Here's what I mean by that: For some species the survivability rate is inverse to the population density- the habitat fills up and a natural equilibrium results. But for other species there is a range of population densities where the survivability rate is positively correlated with population- so the more you have, the better they survive. Any species that moves in large herds is like that- the predators get full, they can't eat any more, and the dent the predators can make on the population is negligible. We are like that too; the more of us there are, the more we can specialize tasks and support each other, and the faster our population increases. (Up to some limit).
Swarming species are more delicate than they look though- passenger pigeons are a good example. Once some external force pushes the population below a certain level, the remainder have a much much harder time surviving. I don't know if people appreciate how easily a good nuclear war could cause the total extinction of humanity- once we lose the big interconnected economy, with all the easy to find resources gone, we could really be f*ed.