The aim of this investigation is to answer the question of why it is prima facie morally wrong to cause or contribute to the extinction of species. The first potential answer investigated in the book is that other species are instrumentally valuable for human beings. The results of this part of the investigation are that many species are instrumentally valuable for human beings but that not all species are equally valuable in all cases. The instrumental values of different species also (...) have to compete with other human values. Sometimes these other values probably outweigh the value of the continued existence of the species. In general the degree of uncertainty is very high and the precautionaty principle is recommended to deal with these uncertainties. We also found that we have a duty to consider the interests of future generations of human beings and that these duties, in general, speak in favour of preservation. Anthropocentric instrumentalism therefore provides us with rather strong reasons to consider many cases of human caused extinction as prima facie morally wrong. Even so, anthropocentric instrumentalism does not fully account for the moral intuition we set out to investigate. The next potential answer that is investigated in the book is that species have a moral standing in their own right. The result of this part of the investigation is that this idea is highly unlikely, in particular because species cannot have any interests to consider. Anotgher potential answer is that species have intrinsic value in some other meaning that does not imply moral standing. We concluded that it is possible to be subjectively valued as an end and that many species have properties that make them highly suitable for being valued as ends by human beings. Finally, we found that our contributions to the extinction of species in most cases frustrate the interests of many non-human sentient beings. This is true if the species in question is made up of sentient individuals, and it is also true when the species in question is made up of non-sentient individuals that have instrumental value for sentient individuals of other species. There are exceptions to this rule, but all in all it seems that the inclusion of non-human sentient individuals together with us humans as moral objects, in most cases, tip the scale drastically in favour of preservation. The main result of the investigation is that there is not one but several explanations to why it is prima facie morally wrong to contribute to the extinction of species – and all of them are about duties to respect the interests of individual sentient animals, including human beings. (shrink)