Sometimes an introduced or invasive species of animal threatens to displace native species. Canes toads in Australia are an example of this, and it has been Australia’s policy to kill cane toads to protect native species.
Here is what Peter Singer had to say about this: "I think we should be concerned about all animals, not only those that happened to arrive here before 1788. Where killing can be done humanely, and is necessary to preserve endangered species, it may be defensible." The quote taken from: http://www.abc.net.au/environment/articles/2011/01/17/3113142.htm
With so many species on the edge of extinction, it is hardly surprising that people wish to protect biodiversity. A list of Evolutionarily Distinct and Globally Endangered (EDGE) species can be found here: http://www.edgeofexistence.org/species/
Based on what Peter Singer said, it would appear that he may approve of the humane killing of invasive species like cane toads to protect biodiversity. It is an interesting statement, since Singer is often quoted by animal rights activists who claim that humans have no right to kill animals.