The blocks are solved on average every 10 minutes, if they happen quicker than that difficulty goes up, if it happens slower then difficulty drops.
So, you can see how adding more and more hash speed to the network makes the difficulty go up.
Ok Ya I can see that now. So if the price of BTC plummeted and miners started pulling the plug because profitability was no longer there then the difficulty level would drop compensating for the reduced amount of Hashing.
Yes, BTC would have to drop to under 2000$ though. Not impossible, but it seem improbable to me, now.