( It ( ( ( ( ( ( is rarely ) , ) however ) , ) ( that ( ( food chains ) ( can ( be ( expressed ( in ( such ( simple terms ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADVP (RB rarely)) (, ,) (ADVP (RB however)) (, ,) (SBAR (IN that) (S (NP (NN food) (NNS chains)) (VP (MD can) (VP (VB be) (VP (VBN expressed) (PP (IN in) (NP (JJ such) (JJ simple) (NNS terms))))))))) (. .)))
It is rarely, however, that food chains can be expressed  in such simple terms.
(ROOT (S (NP (NN Simple) (NNS carbohydrates)) (VP (MD can) (VP (VB be) (VP (VBN classified) (PP (IN in) (NP (NP (NNS terms)) (PP (IN of) (NP (NN chain) (NN length)))))))) (. .)))