It seems that the best way for managing the impact of some activity is to reduce the incidence of that activity. People can stop doing it, or they can do it less. Often, the thing we want to reduce is part of a process; perhaps buried deep in some chain of process, or perhaps used directly. Or the thing might be cultural; it might be part of a belief system, or simply fun to do. In any case, the activity is being used, directly or indirectly, by people. The basic choice of action here is between finding a replacement for the activity, finding replacements for the end products, or reducing demand for the end products without any offering of replacement. People generally don't like loosing things that are part of their lives, so changing demand without any replacement can be a struggle. Business does not like change in any form, and anything that looks like an impingement on business as usual is going to see very strong push-back. Replacement at some level is always going to be the place to start.
For example, let's imagine a perceived need to reduce the use of a weedkiller that is considered a pollutant because of its side effects. Simplifying, there are perhaps two relevant populations here: farmers and gardeners, and these populations are different because they might need different approaches. In either case we might want the farmer or gardener to reduce use of the weed killer to zero, in which case gardeners might be persuaded one at a time, while farmers might want a more all or none approach. Assuming there is a reasonable way of managing without this weedkiller we can apply persuasions of various sorts, including outright banning, and, one way or another, we can say that the incidence of the problem has been reduced.
A more challenging example comes from the destruction of forest environments (destroying habitat) to supply the demand for palm oil. This happens as part of the process for producing the rather large amounts of palm oil that the world demands, and looking for a replacement is clearly going to be the way to go. The action we want to reduce is the destruction of the forest. However, palms are being grown in countries that have few resources other than land, albeit land covered in trees. They are, unsurprisingly, reluctant to contain production. There is also the problem of the cost of operating more sustainable plantations and the impact that reducing forest clearance might have on prices. So, looking at the immediate end product, can we replace palm oil? Unfortunately, it turns out that palm oil is already a replacement for soybean, and other, oils as it needs significantly less land and any replacement could have worse effects. So, demand reduction then? It is used as a biofuel, so there is an opportunity. Biofuels compete with food for land use, so an examination of transport options might be useful. It is used in cosmetics. Is that an opportunity? Perhaps not. The real problem, though is that palm oil is used in food production. And at this point the issue goes onto the 'too difficult' pile.
Global scale problems, particularly those relating to food and water, tend to drive people into distinct camps. There are those who think that the global population is too large and we are all going to hell in a handcart. There are those who think that technology will solve the problem and we can all eat vat grown meat. And there are those who think that there is enough to go around and some form of redistribution will sort it. The redistribution view comes from noting that the richest people in the world use resources out of proportion to their numbers. It is, after all, consumers who drive the demand. On the other hand, capitalist production, relying as it does on competition and a race to produce more than the next guy, can also be seen as a problem. It is, after all, advertising that drives the consumers1.
So, does population matter? Up to a point. But, though scale is always an issue, we cannot afford to throw in the towel just because the problem got too big. We really don't need the rich countries to impose population sanctions on poor ones - the statistics don't warrant it. What we do need to do is recognise that life is way more complicated than looking at one global population figure and coming up with the simplistic view that this big number must be the cause of everything2.