Why are lion numbers decimated? Human hunting. Seems quite straightforward.
How do they repair? Stop hunting.
Any other answer is selfish grredy spin.
If you stop hunting lions in the RSA, or whitetail in middle America for that matter, the populations will exceed the carrying capacity of the habitat available, and crash: massive die-offs, mostly due to disease (like the EHD outbreak in my area, which killed an estimated 60% of the whitetailed deer in my county in the space of 2 months last fall).
leadcounsel, I don't think you understand the problem, and are making judgements based on emotion rather than logic.
I hunt animals.
Some I hunt so that I might eat them, like deer and wild turkey, both of which all but completely disappeared from my state when there was no game management/sport hunting. Once hunting was regulated, and then actively supported and promoted by the State Game and Parks Commission, populations exploded: there are now more whitetailed deer and turkeys in Nebraska then ever, including before Columbus ever hit Hispanolia (because of modern, irrigated row crop farming, the land produces hundreds hundreds of times more forage than it did before, and fire suppression means there are trees here that could not exist in the "sea of grass" of 150 years ago- so there is cover- shelter from the wind and snow that did not exist ......
Some animals I hunt for pure sport- because I want to. You may not like that, but here's the deal: tough. It's legal. Don't like it? Seek to change it ...... but take care that unintended consequenseces don't bite the species you are intending to "save": take the case of the Black Tailed Prairie Dog in western Nebraska......
I used to go out and shoot prairie dogs several times every summer at a distant relative's farm ....... (when I was a kid, this was common weekend activity, and kept the populations in check) ...... then one year, the landowner (we'll call him Frank D. Farmer) tells me that due to pressure by animal rights activists in Colorado, the US Fish & Wildlife Service is making noises to him that they are "endangered" and they are going to outlaw so much as harrassing them, let alone controlling their numbers ...... now Frank only had one pasture (and his brother had one, and his niece, and they rotated the cattle between these pastures)and his cows had to share the grass with the prairie dogs in these pastures ..... and had done so for all my life. Faced with the possibility of not being allowed to manage his own pasture, Melvin went out and purchased, through his County Extension Office, IIRC, a large sack of phosgene tablets, and dropped a few down every hole in the 'dog town. Every other landowner in the area did the same. Now there are NO prairie dogs for me to shoot ...... are you happy now?
Everything has a purpose in Nature ...... until large herds of overly emotional and under-informed ...... people ....... who do not understand things come along and start telling people that actually live on the land how they must do things ...... then things get pear shaped in a hurry.