A common justification for buying meat is that one's individual consumer choices won't make any difference: the supermarket buys steaks by the crate, and so isn't sensitive to such tiny changes in demand. It'd take (say) a hundred boycotters to make a difference. But there's a subtle fallacy here. It might typically take 100 boycotters to ensure that one less crate of 100 steaks is bought. But one of those hundred individual choices must have made the difference between the store choosing to buy X crates or X-1. We just don't know which one -- where the tipping point lies -- whether we just need to decrease demand by 1 more steak, or 36, or 99, before the store will respond. So, in the absence of any further information, any individual consumer should see their personal steak boycott as having a 1/100 chance of reducing the store's purchasing by 100 steaks. (And so on up the supply chain.) That's an expected impact of (ta-da) one steak. The "chunkiness" of the market's sensitivity thus makes no difference. Your lessened chance of making an individual impact is exactly counterbalanced by the higherI hope nobody can find an objection because I make much the same argument in my book.
steakspayoff if you happen to succeed in influencing an entire 'chunk' of demand.
The point generalizes to many other 'chunky' impacts, e.g. life-saving charities, or situations where each of many individuals has an equal chance of making the decisive difference for all of them. For a slightly different case, Derek Parfit made a similar argument in defense of voting: even if your individual vote only has a 1 in a million chance of making the difference between electing Inferior and Superior, it's worth it if the election of Superior would raise average welfare by more than what it cost you to vote. (The structure of this situation is different from the sort of consumer 'chunking' discussed previously, but hopefully the similarity I'm highlighting is clear enough.)
P.S. Technically, the consumer impacts aren't quite so straightforward as all this. For example, reducing demand might lower prices, causing some others on the margin to buy slightly more meat than they otherwise would have. But this presumably won't completely counteract the good done by one's own abstinence -- so we're not in "moral dupe" territory yet. Anyway, my point here is just that chunking doesn't undermine the expected efficacy of our individual decisions. Other things might, but evaluating other objections is a job for another day.
The Tipping Point
Richard Chappell offers this solution to the "causal impotence" problem (see also Alastair Norcross here):