Despite discussing animal welfare, Singer doesn't really go into wild animal suffering much in Practical Ethics, so I didn't really go into it in yesterday's review. However, in the EA memes group it is one of the main topics of discussion, and it's potentially a genuinely very important consideration in how we ought to behave towards the natural environment. While writing this, it occurred to me that much of it probably applies to domesticated animals as well.
There is a long tradition of people arguing that human life is universally terrible. The most famous person to have argued this is probably Arthur Schopenhauer, although David Benatar has also made an argument of this sort as part of his general case against child-bearing. (At some point I will write a post going into detail on why Benatar is wrong, for he is and it's quite obvious that he is once you see the problem. But enough about my MA dissertation...)
Sometimes they appeal to the great sufferings that we endure in life. Sometimes they start with an axiological approach to "what the good life consists in" and argue that it is rarely met. Either way, the fact is that plenty of people have made these arguments, and yet the fact is that the vast majority of us are glad to be living.
I think a large problem with these kinds of arguments is that while they may allow us to come up with a rough ordinal ranking of lives, there is no cardinal value of a life - and therefore no objective "zero point" at which one should be indifferent between existing and not existing. That's not to say there is no such point, indeed I think there clearly is: rather, it is to say that where this point lies is a subjective issue. A life which I consider worth living may not be one that anyone else would consider worth living, even if there is no disagreement either upon what that life involves or on the value of the life and its components.
(The obvious response to this claim is to ask whether it is really plausible that one could rationally prefer an existence of utter misery to non-existence? I would say two things in response: first, I would be inclined to revise to the slightly weaker claim that the extent to which pleasure balances out pain is subjective, hence a life must include at least some pleasure to outweigh the inevitable sufferings. Second, quoth Hume: "'Tis not contrary to reason to prefer the destruction of the wntire world to the scratching of my finger. 'Tis not contrary to reason for me to choose my total ruin, to prevent the least uneasiness of an Indian or person wholly unknown to me. 'Tis as little contrary to reason to prefer even my own acknowledged lesser good to my greater, and have a more ardent affection for the former than the latter." - A Treatise of Human Nature, book II, part iii, section 3. To say someone ought not to consider a life of almost complete suffering to non-existence is to accuse them of irrationality, but rationality is at least primarily, if not entirely, concerned not with what one takes as goals but rather how one seeks to achieve them. Such a preference ranking would be strange, but not irrational).
Arguments for the importance of animal suffering tend to presuppose a roughly hedonic view of the good life - which is fair enough, given that most animals lack the requisite cognitive resources to have a conception of whether their lives are happy overall, rather than merely whether they are happy in any given moment. They then proceed to detail the many sufferings undergone by animals: cold, starvation, being eaten alive, etc. What we have, then, are two challenges to this kind of argument:
(1) it is impossible to simply compare pleasure with pain. These two phenomena may occur in the same subject, but unless that subject is sufficiently advanced to see itself as persisting over time and have opinions on this, there is simply no fact of the matter as to whether the pleasure outweighs the pain or vice versa. Hence while we might think that the improvement of animal conditions is a morally good thing, and the worsening of their conditions a morally bad thing, one cannot say that an animal life containing both pleasure and pain is either good or bad as a whole.
(2) philosophers have tended to make a particular kind of argument to the effect that humans generally suffer by existing, yet we know empirically that these philosophers are wrong. We should therefore be highly skeptical of such arguments when made in relation to animals.