Many species and plants possess a treasure trove of genetic and medicinal knowledge. Given how many poisonous/venomous animal species there are, would it not be pertinent to keep them around to synthesize antitoxins? I hope you don't go swimming and get stung by a Stonefish or if you come across a Black Mamba or a Rattlesnake. (And while you probably won't, there are millions of people that live in areas of the world populated by such creatures.) Or the hundreds of plant species that we use in medicines? That alone is impetus enough to save them, but there's also intrinsic value in saving biodiversity. I pity the generation that doesn't get to see amazing creatures like Polar Bears, or Tigers, or Rhinoceros in the wild, or at all.