okay so I spent a long time thinking about this and I think I have some answers https://twitter.com/AwfulReddit/status/1385581603259404289

okay, so, first, what does it mean to say something is an "infinite McDonalds?" i propose that:

(a) every instantiation of a mcdonald's location is as though it was abandoned twenty minutes ago

(b) there is a normal amount of mcdonald's food in the refrigerators and kitchens

(c) all supplies are normally stocked

(d) all substances which would have a continuous supply (water, air) are available infinite quantity but normal rate -- no magic pipes, but water is available

(e) plant life, bacteria, fungus, etc. normally present in a mcdonalds are there

(f) all McLife spreads from a central point where a full Noah's Ark worth of animals, plants, bacteria, etc. got dumped

(g) as a consequence everything is sterile until the first life reaches it

(h) there is power, AC, and fluorescent lighting

yes, I realize that (e) and (g) appear to conflict, but all life, both animal (ten million of each animal) and microbial (normal amount) begins in the same central zone

pretty clearly you'd have a couple major survival strategies within a couple generations, and none of them would be invertebrates. migration away from the center point would be a dominant strategy: the faster you could get to uneaten kitchens, the more food you have available.

there would be major variability in the thoroughness with which animals picked over the kitchens. so you'd start with waves of fast, picky animals -- e.g., hummingbirds which only drink coca-cola. insects, with their small bodies and energy-conserving metabolisms, don't work here

these would be followed by packs of animals capable of quickly reaching new kitchens and raiding various types of packaging. i honestly think apes would do amazingly in an infinite mcdonalds. probably something like chimpanzees -- omnivores are better off in McWorld.

the final wave would be large specialist freezer raiders. something like bears. they need to be able to eat both patties and veggies. they would probably also be largely responsible for wrecking up restrooms to make watering holes for the next wave of life.

then some years later, you'd have the next wave of life: first superspecialists who crack into the syrup bottles in the soda machines and ice cream concentrate, then bacteria breaking down napkins, plastics, and other organic materials.

finally, much of the inner circle of McWorld would flood, and until the lighting broke down, algal mats would grow under the fluorescent lights. the darkest regions would be pitch-black swamps where bacterial mats took advantage of electron gradients created by wall current.

so we've got the inner circle and outer ring, but what about the picked-over area where the floods haven't yet come but the herds of McGrazers have already passed? here, we have only the detritus left behind after the grazers have passed.

digestion in early-stage McGrazers is likely extremely inefficient: you want to get as much energy as possible out of the food and not carry around much indigestible mass at all -- digestion is quick and dirty and leaves behind nutrients for future McLife colonization.

here we have beetles, flies, springtails, and other small animals which feed on the waste left behind by the McGrazers. note that i left out worms -- there is a much greater concentration of macronutrients left behind in the aftermath of a grazer pass, but lower total quantity.

then, of course, we end up with a food web based on the animals which consume the waste -- predators on the detritivores. after the chemical gradient in that detritus is spent, McBeavers channel water from broken plumbing fixtures in the central region to pile up mounds of earth.

here on these tiny McIslands in the kitchen and dining room swamplands, swimming rodents cultivate hydroponic gardens under fluorescent lights, harvesting the energy from the light fixtures. in the dark regions, something similar likely happens with bacterial electrovore mats.

predation is uncommon at the outer ring, but becomes more and more common towards the center as the animals themselves become the densest nutrient sources. parasitism -- and particularly phoresy -- is common throughout.

animals not fast enough to seek out new sources of food on the outer ring -- mites and springtails in particular -- can cling to larger animals, depositing gravid females in kitchens and establishing new detritivore populations long before any other populations show up.