It is all a matter of definition and scientists disagree. For some if animals can interbreed naturally and produce fertile offspring then there is there only one species. For others any distinct population is a separate species and it is terrible when they interbreed and mess up the charts.
Tigers and Lions can and do produce fertile offspring.
Reality doesn't conform as well as some would like to neat little boxes. Science likes to put things into neat little boxes - but putting things in boxes is not itself science.