Sentence:  Plastics are energy-rich substances, which is <WSD>why</WSD> many of them burn so readily. Any organism that could unlock and use that energy would do well in the Anthropocene. Terrestrial bacteria and fungi which can manage this trick are already familiar to experts in the field. 
Meaning of the word marked with <WSD> tags:  For which cause, reason, or purpose (relative adverb).