環境中の様々な状態(pH, 気温, 湿度, 栄養塩濃度など)が生物の適応を支配してきたのと同時に、生物も様々な過程を経てそれらの環境の状態に寄与してきた。環境から生物へ、生物から環境へ、それぞれの関わり方に関する研究は数多く存在するが、この双方における相互作用を包括的に取り扱うことは非常に複雑で困難である。
これに対しLovelockは「生物が存在することにより地球の恒常性が保たれている」という見解を"ガイア仮説"として発表した。この仮定が満たされるためには(1)生物が環境に影響を与える、(2)環境が撹乱された際にそれを制御する振る舞いをする生物(調節種)が選択される、という2つの条件が満たされる必要がある。しかしながら、調節的な振る舞いは利他的であるため(2)は生じ難いとされ、この仮説は多くの生物学者から否定的に扱われてきた。
我々は(2)が生じるメカニズムを模索し、環境-生物-資源の抽象化モデルでその検証を行なってきた。その結果、資源の循環と生物の個体群数に揺らぎが存在する場合、もしくは生物間に資源利用に対するトレードオフが存在する場合に調節種が選
択されうることが明らかになった。これらのモデル系は現実の生態系に適用できる可能性があり、生物
によって地球の恒常性が保たれているという見解に対してより強い現実性を与える。
Environmental conditions (pH, temperature, salinity, etc.) affect the
growth and adaptation of organisms and organisms also alter their
environmental conditions via various processes (e.g. emission or
fixation of greenhouse gasses, consumption of nutrients). Although
each process has been well established, interactions between
environmental conditions and environment- affecting species have been
poorly known. The Gaia hypothesis was proposed to explain that
organisms contribute to homeostasis of the environment, which is kept
habitable for life. The hypothesis views feedback between biota and
environmental conditions as a key role of such homeostatic control.
Supposedly, regulative feedback by biota needs the following two
prerequisites: (i) emergence of an environment-affecting species and
(ii) a mechanism in the system whereby a species that is favorable to
the whole biota (e.g., that generates a moderate temperature, or
stabilizes a nutrient supply on land in a moderate range) is selected
over a species with an adverse effect on it (e.g., that causes
extreme temperatures, or decreases a nutrient supply). Because of
the theoretical difficulty to select a favorable or altruistic
species, however, many biologists dismissed the idea of self-
regulation by biota. We have sought to understand how the second
prerequisite is met using simple abstract models which are composed
of environmental-condition, two species and resource. As a result,
we found that the selection of regulatory species may emerge from
fluctuation in the growth rate and trade-off in ability to utilize a
resource. It may provide a stronger support to the biotic
homeostasis of the earth.
-- 引用文献 --
[1] Lovelock, J. E., 1988.
The Ages of Gaia - a biography of ourliving Earth. The Commonwealth Fund Book Program. W. W. Norton & Co.,New York
[2] Watson, A. J., Lovelock, J. E., 1983.
Biological homeostasis ofthe global environment: the parable of daisyworld. Tellus 35B, 284 - 289.
[3] Akagi, T., 2006.
Maintenance of environmental homeostasis by biota, selected non- locally by circulation and fluctuation mechanisms. Artificial life 12 (1), 135 -152.
[4] Seto, M., Akagi, T., 2005.
Daisyworld inhabited with daisies incorporating a seed size/number trade-off: the mechanism of negative feedback on selection from a standpoint of the competition theory. Journal of Theoretical Biology 234, 167– 172.
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