Complex sentence: Some species of petrels did become extinct on Marion Island, and thus a "cat eradication program" was set up: a few cats were infected with the highly specific feline panleukopenia virus, which reduced the number of cats to about 600 by 1982.
Split into simpler sentences: Some species of petrels became extinct on Marion Island, and a "cat eradication program" was established. A few cats were infected with the highly specific feline panleukopenia virus, which reduced the number of cats to about 600 by 1982.