Hiromune Mitsuhashi

Influence of leaf litter quality on the colonization and consumption of stream invertebrate shredders

The importance of leaf litter quality for colonization and consumption by two caddisflies (Goerodes satoi and Hydatophylax festivus) and an amphipod (Sternomoera yezoensis) was examined by laboratory and field experiments in a forested headwater stream in northern Japan. Initial values of leaf litter toughness and the carbon to nitrogen (C : N) ratio, both of which varied among the three most common tree species, oak (Quercus crispula), maple (Acer mono) and alder (Alnus hirsuta), decreased dramatically in the field experiment. The density of Hydatophylax was greatest in alder leaf litter characterized by the lowest C : N ratio among the three, with C : N ratio being the only predictor of their density. In contrast, lesser toughness was the only factor facilitating colonization by Sternomoera, the density of which was greatest in the softest maple litter. Neither toughness nor C : N ratio could be used to predict colonization by Goerodes. In the laboratory experiment, in which the shredders were reared on the aforementioned types of leaf litter, following conditioning periods of 1 week and 1 month (six separate trials), the litter consumption rate decreased with toughness in both Sternomoera and Hydatophylax, and with C : N ratio in the latter. However, the consumption rate of Goerodes was influenced by neither of the above, indicating that Sternomoera and Hydatophylax, but not Goerodes, selectively colonized the leaf litter available as a food resource in the stream. Both chemical and physical qualities of leaf litter play an important role in determining species-specific colonization by invertebrate shredders.

Areas of increasing agricultural abandonment overlap the distribution of previously common, currently threatened plant species.

Human-driven land-use changes increasingly threaten biodiversity. In agricultural ecosystems, abandonment of former farmlands constitutes a major land-use shift. We examined the relationships between areas in which agriculture has been abandoned and the distribution records of threatened plant species across Japan. We selected 23 plant species that are currently identified as threatened but were previously common in the country as indicators of threatened plant species. The areas of abandoned farmlands within the distribution ranges of the indicator species were significantly larger than the proportion of abandoned farmland area across the whole country. Also, abandoned farmland areas were positively correlated with the occurrence of indicator species. Therefore, sections of agricultural landscape that are increasingly becoming abandoned and the distribution ranges of indicator species overlapped. These results suggest that abandoned farmland areas contain degraded or preferred habitats of threatened plant species. We propose that areas experiencing increased abandonment of farmland can be divided into at least two categories: those that threaten the existence of threatened species and those that provide habitats for these threatened species.

River confluences enhance riparian plant species diversity

In riparian zones along the banks of streams and rivers, flooding often causes large changes in environmental conditions immediately downstream of confluences. In turn, spatial heterogeneity in flooding along rivers and streams likely affects local species diversity. Furthermore, flooding during the plant growing season can strongly affect plant survival. In this study, we hypothesized that confluences have impacts on plant species diversity, and that these impacts are larger during the plant growing season. To test this hypothesis, we measured plant species diversity and the extent of natural bare ground at 11 river confluences during two different seasons (summer and spring) within the Mukogawa River basin system, Japan. Species diversity was highest at down-confluence areas in the summer. We linked the pattern of species diversity to that of bare ground creation by floods around the confluences and to the seasonality of annual plant recruitment. The extent of bare ground was significantly greater at down-confluence areas than at up-confluence areas. The recruitment of annual species was higher in the summer than in the spring and included rapid occupancy of bare ground in the summer. We suggest that within river systems, spatial and seasonal differences in patterns of flooding function together to regulate plant species diversity.

Push-up response of stonefly larvae in low-oxygen conditions

Under conditions of low oxygen availability, the larvae of the stonefly Oyamia lugubris McLachlan demonstrate a ‘push-up’ behavior that is thought to enhance respiratory efficiency. We conducted an experiment to investigate the effect of the oxygen supply on this behavior in winter and summer by using a lotic chamber and natural water. From the experiment, we determined the critical oxygen supply level below which the stonefly larvae are compelled to do push-ups. There was a small difference in the critical oxygen supply level between the seasons. This result emphasizes that a novel measurement of the oxygen availability, that is, the oxygen supply, could be an important determinant of the distribution of aquatic insects.

A seasonal change in the distribution of a stream-dwelling stonefly nymph reflects oxygen supply and water flow

We examined the effects of oxygen availability, which has been viewed as a minor factor in streams, on the seasonal and spatial microhabitat distribution of a stonefly. Surveys were conducted in winter and summer in a mountain stream by collecting stones from the streambed and determining the presence or absence of the insect. At each stone sampling, we also measured physical conditions. The probability of the stonefly presence increased significantly with current velocity in summer, but not in winter. Because current influences oxygen renewal rates, our results suggest that the distribution of the insect could be restricted by oxygen.

A Simple Acrylate Fiber Sampler for Stream Periphyton.

河川などの石礫付着藻類の現存量の指標として頻繁に使われているクロロフィル色素の定量的採取法として,ミクロクロス(アクリル特殊繊維)を用いた方法を開発した。従来の歯ブラシによる方形枠の剥ぎとり法と本法とを比較した結果,実用的には同一の測定値が得られることが確認された。この方法は,現場における採取の手間が少ないうえに,色素定量に先立つ濾過あるいは沈澱操作が不要で,著しく効率のよい方法であった。大量の標本を処理する必要のある藻類の微細分布調査などには,とくに適していると思われる。

Bagging GLM: Improved generalized linear model for the analysis of zero-inflated data

article i nfo Species-occurrence data sets tend to contain a large proportion of zero values, i.e., absence values (zero- inflated). Statistical inference using such data sets is likely to be inefficient or lead to incorrect conclusions unless the data are treated carefully. In this study, we propose a new modeling method to overcome the problems caused by zero-inflated data sets that involves a regression model and a machine-learning technique. We combined a generalized liner model (GLM), which is widely used in ecology, and bootstrap aggregation (bagging), a machine-learning technique. We established distribution models of Vincetoxicum pycnostelma (a vascular plant) and Ninox scutulata (an owl), both of which are endangered and have zero- inflated distribution patterns, using our new method and traditional GLM and compared model performances. At the same time we modeled four theoretical data sets that contained different ratios of presence/absence values using new and traditional methods and also compared model performances. For distribution models, our new method showed good performance compared to traditional GLMs. After bagging, area under the curve (AUC) values were almost the same as with traditional methods, but sensitivity values were higher. Additionally, our new method showed high sensitivity values compared to the traditional GLM when modeling a theoretical data set containing a large proportion of zero values. These results indicate that our new method has high predictive ability with presence data when analyzing zero-inflated data sets. Generally, predicting presence data is more difficult than predicting absence data. Our new modeling method has potential for advancing species distribution modeling.

Enhanced Diversity at Network Nodes: River Confluences Increase Vegetation-Patch Diversity

Although dendritic networks within ecosystems have typically been considered a special case of network topology, they have attracted a great deal of attention in recent years. These systems exhibit unique features in that both the nodes and branches provide distinct habitats. Within a river discontinuum context, river confluences, which are nodes of dendritic river networks, are hypothesised to have particular hydrodynamic traits that create heterogeneous habitats through a unique disturbance regime, although this hypothesis has not yet been tested. We tested this hypothesis using a vegetation data set collected from 14 river basin systems in Hyogo Prefecture, Japan. We compared vegetation-patch diversity between confluence and single-flow areas using hierarchical Bayesian models. Our results demonstrated greater vegetation-patch diversity in confluence areas compared to single-flow areas. Our findings support the hypothesis that confluences result in highly heterogeneous habitats. To the best of our knowledge, this is the first empirical report to demonstrate that river confluences have high vegetation-patch diversity. We conclude that network nodes play an important role in maintaining the biodiversity of river networks.

Environmental and biotic characteristics to discriminate farm ponds with and without exotic largemouth bass and bluegill in western Japan

We compared the environmental and biotic characteristics of farm ponds with and without the invasive fish, largemouth bass (Micropterus salmoides), and bluegill (Lepomis macrochirus), with varying degrees of aquatic vegetation cover in western Japan. Redundancy analysis (RDA) revealed that aquatic vegetation cover and pond area were significant environmental variables in explaining the variance in aquatic organisms. Aquatic vegetation cover predominantly affected Odonata and Hemiptera larvae, and the native cyprinid, Hemigrammocypris rasborella, while the pond area mainly affected the two exotic fishes (largemouth bass and bluegill), Viviparidae, Oligocheata, Ephemeroptera, and chironomid larvae. In the RDA biplot for aquatic organisms, the RDA1 axis appeared to separate the exotic fish group (bluegill, largemouth bass, Gammaridae, Oligochaeta, Viviparidae, Ephemeroptera, Trichoptera, and chironomid larvae) from the native fish group (H. rasborella, Oryzias latipes, Rhinogobius sp., Odonata, shrimps, and Hemiptera larvae). The best path model results indicated that the presence of piscivorous largemouth bass had a significantly negative effect on native fish numbers; largemouth bass also had a positive indirect effect on benthic organism numbers. Our data suggest that the depletion of native fishes via top-down effects by exotic largemouth bass may indirectly increase the number of benthic organisms as a result of trophic-cascading effects.

Differences in distribution patterns around river confluences among hydrophilic vegetation groups

In riparian areas, the distribution patterns of plant species are generally considered to depend on their flooding tolerance. Areas around river confluences are known to experience frequent and/or strong flooding events and provide diverse habitats for plants in riparian areas. However, the degree to which hydrophilic vegetation types increase their distribution around confluences may depend on their flooding tolerance. To test this hypothesis, we compared patch numbers and total areas of ten vegetation groups between confluences and single-flow areas. The vegetation groups were classified on the basis of life form and morphology of dominant species. Additionally, we compared total area of natural bare ground (an index of flooding disturbance) between confluences and single-flow areas. We found that patch numbers of annual grass, forb, and vine, perennial grass and forb, and riparian forest vegetation, as well as total areas of annual forb and vine, perennial grass and forb, bamboo and riparian forest vegetation, and natural bare ground, were greater around river confluences than in single-flow areas. On the other hand, patch numbers of shrub vegetation and total areas of annual grass, perennial vine, willow, and shrub vegetation decreased around confluences. These results suggest that confluences enhance diverse, but not all, types of habitat for hydrophilic vegetation. Thus, river confluences are a key element in maintaining diverse riparian vegetation.