Nematodes are a common component of all biocenoses,
They are either common to many biocenoses or a component of all.
"common' means 'often found' (but not in 'all'); and you then say 'they (are found) in all biocenoses"
Before proofreading further: is this an Abstract? This will determine whether definite articles are used or not. As it stands, it is written as an Abstract.
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