By killing mice and rats in barns and grain storage areas, cats are vital for keeping those pests in check.
Rodents eat and contaminate grain stored for human consumption. Without cats to control their population, rodents might significantly impact our usable supply of grains. Furthermore, with a dramatic increase in the global rodent population, we would likely see a decline in the number of seabirds since rodents eat their eggs. Species who prey upon rats would probably grow in numbers. The effects would ripple across the globe.
But don’t worry. I just checked Petfinder to see how many cats are available for adoption within 500 miles of me and there are 40, 028. Global meltdown averted.