Studies of the population dynamics of Trypoxylus dichotomus (Coleoptera: Scarabaeidae), a bioindicator species of the Satoyama landscape, for determination of its baseline reference density

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  • 里地里山の生物指標としてのカブトムシの基準個体群密度の設定と個体群動態
  • リチ サトヤマ ノ セイブツ シヒョウ ト シテ ノ カブトムシ ノ キジュン コタイグン ミツド ノ セッテイ ト コタイグン ドウタイ

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Abstract

Bioindicators are important for evaluating desired outcomes accurately and inexpensively, such as increases in biological diversity. Reasonable baselines are needed to monitor changes in Japanese Satoyama landscapes. At each locality, a baseline density should be determined for every bioindicator species in relation to both surrounding land-use and monitoring method. The Japanese horned beetle Trypoxylus dichotomus septentrionalis is one of the most emblematic and charismatic species in the Satoyama landscape. The number of beetles attracted to fixed light sources was surveyed every day except for rainy days from 2004 to 2011 at a location in the Izu Peninsula. Annual net replacement rate (Nt+1/Nt) was significantly inversely related to the number of beetles collected in the previous year (Nt) (R2=0.601, P=0.0406). Populations fluctuated around an equilibrium value of 73, and the deviations from the equilibrium value were related to the mean monthly temperature in June (T) when the beetles were pupae or teneral adults. A high negative correlation (R2=0.4570, P=0.0657) between Nt and T was observed. Female body length, but not male's, was mainly determined by the mean monthly temperature in April(R2=0.8470, P=0.0033). A warm early spring seemed to encourage an earlier development of overwintered larvae and contribute to the production of larger adult females. Hot summers negatively affected the number of beetles, but the density dependent population processes provides a resilience that may prevent the beetle from an ever more rapid decline. The equilibrium density of a bioindicator species could be used as a baseline density in biodiversity conservation projects.

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