Earth is not a thermodynamically closed system; all bio-geo-chemical cycles within its boundaries are driven by the continuous influx of solar radiation. The complex ecological systems in the biosphere as well as nature's recycling processes for the key elements such as carbon are all fueled by this single source of energy. The primary anabolic activity in the biosphere uses solar energy to create products such as carbohydrates via photosynthesis, and dissipates low-grade thermal energy (energy that is difficult to collect and put to use) in the process. The second law of thermodynamics requires that this conversion of photon energy into other forms be less than 100% efficient and some release of thermal energy is inevitable.
Nature is able to methodically pass on food products throughout the ecosystem employing a complicated web of nutritionally interdependent life forms. This allows, for instance, the phytoplankton in a marine ecosystem to be consumed by zooplanktons that are in turn consumed by fish, and these in turn by birds, and so on. Depending on how complicated the ecosystem of interest is, it is possible to discern linear food chains or complicated food webs along which photosyn-thetically trapped energy flows through the system to sustain higher organisms. Dead organic matter (leaves or animal carcasses) are a valuable carbon source to soil microbiota and are readily biodegraded and their constituent elements returned promptly to their respective bio-geo-chemical cycles. The cycles work so well because the occupants of various niches in the ecosystem have evolved and modified themselves to best fit those individual roles. This makes it possible for nature to efficiently create low-entropy materials using solar energy, to sustain a complex web of life, and to effectively reuse the same elemental building blocks over and over again.
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