When talking to visitors of our museum/zoo, some of the most frequent questions asked are "Why is keeping species from going extinct an activity worthwhile to pursue?" or "Why does it matter if this midge or that tick is going extinct?".
While most biologists will probably reply that species have very specific roles in their ecosystems and thus the extinction of one species can probably affect others, too, or that each species is part of the richness of our nature (a value in itself so to speak), some non-biologists are often not that easily convinced (especially when you refer to species not very attractive to the human eye). What even more hampers an easy answer is that more than 99% of species that ever existed have died out in the course of evolution. So what is all that conservationist effort, to 'preserve the current state' all about? Changes in biodiversity and mass extinctions have always happened, so why don't we let it just happen now (to put it insensibly)? There are also some human related arguments, for example "we breathe air and thus need plants of some sort". But that does not mean I need every plant species on the planet to produce oxygen, crops will do that, too...
I would like to hear your thoughts on this to have some good arguments at hand when dealing with the next inquisitive visitor. ;-)