Consumers are usually recognized as animals reducing abundance of prey species or basal resources in the food web. The existence of a certain consumer undoubtedly causes negative effects on other members of the web through consumption, predation or competition. However, it has been revealed that consumers can positively affect the resources and enhance their biomass by releasing nutrients through their activity. This consumer-mediated nutrient recycling process is called nutrient regeneration, and has been getting focused on as important pathways in a food web.

Omnivores can regenerate nutrients from herbivory, detritivory, and carnivory, feeding basal resource such as algae and detritus as well as their prey animals. The regenerated nutrient can be incorporated into algal assemblages and microbes on detritus, and can indirectly affect herbivores and detritivores. However, when their herbivory or detritivory reduce resource that can be positively affected by their nutrient regeneration, the consumption effect may mask or override the direct/indirect effects via nutrient regeneration on the basal resources/consumers. Therefore, in order to understand the roles of an omnivore as a nutrient regenerator in structuring forest aquatic food webs, one should examine the effects of nutrient regeneration on different components of the food web, separately from the overall effect on them.

Tadpoles are omnivores comprising a large portion of the biomass in freshwater habitats including oligotrophic pools, where nutrient regeneration effects may be more important. They consume a variety of foods, such as algae, plant fragments, small animals, and detritus, and should have complicated role in aquatic food webs. Our aim was to understand the effects and relative importance of nutrient regeneration caused by omnivorous tadpoles on other components in the ecosystem.

The strength of the indirect effects of consumption of a particular food item may differ among tadpoles of different species, because the suitability of food items varies among species, and the suitability of the food can affect the consumption rate and/or assimilation efficiency. Therefore, in chapter 2, we evaluated relative suitability of different food items (leaf litter, algae, and worm) for tadpoles of four Japanese common frog species; Rana japonica, Rana ornativentris, Rhacophorus arboreus, and Bufo japonicus formosus. We compared their survival, growth and development by feeding individuals on leaf litter, algae or animal materials (sludgeworms). The suitability of algae and animal materials was different for each species of tadpoles: algae was the best food for Ra. japonica and Rh. arboreus, but worm was the best for Ra. ornativentris, and leaf litter was not suitable for all species. It was predicted that leaf litter might be a supplemental food for these tadpoles.

No study has examined the effect of nutrient regeneration by consumers on microbial conditioning of leaves and further indirectly on leaf-consuming detritivores. We conducted a laboratory experiment in chapter 3, feeding the three food items on the four species of tadpoles we used in chapter 2 to examine the effects of nutrient regeneration by tadpoles on leaf-eating detritivores, and to compare the difference of effects among food items. We conditioned terrestrial dead leaves with water from reared tadpoles (treatments) or food items alone (controls), and compared the C:N ratios of the conditioned leaves and the growth of the isopod Asellus hilgendorfii fed the conditioned leaves. Tadpole feeding activity reduced the C:N ratio of conditioned leaves, and the effect was greatest when tadpoles were fed algae. Isopod growth rates were often higher when they were fed the litter conditioned with water from reared tadpoles. Thus, nutrient regeneration by tadpoles had a positive indirect effect on detritivores by enhancing leaf quality. Tadpoles often occur in nutrient-limited habitats where leaf litter is the major energy source, and their facilitative effects on leaf-eating detritivores may be of great significance in food webs by enhancing litter decomposition.

In chapter 4, we examined the effects of nutrient regeneration and its relative importance in the consumers' overall effect. We conducted a manipulated experiment using tank mesocosms imitating natural ponds with three tadpole densities (zero, low, high). Leaf bundles of three species and tiles as substrate for algae and benthic invertebrates, exclosure and control, were added into each tank. Nutrient concentration in water, phytoplankton abundance, and emerged number of chironomid were measured every week. Tadpoles were retrieved in two weeks. After the introduction of tadpoles, phytoplankton and phosphate concentration in tank water, and epilithic algae and chironomid nest tube on exclosure tiles but not on control ones increased with higher density of tadpoles. Emerged number of adult chironomid was not affected by tadpole presence. Tadpole nutrient regeneration did not affect NP content and mass loss of leaf litter, but mineralization of leaf litter was enhanced in two out of three species of leaves. Our results showed that tadpoles had positive effects on algae, chironomid larvae, and microbial mineralization via nutrient regeneration and that their overall effects were positive on phytoplankton but canceling on epilithic algae and chironomid. Algae - herbivory system was the important path, leading to positive overall effect on phytoplankton, and this path further had an indirect positive effect on chironomid larvae that compensated the negative effects on them.

We found positive nutrient regeneration effects by consumers both on algae and microbes on detritus. The magnitude of the effects was comparable to the negative consumption effects, sometimes exceeding it. The positive effects on basal resources further had positive indirect effects on detritivores and herbivores. We also showed the cross-resource path of nutrient regeneration effects, from epilithic algae-chironomid complex to microbes on leaf litter as well as within-resource effects. These findings showed the importance of considering direct and indirect nutrient regeneration effects on multiple components for more detailed and realistic understating of consumers' role in food web. The magnitude and direction of nutrient regeneration as in from which resource to which recipient should depend on the consumers' food habit, nutrient regeneration efficiency, and environmental conditions. We should recognize the importance of nutrient regeneration effects constituting a complicated food web, in which each consumer has its own direction and magnitude of positive and negative, direct and indirect effects on other components in the ecosystem.

生態系内の生物は生産者(主に植物)、消費者(動物)、分解者(微生物)に分けられ、生産者は光合成などによって有機物を作り、消費者は生産者や他の消費者あるいは生物遺体や排泄物などのデトリタスから栄養摂取を行い、分解者はデトリタスを無機化する。生態系内の「食う食われる」の関係は食物網と呼ばれ、その中には生産者が食べられることから始まる生食連鎖系とデトリタスが食べられることから始まる腐食連鎖系がある。消費者は摂食に伴って栄養塩の放出(栄養塩回帰)を行い、放出された栄養塩は植物の生産や微生物の増殖率を高め、植物やデトリタスの量や質の改変を介して、植食者や腐食者のような消費者に間接的に影響を与える可能性がある。消費者が雑食性の場合、複数の摂食対象から複数の受容者へ様々な栄養塩の流れが起こりうる。しかし、水域においては、消費者の栄養塩回帰による影響は生産者に対する生食連鎖系への直接的な効果のみがこれまで注目され、腐食連鎖系への影響と間接的な効果については研究されてこなかった。

森林の水域の生態系では、藻類が主な生産者であり、林からの落葉が主要なデトリタスで落葉リターと呼ばれる。糸状菌や細菌は葉の表面やその中でそれらを分解し、藻類や微生物に利用可能な栄養塩に変える。これに対して、消費者はその摂食活動を通して藻類や微生物を減少させるだけでなく、摂食に伴う栄養塩の放出を行い、水中における栄養塩の拡散によって藻類や微生物の増殖率を高めると考えられる。申請者は、森林池沼の雑食者であるオタマジャクシ(カエル目幼生)を用いて、栄養塩回帰を介して食物網の他の要素に与える影響を明らかにするために実験を行った。

本論文は5章からなり、1章は序章であり、栄養塩回帰についての概論と問題を提起している。2章では、雑食性の程度を種間比較するために、室内でニホンアカガエル、ヤマアカガエル、モリアオガエル、アズマヒキガエルの4種の幼生に落葉リター、付着性藻類、またはイトミミズを餌として単独で与えて、幼生の生残と発育を調べた。その結果、4種において餌としての好適性は落葉リターよりも藻類と動物性食物が高いことが示された。3章では、幼生の栄養塩の放出が腐食者に及ぼす効果を実験的に明らかにするために、4種の幼生に落葉リター、藻類、またはイトミミズを摂食させた水にニセアカシアまたはケヤキの葉を10日間漬けておき、そのC:N比を調べるとともに、それを腐食性等脚類のミズムシに与えて成長を調べた。対照は幼生を加えなかった水であった。その結果、幼生が関与した葉ではC:N比が減少し、ミズムシの成長は早くなる場合が多かった。このことから、幼生の摂食による栄養塩放出が葉表面の微生物に取り込まれ、その栄養価を高めたと考えられ、栄養塩回帰による正の間接効果を示した。4章では、付着藻類や餌動物に対するCyclorana spp.幼生の摂食による負の効果と栄養塩回帰による正の効果を分離するために、野外の大型タンク内に網で栄養塩を通すが幼生を通さない小区画を作り、その内外の藻類と昆虫の動態を調べた。その結果、全体として幼生は付着藻類と固着性ユスリカを摂食し、植物プランクトンの成長と落葉リターの無機化を促進した。排除区では幼生密度の増加につれて、付着藻類の成長量と植食性ユスリカの生育量が増加したが、幼生の摂食可能な区域では、付着藻類量とユスリカ生育量は低く、幼生密度の影響は見られないことを示した。5章は森林池沼におけるカエル幼生の栄養塩回帰効果を池沼内と森林生態系内に分けて論議している。

このように、本論文は、森林の池沼生態系におけるカエル幼生の機能を栄養塩回帰と腐食連鎖系を通して実験的に解析し、1)幼生による栄養塩の回帰源は主に藻類と動物性食物であること、2)栄養塩回帰を通して幼生は落葉リターの栄養的質を高め、腐食者の成長を促進しうること、また、微生物の活性を高めて落葉リターの無機化を促進しうること、3)栄養塩回帰を通して幼生は藻類の成長を促進して藻類食者に、また、落葉リターの栄養的質を増加させてリター食者に、それぞれ正の間接効果を生じうること、4)付着藻類の生産および、植食性ユスリカの生育量に対する幼生の栄養塩回帰による正の効果は、消費による負の効果に匹敵することを示した。この成果は、森林生態系における生物間の間接効果の重要性と森林管理における生物多様性の意義を明らかにしたものであり、審査員一同は,本論文が学術的にも応用的にも価値が高く,博士(農学)の学位論文に値すると判断した。