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Artificial Refugia
(slrpnk.net)
If this isn't the most dystopian thing you've read today I don't know what is.
Artificial Refugia (such as bat boxes) can be used to somewhat offset lost habitat, but it's like replacing a rainforest with a plastic plant and calling it conservation. Further, unless they are meticulously designed, they can act as traps (trap animals inside), snack plates for predators, or even 'cook' the animals inside them.
To be fair, a metal subway car (provided they removed every piece of rubber, plastic and electronic component) is environmental quite neutral, as metal and glass are inert. In this case though, I'm pretty sure tennis ball contain plastic so it wouldn't be whise to just leave them in the open. If they were glass balls, it would be better.