Of course I'm thinking of rodents, not the two-legged parasites that react in many of the same ways. When something as simple as construction work can cause mice and rats to vacate a building and look for new homes, imagine what a major disaster will do. We've all heard the phrase "rats leaving a sinking ship", and it is a real phenomenon; they flee burning buildings as well.
Rats and mice are opportunistic feeders, meaning they will eat whatever they can find. If you can eat it, so can they; in developing countries where construction and sanitation standards are not as strictly enforced, rodents can destroy or contaminate up to 25% of every harvest. That's not counting the documented "swarming" of rats that can entirely destroy crops in a matter of days.
Rodents have front teeth that grow as long as they are alive, which is why they are constantly chewing on things. If they don't chew and wear down their teeth, the teeth will grow through their skulls and kill them. Rats in particular can chew through anything, given enough time. And I mean anything: concrete, lead, steel, glass, and plastics are not enough to keep them out if they are hungry or just need to chew.
Mice and rats also have very flexible skeletons. A fully-grown rat can squeeze through a hole the size of a quarter and a mouse through one the size of a dime. Openings for pipes and wires are the most common entry for rodents into a house. Keep things sealed up and make sure your wall openings are as tight as possible.
Rodents are very hard to get rid of once they have infested an area. There are a few islands that have been cleared of rats (brought in on visiting ships hundreds of years ago), but it is an expensive and time-consuming process. Some areas of the Arctic and all of Antarctica are rat-free only because the conditions are so severe that they can't live there (nor can much of anything else).
Controlling a rodent population is possible. Proper sanitation and storage procedures can keep them out of certain areas, and poisons, traps, and predators work after they've moved in.
- Mice are very near-sighted, which is why they tend to stay close to walls. They also leak urine as they run around, leaving a scent trail for other mice. Now you have another reason to scrub the floor.
- Food must be kept cleaned up to eliminate easy meals that will keep them coming back for more. Bulk storage in particular gets messy when you're transferring a portion to a smaller container for use. Crumbs and kernels of grain on the floor may not look like much to a 200# adult human, but to a mouse they're a full pantry.
- Storage requirements are simple: Keep everything in the best container you can afford until you need it. Those quaint bread boxes and pie safes that your grandma had in her kitchen? Those were there for a reason. Keeping everything covered kept the bugs and rodents out of it.
- Keeping your area clear of debris removes material that rodents can use to make nests. Paper and cloth need to be kept off of the floor and neatly stacked. Never store anything leaned against the side of a building, because that creates pathways and nesting areas for them.
- Poisons: Most rodent poisons are anti-coagulants. This class of drug is very hard to develop an immunity to, but you'll still have to change the active ingredient once in a while. Warfarin used to be the most commonly used blood thinner, but the brown rat (Norway rat) has become resistant in some areas. Fumigation is an option, but is not something for the DIY crowd; the chemicals are extremely hard to obtain without the proper paperwork, and they're not very forgiving of mistakes since they'll kill anything that breathes.
- Traps: The standard mouse trap still works after many years. Cheap, simple, and useful for other purposes, everyone should have a few set aside. The more elaborate traps work as well, but I'm a fan of the KISS principle.
- Predators: Cats and small dogs have hunted rodents for as long as they've both been around. Most of the smaller Terrier breeds of dogs are excellent rat hunters and any cat will hunt mice if they're hungry enough. That's the trick with cats-- keep them hungry and they'll hunt for their own food. Owls and hawks also find rodents quite tasty, but they're a bit harder to train and care for. Do what you can to foster their choosing to live near you and they'll help keep the rodent population down.
Just having things isn't enough; you also need a plan on how to keep them. Rodents and other vermin are not as common in modern society as they were a hundred years ago, but if things start to break down, their rapid breeding will put them back to where they were within a few months.
Something for you to think about, in case it had never crossed your mind.