Ok, so let’s assume that there is no outdoors, only the man-made structure of a mcdonalds, connected via the exits to other mcdonalds. We can also assume that humans are not an intrinsic part of mcdonalds, but are inserted as part of the 10 million of each animal. Let’s assume that the animals are comfortably distributed, grouped in breeding populations, occupying mcdonalds within a radius of a central, “starting mcdonalds”. Let’s assume that each mcdonalds has a full store of fresh food at the start of the scenario, but it does not replenish magically. Once a mcdonalds is empty, it stays empty.
We must ask, is there electricity, and are the mcdonaldses all wired together, or are they cut off? Are the lights on any kind of timer, or are they all manual and static? The big question here is: are there mcdonalds with multiple floors? And if so, are there more than one “plane” of mcdonaldses? Can you travel up and down infinitely, is there a “floor” level of mcdonalds, or is it just one thin, infinitely wide layer of mcdonalds?
Naturally, these variations will be vital in determining 1) which species have a high chance of surviving the first few years and 2) What long-term selective pressures will emerge after the initial bottleneck event.
If there is a floor level, and the mcdonalds extend upwards as well as outwards, then it will eventually fill up with soda, sewage, and waste from higher levels. The “starting” mcdonalds will be buried and lost relatively quickly, the vile ocean that drips from each level downward and then spreads, pouring out in every direction, along the infinite soggy bottom layer of Mcdonalds.
Now, in our dimension, Earth is only so big. You can only walk so far before something blocks your way, or you’re back where you started. The Sun and geothermal energy introduce energy into the food chain, which becomes chemical energy for plants, herbivores, carnivores, etc. This is the basis for biological competition. But in Mcdonalds, these rules go out the window…and into the adjacent mcdonalds.
I think you would see that the nature of an infinite plane which always has more resources if you walk far enough, things would evolve to either be fast enough to get to the next unlooted Mcdonalds first, rushing in as loners, or swarming through as a stampede on a perpetual warpath. Any life that stays close to the starting point, or lives in de-stocked areas is going to have to be very efficient with the energy they have, because any “new” energy introduced to the ecology is solely through miraculously unspoiled big macs, cold fries, and soda slime bogs and rivers from the more and more distant untouched McDonalds, or alternatively through the electricity or flowing water of the building, if they have any.
I’d have to say that while these evolving organisms may look superficially familiar to us, we’d find that they’d quickly start to behave very differently, adopting new strategies to survive, and thrive in an infinite McDonald’s.
