Naotoshi Kuhara

TERRESTRIAL–AQUATIC LINKAGES: RIPARIAN ARTHROPOD INPUTS ALTER TROPHIC CASCADES IN A STREAM FOOD WEB

Dynamics of headwater stream ecosystems are generally regarded as occurring at the interface of aquatic and terrestrial ecosystems. Terrestrial arthropod inputs can provide an energy subsidy and increase the abundance of predatory fish, and the ensuing effects potentially can cascade through the food web and ultimately affect primary producers. Nevertheless, the community-based effects of such inputs on stream food web dynamics are still poorly understood. We present experimental evidence that terrestrial arthropod inputs have an indirect but prominent effect on a stream benthic community by altering the intensity of fish predation in the food web. Two key elements of the stream food web, terrestrial arthropod inputs and the presence of predatory fish, were experimentally manipulated by using greenhouse-type covers and enclosures (or exclosures) in a forest stream located in northern Japan. When terrestrial arthropod inputs to the stream were experimentally reduced, fish predation pressure shifted dramatically from terrestrial to aquatic arthropods. The ensuing depletion of aquatic arthropods resulted in a subsequent increase in periphyton biomass. This field experiment revealed that terrestrial arthropod inputs were a primary factor controlling cascading trophic interactions among predatory fish, herbivorous aquatic arthropods, and benthic periphyton. These results provide empirical support for the perspective that transfers of energy and biomass from donor systems are frequently significant for the maintenance of biotic communities in recipient systems.

Selective foraging on terrestrial invertebrates by rainbow trout in a forested headwater stream in northern Japan

The important contribution of terrestrial invertebrates to the energy budget of drift-foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m−2 day−1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m−2 day−1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.

Interspecific competition between two stream insect grazers mediated by non-feeding predatory fish

Two periphyton-grazing stream insects, a baetid mayfly and a glossosomatid caddisfly, were found to be strong exploitative competitors. The presence of predatory fish was found to alter the foraging activities of the baetid. but not of the glossosomatid. We investigated the local distribution of Baetis thermicus and Glossosoma sp. to examine the potential for competition between the two grazers in a northern Japanese stream. Further, we carried our a laboratory experiment to test the hypothesis that the freshwater sculpin Cottus nozawae, when not allowed to attack prey, mediates the resource-limited competitive influences of Baetly on the growth of Glossosoma, but not the reverse. We found a negative correlation between the densities of Baetis and Glossosoma in the stream, suggesting the strong potential for resource-limiting competitive interactions to occur between the grazers. Using a laboratory channel experiment employing a target-neighbor design, neither the presence of sculpin nor the density of the two grazers altered any life-history traits measured for the grazers. However, the proportion of Baetis individuals positioned on the upper surface of a ceramic plate substrate (with greater periphyton biomass) largely decreased in the presence of sculpin, with only a minor behavioral shift being detected in Glossasoma. This difference in behavioral shift resulted in greater growth suppression in Baetis than in Glossosoma. In the absence of sculpin, growth of both target grazers was lowered by experimentally increasing the density of neighboring competitors. In partial contradiction to the above hypothesis, the growth of not only Glossosoma but also Baetis was never influenced by the density of neighbor competitors when sculpin were present. The biomass of periphyton in the presence of sculpin was greater than when sculpin were absent, and was lower with higher densities of either grazer. Predator-avoidance behavioral modifications in both grazers may be the mechanism responsible for mediation of their exploitative competition.

Relationships between length and weight of freshwater macroinvertebrates in Japan

Relationships between weight (W; dry weight) and length (L; head capsule width, total body length or head carapace length) were examined in 31 Japanese freshwater macroinvertebrate taxa, using the form W = aLb. The relationships were expressed as data of the lowest taxonomic level and data of higher taxonomic levels. The length–weight relationships obtained in this study were similar to those obtained in North America and Europe at the lowest taxonomic level, whereas they could be different from those obtained in North America and Europe at the higher taxonomic levels. We suggest that researchers should make their own regressions for a target taxon or use the regression for the same taxon as possible lower taxonomic level in the local area.

Flow rate mediates the competitive influence of a grazing caddisfly on mayflies

The Glossosomatid caddisflies, which are stream-insect grazers, have been found to negatively affect the densities of mayfly grazers through intensive periphyton exploitation. The grazing efficiency of a Glossomatid caddisfly, however, is known to decrease with increasing current velocity. A manipulative field experiment was conducted to test the hypotheses that the presence of Glossosoma sp. decreased the densities of three mayfly grazers, Baetis thermicus, Cinygmula sp. and Paraleptophlebia chocorata, and that the decrease was stronger in slower stream-flow conditions. The experiment followed a two-factorial design with flow conditions and Glossosoma densities as the main factors. The periphyton biomass in conditions of reduced Glossosoma density was greater than in natural Glossosoma densities, but was not affected by flow conditions. In partial contradiction of the experimental hypothesis, the experimental reduction in Glossosoma density resulted in a higher density of Baetis, irrespective of flow conditions. The grazing activity of Glossosoma would not be affected by the flow variations that were employed in the present experiment. The reduction in Glossosoma density resulted in higher densities of Cinygmula and Paraleptophlebia only in fast-flow conditions, a result that did not support the experimental hypotheses. The competitive influence of Glossosoma on these mayflies might be altered by flow conditions causing differences in the resource structure.

Alterations in the grazing activities of cased caddisfly larvae in response to variations in predation risk and resource level

Flexible behavioral response to avoid encountering predators has never been reported in lotic, cased caddisfly larvae with low mobility. However, an earlier laboratory experiment found the growth of such a caddisfly species, Glossosoma sp., decreased in the presence of a predatory sculpin, Cottus nozawae. We conducted laboratory experiments to test whether Glossosoma responses to sculpin varied according to resource level and/or time of day. Lower periphyton biomass resulted in increased movement at any time of day, but was followed by decreased grazing success of Glossosoma in the morning when they were sampled. Although sculpins were active in the morning, evening and night but never in the afternoon, movements of Glossosoma were suppressed by sculpin stimuli only in the morning, when food intake of Glossosoma also decreased, regardless of periphyton biomass. Glossosoma reduced the risk of predation by sculpin by flexibly controlling its grazing activities during the most risky period of the day (i.e. morning). Even in the morning in the presence of sculpin, lower resource levels resulted in longer movement distances similar to those in predator-free conditions, suggesting that lower resource availability promotes predation risk in natural streams inhabited by predatory sculpin.

Succession in a stream invertebrate community: A transition in species dominance through colonization

A field experiment was conducted using ceramic plates as experimental substrates to describe the colonizing pattern of a stream invertebrate community after disturbance, and to ascertain the importance of colonizing ability for succession in a Japanese stream. We employed the simultaneous removal design in which plates were periodically set in place (1, 2, 4, 8, 16 and 32 days before sampling) and all plates were sampled on the last day of the experiment, to diminish the influence of seasonal change in stream invertebrates. Total abundance and taxon richness reached a plateau after 4–16 days of colonization. In contrast, the relative abundance of six common taxa and community structure changed throughout the 32 days of colonization. Differences in the colonizing ability of the stream invertebrates were evident. Taxa with high mobility, such as the mayflies Baetis thermicus, Paraleptophlebia japonica, Cinygmula sp. and Drunella sachalinensis, colonized faster than those with low mobility (e.g. the caddisfly species Brachycentrus americanus and the chironomid midge Diamesinae spp.). The abundance of the most common taxon, Baetis, decreased at late stages of colonization, possibly because of low periphyton biomass. Consequently, we concluded that a difference in colonization ability among taxa is an important factor causing succession in stream invertebrate communities in local habitats.

Thermal changes in the gastric evacuation rate of the freshwater sculpin Cottus nozawae Snyder

We experimentally measured thermal changes in the gastric evacuation rate (GER) of the freshwater sculpin Cottus nozawae Snyder under three water temperature regimes (2°, 7°, and 12°C). Laboratory experiments showed that the GER was accelerated with increasing water temperature. This result suggests that the daily food rations of fish are more likely to be underestimated at higher water temperatures if estimation is simply based on the stomach content weight alone. By comparing the GER for various fish species from subfrigid to temperate streams, we found a general pattern that the GER increases with water temperature, regardless of taxonomic group or foraging mode. However, the reaction norms of the GER against water temperature showed considerable interspecific variation. This means that stomach content weight is not comparable as a simple measure for determining the daily rations among fish species when water temperature regimes are different. To consider the temperature-dependent pattern of such a physiological phenomenon is important in understanding the feeding ecology of fishes and their roles in material cycles through food webs in aquatic ecosystems.

Revision of Japanese species of the genus Ecnomus McLachlan (Trichoptera: Ecnomidae), with descriptions of two new species

Five species of the genus Ecnomus McLachlan (Ecnomidae), including 2 new species, are recognized from Japan: E. hokkaidensis sp. nov. from Hokkaidô and E. sakishimensis sp. nov. from Ishigaki-jima and Iriomote-jima, Ryûkyû Islands, are described. Ecnomus japonicus Fischer, originally described from Kyûshû, is re-described. In addition, illustrations of the male and female genitalia are provided for E. tenellus (Rambur) and E. yamashironis Tsuda, which are common species in Japan. Ecnomus kososiensis Kobayashi, originally described from Honshû, and E. tsudai Kumanski, originally described from Korea, are synonymized under E. tenellus and E. japonicus, respectively.

A new lotic species of the genus Leptocerus Leach (Trichoptera, Leptoceridae) from Japan

A new lotic species of leptocerid caddisfly, Leptocerus fluminalis sp. nov., is described based on adult and immature stages from Japan. The habitat and gut contents of this species are noted.