I was going over Google Analytics with one of our clients the other day and noticed that the traffic they were getting from Google search had dropped by about half over the past year. Their site was still the same, but wasn't coming up on the first page on certain key search terms any more. What was wrong? What was wrong was that their site was the same, and the competition's had changed. A good analogy comes from Alice's adventures through the looking-glass where...
Alice first meets the Red Queen. In the middle of the conversation the Queen grabs Alice's hand and they run as fast as they can for a long time but never move. After they stop under the same tree where they started, part of the dialog goes as:
'Well, in our country,' said Alice, still panting a little, 'you'd generally get to somewhere else — if you ran very fast for a long time, as we've been doing.'
'A slow sort of country!' said the Queen. 'Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!'
The moral of the story for SEO: if you want to maintain your search engine rankings, if you want to stay where you are, start running. Hard and fast.