I'd say deforestation is a main cause of extinction for animal species... loss of habitat. Not everything is reversable.
Agreed there.
I was thinking in terms of the very public debate about forestry. Given that there have been instances of previously logged forest being nominated (by environmentalists) as an area of high conservation value now that it has regenerated, the regeneration can't be that bad. Not perfect maybe, and I certainly don't claim that replacing a natural forest with a monoculture plantation is legitimate "regeneration", but it's possible to replant mixed species and put back something that, whilst different, doesn't really constitute the environment having been "wrecked".
Agreed about animals. The studies that I have seen found that firewood collection is a far bigger problem on a per tonne of wood taken basis than the contentious clearfelling for woodchips since firewood targets the dry, hollow logs thus maximising habitat loss. I don't recall the exact figure, but the impact on habitat loss was an order of magnitude difference per tonne of wood taken.
Overall, I'd put it this way. If a forest was logged in 1950 and then properly regenerated then what we have today is likely to be a reasonable forest. Perhaps not perfect, but it's a forest with mixed species, animals and so on. Done properly, we could keep taking some wood from that forest every so often and nothing drastic is likely to happen. A change in the environment certainly, but the underlying activity is reasonably sustainable if done properly (doing it properly being the key point).
Contrast that with an oil field put on production in 1950. With the exception of the very large fields such as Ghawar, Burgan etc (Saudi Arabia and Kuwait respectively) which actually do date back that far, odds are it's all but dried up now. The oil's gone, in practice, forever. And the carbon is in the air for longer than it's worth calculating. We can't undo the damage. And no matter how it's done the end result is the same - the oil's gone and the carbon's in the air. Extracting a non-renewable resource, or one that renews incredibly slowly, just isn't sustainable when done on an industrial scale no matter what technology is used and how carefully the operation is done. Sooner or later it's all gone.
Dams are much like forests in terms of ability to restore the landscape. Studies in Tasmania have found no real problem with restoration of even large (240km²) man-made water storage sites. That said, I'm absolutely opposed to actually doing such a restoration right now (as is essentially everyone involved with the issue) as I see the value of renewable hydro power as vastly exceeding any benefit in restoration at the present time. We need to do something about fossil fuel energy with a degree of urgency given the depletion, pollution and climate issues. In contrast, restoration of a water storage is likely to be no more difficult (that is, quite doable) in 20 or 100 years time than it is today. Sustainability versus conservation (or in this case restoration) of an untouched natural environment.
In contrast to water storages and forests, a future generation has absolutely no chance of getting back the oil and gas we're burning at such an incredible rate. And they've got close to zero chance of doing anything about the carbon we're putting in the air, the nuclear waste we're piling up or the persistent pollutants we've added to the land and oceans. Hence I'm not too worried about dams or a
properly managed forestry operation with proper regeneration.
Note that there's a big difference between proper forest management as a long term source of wood (fairly uncommon) and simply clearing land or putting in plantations (all too common). Forestry's one area where I think we did a lot better environmentally 50 years ago than we do today. The trouble now is the pursuit of efficiency - a concept that doesn't fit well with the natural environment.