Hi Gedusa,
you are trying to overcome your bias towards the 'sanctity of life' and other biases? It is an important part of our quest for knowledge to overcome these biases. Good luck. I try too to be aware of my biases. I hope they are not working their spell in these utterances.
When you say that humans have the capacity to increase the utility of the biosphere, if you mean by this that we have the capacity to genetically design wild animals and thereby increase their hedonic experience per individual, there is doubt among scientists that this will ever work. If we try to raise the hedonic setpoint of 'wild' animals, these animals might not survive. In 'The Extended Phenotype' Richard Dawkins points out that nature might have provided any given species with an optimum level of mutations already. Therefore, for example, the range of the hedonic setpoint of the gazelle might already be as high as nature will allow before the species has such a high pleasure to discomfort ratio that it gets killed too easily and becomes extinct. Dawkins provides premises for this suspicion by pointing out that there are various species separated by continents who have independently developed similar mutations as their cross continent cousins. This gives us reason to doubt that nature has been inefficient in trying to give wild animals a higher hedonic set point. If nature has experimented with the hedonic setpoint of gazelles, for example, then as low as the pleasure to pain ratio is for an individual gazelle is, it might be as high as nature will allow without sacrificing the gazelle species ('sacrifice' being a metaphor of course, the species would not feel pain during its 'sacrifice').
Furthermore, there has been no precedent for genetically altering the hedonic set point of wild animals. Even predictions of the future that have a precedent are logically considered weak inductive arguments, never mind predictions that have no precedent. Therefore, predicting that humans will one day be able to alter the hedonic setpoint of wild animals, an unprecedented prediction of the future, is tenuous.
If on the other hand, you mean that humans have the capacity to increase utility in the biosphere by breeding animals, then how do we have that capacity, when we do not know whether an individual animal's life is worth living. Bear in mind that evolution cares for the species, not for the organisms. Many humans make the philosophical choice to breed. Let us assume for argument's sake that we can infer from deliberate human reproduction that up to a point human life is worth living. Non-human animals do not reproduce through philosophical choice. They do not reproduce because their lives are worth living. They might be blindly reproducing despite their lives not being worth it. (I doubt, however, that we can infer that human life is worth living from deliberate human reproduction, because humans who reproduce have not yet lived through old age and so they have not experienced what is in most cases the worst part, whose trials I will not detail here).
We have been given insight into non-human animal experience through the autistic animal welfare campaigner Temple Grandin who has been observed to be and believes she is akin to non-human animals in her hyper-sensitivity, and she describes the experience she feels when she passes suspicious strangers as 'hell'. I hope it is not that bad for non-human animals.
Ahhhh, the 'natural' world. (I prefer to put 'natural' in commas because I prefer to use abstract nouns like 'natural' in their broadest sense unless I specify otherwise or put them in inverted commas. The broadest sense of natural of course includes humans; the definition I am using here does not.) To remind us of how painful life can be, (here quoted from Wikipedia) Schopenhauer says, 'A quick test of the assertion that enjoyment outweighs pain in this world, or that they are at any rate balanced, would be to compare the feelings of an animal engaged in eating another with those of the animal being eaten.' So, I doubt that the 'natural' world has positive utility.
My last point is that you would find it difficult to justify ending a life because you think that it is not worth living. I did not say, however, that that would necessarily be justifiable. My point was that if our species were to become extinct, there would be much less suffering on earth. Although a human free earth would have less suffering, that does not mean that we should try to achieve that. One need not prescribe venturing out at night with a body bag and shovel. Rather, tell your friends, if you see an asteroid the size of Texas coming, don't call the police.