Complex sentence: The main threats it faces are destruction of the pine forests in which it lives and building work but its steep decline is unexplained.it may be the victim of disease such as chytridiomycosis but this had not been reported in Chile when the decline started.
Split into simpler sentences: The main threats it faces are destruction of the pine forests in which it lives and building work but its steep decline is unexplained. It may be the victim of disease such as chytridiomycosis but this had not been reported in Chile when the decline started.