My rather flippant, off the cuff remark did have a serious intent. For the purposes of taxonomy, species may well be distinguished by the number of spots on their wing cases, the colour of their winter plummage, the number of petals...
The other definition is "a group of organisms capable of interbreeding and producing fertile offspring.". In this sense, it is not a "structure we humans impose on the world"" but a practical reality and has nothing to do with the human ability to distinguish between species.
Nothing is ever that simple of course. Lions and Tigers don't mate in nature because, under normal circumstances, they never meet. When they do (thanks to human interference), the offspring are of limited viability. (Google on Ligers and Tiglons). Other obvious examples are mules and zebroids. On the other hand, the Ruddy Duck from the Americas has caused problems by crossbreeding with the European White Headed Duck.