The Effect of Organic Farming on Food Intake and Abundance of Egrets and Herons in Rice fields

27 Mar The Effect of Organic Farming on Food Intake and Abundance of Egrets and Herons in Rice fields

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Abstract

水田の有機農法がサギ類（ダイサギArdea alba，チュウサギEgretta intermedia，アオサギA. cinerea）にもたらす効果を検証するため，2013-2014年の5-7月に茨城県と栃木県の有機および慣行水田で野外調査を行った．採食行動調査の結果，ドジョウなどの魚類とカエル類が主食だったが，その割合は種ごとに異なっていた；ダイサギ，チュウサギ，アオサギの順に魚類の割合が高かった．また，いずれの種でも有機水田では慣行水田よりも魚類の割合が高くなっていた．一般化線形モデルを用いて解析した結果，有機農法は採食効率（g/min）に正の影響をもっていたが，その傾向はダイサギでのみ明瞭であるようだった．また有機農法は，ダイサギとアオサギの採食個体数に正の影響をもっていた．以上を踏まえ，本研究では有機農法の正の効果はダイサギでは一貫して認められたが，チュウサギとアオサギでは不明瞭であると結論づけた．この理由としては，食物種（魚類やカエル類）ごとの水田農薬の影響の違いや，有機農法の実施面積割合の少なさ（日本全国では総水田面積の約0.28％，本研究の鳥類センサス地点から周囲200 m以内では18-68％）などが考えられた

English Abstract

To investigate the effect of organic farming on food intake and abundance of three egret and heron species (Great Egret Ardea alba, Intermediate Egret Egretta intermedia, Grey Heron A. cinerea), field surveys were conducted in conventio l and organic rice fields in five cities in Ibaraki and Tochigi prefectures, Japan, from May to July 2013 and 2014. Direct observation of prey captures revealed that fish (mainly Oriental Weather Loach Misgurnus anguillicaudatus) and frogs Ra spp. are the primary food sources, although their compositions differed among bird species; proportion of fish biomass was higher in the order of Great Egret, Intermediate Egret and Grey Heron. In addition, for all three species, composition of fish mass in organic fields was higher than that in conventio l ones. Generalized linear models showed that organic farming had a positive effect on food intake rates (g/min) although the effect seemed to be clear only for Great Egret. Organic farming also had a positive effect on the abundance of foraging Great Egrets and Grey Herons. Therefore, we concluded that the benefit of organic farming was consistently shown for Great Egret, but not for Intermediate Egret and Grey Heron, possibly due to differential effects of rice pesticides on prey species (e.g., fish and frogs) and the small area of organic rice fields in Japan (only 0.28% of rice fields are ma ged for organic farming in Japan amounting to 18–68% of census plots in this study).