For example, early successional forests gain biomass as trees grow and thus act as a carbon sink (Figure 3). Because of mass balance, over time the amount of any element in any one of these compartments could hold steady (if inputs = outputs), increase (if inputs > outputs), or decrease (if inputs < outputs). Any compartment could represent a biotic or abiotic component: a fish, a school of fish, a forest, or a pool of carbon. Scientists conceptualize ecosystems as a set of compartments (Figure 2) that are connected by flows of material and energy. Though no real ecosystem is a truly closed system, we use the same conservation law by accounting for all inputs and all outputs. Though most ecosystems contain so many individual reactions, it would be impossible to identify them all, each of these reactions must obey the Law of Conservation of Mass - the entire ecosystem must also follow this same constraint. And each of these important elements cycle through the Earth system.Įcosystems can be thought of as a battleground for these elements, in which species that are more efficient competitors can often exclude inferior competitors. Living organisms are primarily made of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. The biomolecules that are the building blocks of life (proteins, lipids, carbohydrates, and nucleic acids) are composed of a relatively small subset of the hundred or so naturally occurring elements. Life involves obtaining, utilizing, and disposing of elements. These analyses are as important to the progress of ecology as Lavoisier's findings were to chemistry.

Ecologists can apply the law of conservation of mass to the analysis of elemental cycles by conducting a mass balance. The atom itself is neither created nor destroyed but cycles among chemical compounds. An individual atom of a biologically important element, such as carbon, may have spent 65 million years buried as coal before being burned in a power plant, followed by two decades in Earth's atmosphere before being dissolved in the ocean, and then taken up by an algal cell that was consumed by a copepod before being respired and again entering Earth's atmosphere (Figure 1). Because of this, individual atoms that make up living and nonliving matter are very old and each atom has a history. Therefore, in the everyday world of Earth, from the peak of the highest mountain to the depths of the deepest ocean, atoms are not converted to other elements during chemical reactions. Most elements come from fusion reactions found only in stars or supernovae. The Law of Conservation of Mass holds true because naturally occurring elements are very stable at the conditions found on the surface of the Earth.

Lavoisier's finding laid the foundation for modern chemistry and revolutionized science. If we account for all reactants and products in a chemical reaction, the total mass will be the same at any point in time in any closed system.

In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions.

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