Past events disappear from subscribed calendars in iCal

I've searched both in this community and online for a couple of hours but have not found a solution to this problem. In short, I subscribe to many calendars and would like all past events to stay in iCal so that I can search for them if I need to in the future (as I've done for years with calendars I've created on my own). I want iCal to keep a history of my life, so to speak. However, more often than not, past events disappear in my subscribed calendars. I'm assuming this is because the "owner" of these calendars deletes them for some reason (though this is just a guess on my part).

So, how can I prevent past events in subscribed calendards from disappearing in iCal, regardless of the changes the calendar "owner" might make? In other words, is there a way for me to save the past events as my own so that subscription updates do not delete them?

To clarify, I am specifically referring to past events from subscribed calendars in iCal 5.0.3 running on an iMac with OS X 10.7.5. I've already checked my iPhone, iTouch and iPad to make sure they're all set not to delete older items just in case they were possibly causing a problem, and they're fine. In addition, as long as those devices aren't affecting the content on my iMac, I don't care if they keep an accurate calendar history. I only want to ensure my iCal holds onto all events.

Thanks for your help!