Sentence:  The staminate willow catkins , then , provide their visitors with both nectar and pollen ; a marvelous arrangement , for it provides exactly what the bee queens need to make their beebread , a combination of honey and pollen with which the young of all <WSD>species</WSD> are fed . 
Meaning of the word marked with <WSD> tags:  (biology) taxonomic group whose members can interbreed