The Silph Road has done quite a bit of research into spawning mechanisms and their findings have been corroborated by much of the community. It isn't necessarily guaranteed to be 100% accurate but the evidence supporting it is overwhelming. In either case, all their work is reproducible and peer-reviewed so it's largely considered to be the most accurate non-official information available.
From what I understand of their findings, the world map is littered with "spawn points" (marked by the rustling grass animations), which spawn one pokemon at regular intervals. Most spawn points trigger once per hour but I've heard of some spawning as often as every half hour (although it is possible that the location simply has two spawn points very close together). A pokemon spawned in this way lasts for 15 minutes before despawning and will be available to any player who is in range during that time. Every spawn location has different timings so one might trigger at :02 past the hour and another might trigger at :29.
I have not yet seen any evidence to suggest that these points may cease spawning at any time for any reason. We do know that they are tied to locations that historically have increased mobile data usage, which is why there are tons in the downtowns of larger cities but very few, if any, out in the country.
Assuming this is all correct, it is theoretically possible to momentarily deplete an area of pokemon by catching everything that is currently spawned but that will only affect you and will only last until the next spawn point triggers.
If I were to speculate, I would say that the "dry area" effect is a description of the 45 minutes per hour that there are no pokemon at a given spawn point. If your area only has one or two spawn points within radar range (especially if they all trigger at roughly the same time), the effect would be highly visible, which is likely what that other user was seeing. I don't know whether the map turning grey is related to this but I would suspect it isn't. Instead, I would chalk that up to a graphical glitch but I don't know that for certain.
When a Pokemon spawns in, it does not move. It remains in the same spot for 15 minutes, then despawns.
A Pokemon may also disappear if it flees while you are trying to capture it. Each Pokemon has a certain flee chance.
Best Answer
When talking about IV vs CP,
There are 3 IV's that we know of; Attack, Defense, and Stamina. Defense and Stamina are important when it comes to defending in gym battles, and Attack and the move you are attacking with's base damage and attack speed are important for attacking gyms.
So for defense of gyms, a Pokémon with a lower cp but higher Defense and Stamina may be a better pick than a higher cp Pokémon who just has really high Attack. For attacking, you will want to prioritize Pokémon with higher attack and lower defense, as you can always dodge to avoid taking a lot of damage. See here for more info.
Also, see here or here for an IV calculator.