r and K selection models just refer to the relative number of offspring produced each generation and the number that make it to maturity; it's not really adequate to explain what happens when a species crosses over the carrying capacity line. It's also not particularly useful for a species (like ours) that's operating outside of ecological boundaries. Overpopulation is not an issue per se because we can't assume that each individual in the human population is consuming roughly equal amounts of resources, so population growth doesn't necessarily predict resource consumption growth.