I think the drought and poor habitat it the main cause.. followed by poaching. I really believe a lot of deer are illegally killed year round. There is virtually no big game management in So. Cal zones deer for habitat improvement. I honestly don’t know about the northern zones, but assume the same is true.
One thing that is missing is water... artificial water sources like BG guzzlers and livestock dirt tanks. There used to be cattle in much of the so. Cal area.. MNP, (pre preserve status), the Santa Ana canyon, etc. remove the cattle and the water is also gone because no ranchers are there to maintain it.
Best example I can compare to is AZ. Cattle everywhere. Cattle = water. Water= wildlife. Simple. Additionally, they have 3000 BG guzzlers!. You can’t go a mile or two in any direction in Az. Without a natural or man made water
Source.
Any of you who hunt Az know what I’m talking about... they also have a ton of lions... I know they hunt them and I wish we could..that said if we had the cattle/water like they do we too would have better deer numbers.
Our only deer habitat improvement come from huge wild land fires which initially destroy an area. 2-3 years later they begin to produce better/ good habitat. In 10 years it needs to burn again..