For cycling. Everything is made up of elements, but the compounds which define structure and hence functionality are not the same, hence the elements (which are finite) must cycle and require decomposition to be available.
decomposition occurs during life as well...its just that the rate of new cell formation is higher than the rate of cells decomposing.
decomposition is a natural state reaction, one of the two main reactions- composition and decomposition. Usually you can't have one without the other, just like you can't go to a good school and still keep your wallet.
bacterial decomposition occurs during life as well...but at extremely small rate...since bacteria are faced with lysing cells of the body. bacteria decomposition increases logarithmically as soon as the body no longer is able to supply such lysing cells to kill the bacteria.
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Because the body starts breaking down after death because of it's own chemical processes.. plus there is no defense against these micro-organisms anymore.
No, it just means that ecosystems or more or less closed systems. I don't know whether or not SAM worded it like that on purpose, but there is a false sense of purpose implied by her post.