with time, with the negative density-mediated effect becoming prominent in the long term. Our novel approach is widely applicable in assessing the dynamic impacts of wildlife if the spatio-temporal variability of expansion and/or invasion history is known.

Although patch isolation and patch size have long been considered the two primary parameters affecting metapopulation processes, patch quality is now recognized as a third factor. The extent to which patch quality influences between-patch migration processes and local population sizes, however, is poorly understood. Using a mark-recapture survey for the butterfly Minois dryas, which inhabits grasslands in a fragmented landscape, we investigated factors affecting immigration and emigration of female individuals between patches. For patch quality, we assessed the proportion of forest-edge length along the patch periphery, as well as nectar resource availability, as the former is known to provide shelter from heat stress for grassland butterflies in midsummer. Results revealed that immigration into patches was largely determined by patch quality represented by forest-grassland adjacency, in addition to patch isolation. Emigration was lower from patches with high-quality measured by nectar resource availability as well as the amount of surrounding forest. These results suggest that a forest-grassland boundary is important, even for grassland-dwelling butterflies, in fragmented landscapes, and that conservation priorities should be given to habitats having this structural feature, as well as large patch size and high connectivity.

Winter-flooding of rice paddies without the application of agricultural chemicals is attracting attention as a new agricultural method for enhancing the habitat conditions of wintering waterfowl in rice paddy ecosystems throughout Japan and east Asia. Conditions in these paddies are expected to result in restoration of not only the winter habitats of waterfowl but also those of other taxonomic groups during the rice growing season. In this study, we tested whether the diversity of summer spiders-ubiquitous predators in rice paddies-was higher in the winter-flooded paddies than in the conventional ones by conducting field measurements in 31 winter-flooded and 7 conventional paddies. Limiting factors of spiders in the winter-flooded paddies were then examined. Results revealed that both the density and species richness of spiders were significantly higher in the winter-flooded paddies than in the conventional ones both before and after the insecticide application against pecky rice bug Stenotus rubrovittatus (Matsumura) (Hemiptera: Miridae) to conventional paddies. In addition, spider density and species richness in the winter-flooded paddies correlated with the availability of two prey groups-chironomids and other nematocera. These findings suggest that in the winter-flooded paddies the diversity of generalist predators is higher than in the conventional ones during the rice-growing season and that the combination of management at both the landscape and field level is likely more effective for increasing spider abundance in winter-flooded paddies.

An increasing number of studies are being conducted to examine the density- or trait-mediated indirect effects of large herbivores on phytophagous insects. However, these effects are highly context dependent and no general trends have been made clear. We conducted a meta-analysis focusing on three factors capable of affecting detection of the indirect effects of large herbivores on phytophagous insects: type of response variable, experimental scale, and characteristics of study organisms. Overall, large herbivores exerted a negative effect on insects, a trend that was prominent in studies using insect abundance as a response variable. No particular trends were observed in studies using herbivory rate as a variable, and these studies often focused on plant trait-mediated effects more than density-mediated ones. Experimental scale affected the strength of indirect effects: within-year or individual tree level experiments did not follow any trends, whereas 1-10 year experiments or 0-10 ha scale experiments show a negative impact on insects. Characteristics of large herbivores and growth forms of transmitter plants also contributed to variations in the observed effect size; negative effects were reported in livestock-grassland ecosystems and neutral effects in tree-dominated systems. There was a close association between response variable, experimental scale, and characteristics of study organisms, and these effects jointly contributed to the apparent trends. To predict the impacts of large herbivores at ecosystem level, it is necessary to eliminate these biases arising from study design and to evaluate the effect on insect densities at large spatial and temporal scales.

Trait-mediated effects of predators can impact prey population dynamics by affecting prey behavior,. The mind bug Stenotus rubrovittatus (Matsumura) (Hemiptera: Miridae), a major insect pest in Japanese rice production, usually remains in the upper layer of paddies to feed on rice ears. However, the minds are frequently trapped by horizontal webs of Tetragnatha spp. spiders, which are highly abundant in organic rice paddies, and fall to the bottom layers of paddies where they are preyed upon by ground-dwelling predators. It is hypothesized that Tetragnatha spp. spiders facilitate bug predation by wolf spiders through trait-mediated effects, in which their horizontal Webs force the bugs onto or near the ground and thereby into the hunting zones of wolf spiders. Molecular gut-content analysis of 619 wolf spiders coupled with field measurements revealed that the number of wolf spiders that tested positive for mind bug predation increased significantly with the density of Tetragnatha spp. spiders in the paddies. We also observed a positive relationship between Tetragnatha spp. abundance and total cover by their webs in paddies. We identified the potential for an unexpected interaction between an herbivorous insect pest and ground-dwelling spiders that usually inhabit different microhabitats in paddy fields by focusing on trait-mediated effects of webs built by Tetragnatha spp. Because spider webs occupy a certain proportion of the available space in terrestrial ecosystems, consideration of trait-mediated effects on interactions between flying insects and other predators may lead to a better understanding of local food webs.

Landscape supplementation, which enhances densities of organisms by combination of different landscape elements, is likely common in heterogeneous landscapes, but its prevalence and effects on species richness have been little explored. Using grassland-dwelling spiders in an agricultural landscape, we postulated that richness and abundances of major constituent species are both highest in intermediate mixtures of forests and paddy fields, and that this effect derives from multi-scale landscape heterogeneity. We collected spiders in 35 grasslands in an agricultural landscape in Japan and determined how species richness and abundances of major species related to local and landscape factors across different spatial scales. We used a generalized linear model to fit data, created all possible combinations of variables at 15 spatial scales, and then explored the best models using Akaike's information criterion. Species richness showed a hump-shaped pattern in relation to surrounding forest cover, and the spatial scale determining this relationship was a 300-500-m radius around the study sites. Local variables were of minor importance for species richness. Abundances of major species also exhibited a hump-shaped pattern when plotted against forest cover. Thus, a combination of paddy fields and forests is important for enhancement of grassland spider species richness and abundance, suggesting habitat supplementation. The effective spatial scales determining abundances varied, ranging from 200 to &gt; 1000 m, probably representing different dispersal abilities. Landscape compositional heterogeneity at multiple spatial scales may be thus crucial for the maintenance of species diversity.

Mirid bugs, major insect pests in rice production, have dramatically expanded their range in Japan, resulting in increased economic losses especially for organic rice farmers. In this study, the within-field and landscape-scale factors affecting the bug and its damage to crops were examined in organic rice farms. Bug densities and crop damage levels showed significant positive relationships with weed densities (Schoenoplectus juncoides and Echinochloa crus-galli) within individual fields. Bug densities were negatively associated with Tetragnatha spp. and Pachygnatha clercki densities, indicating that these spiders act as biological control agents that help to decrease crop damage levels. At the landscape scale, bug densities increased with the proportion of the area that was fallow within a 400-m radius around focal fields. Fallows represent important source habitat for the bug and the spatial extent at which bug abundance was affected indicates the mobility of these insects. These results suggest that the combination of biological control by natural enemies, and weed management within individual paddies and surrounding landscapes inside a buffer zone of less than half a kilometre may be effective in controlling mirid bugs and the damage they cause in organic paddy fields. (C) 2011 Elsevier Inc. All rights reserved.

Phytophagous insects can be affected by plant trait-mediated indirect effects of large herbivores, but little is known regarding how these effects change in response to different densities of large herbivores. To assess the response of an insect to plant qualitative change, the response of a woody vine (Aristolochia kaempferi) to browsing by sika deer (Cervus nippon) and utilization of young leaves by a swallowtail butterfly (Byasa alcinous) were investigated across a deer density gradient. Natural and simulated deer browsing stimulated the regrowth of A. kaempferi and improved nutritional and physical quality of leaves. Young leaves were frequently observed in areas with high deer densities. The proportion of young leaves among the leaves selected for oviposition was higher than their proportion of the total number of leaves. In areas with low deer densities, the utilization of young leaves by B. alcinous increased linearly with deer density, whereas in areas with high deer densities, the utilization of young leaves was around 90%.

Spiders are common generalist predators, and understanding their potential in biological control is important for the development of integrated pest management programs. In this study, predation by three groups of spiders on the mind bug Stenotus rubrovittatus (Hemiptera: Miridae) in rice paddies was investigated using DNA-based gut-content analysis. A laboratory feeding study revealed that the detection half-lives of bug DNA in the spider gut at 25 degrees C was 3.4 days for Lycosidae and 1.5 days for Tetragnathidae. Individual spider predation on the mind bug was investigated by detecting DNA of prey in field-collected spiders. In total, 1199 spiders were assayed from three spider groups: Pirata subpiraticus (Lycosidae), Tetragnatha spp. (Tetra-gnathidae), and Pachygnatha clercki (Tetra-gnathidae), which each differ in their preferred microhabitat as well as their predatory habits. Detection rates of prey DNA in spiders increased significantly with the density of prey across all spider groups. P subpiraticus and Tetragnatha spp. predation showed a better fit to a saturated response curve to increasing prey density, while P. clercki showed a simple linear relationship with prey density. Densities of alternative prey species did not affect the detection rates of minds. These results suggest that predation on pests by generalist predators in an agroecosystem is affected not only by prey abundance but also by predator preference for specific prey. Predator preference is therefore an important factor to consider when estimating the role of natural enemies as biological control agents.

Using host plant quality as a diapause-inducing stimulus may be adaptive for herbivorous insects that use host plants whose phenology is unpredictable based on photoperiod or temperature. We examined the effects of leaf toughness of the woody vine Aristolochia kaempferi Willd. on larval performance and pupal diapause induction of the swallowtail butterfly Byasa alcinous Klug (Lepidoptera: Papilionidae). We also conducted field observations to assess whether human disturbance affects leaf phenology of A. kaempferi. Laboratory experiments showed that tough leaves decreased the survival rate of young larvae and lengthened the larval period, although pupal weight was not affected by leaf toughness. The incidence of pupal diapause was greater with tough leaves than with soft leaves and increased with larval duration. In the field, leaf age composition in A. kaempferi was greatly influenced by human mowing, which led to changes in leaf quality. These results suggest that B. alcinous has a plastic diapause strategy using host plant quality as a diapause-inducing stimulus, which may have evolved in frequently disturbed environments.