Density dependent effects impact the population of a species and are dependent upon the density of that population. These density dependent effects can have negative effects on the population, with both extremely high and extremely low densities risking extinction. The main forms of density dependent effects are competition, predation and availability of mates.
Different species have different population densities that work best for their behaviors. In animals that form large herds, high population density is desirable, since their coordinated movements provide some protection for each member. Similarly, pack hunters, in maintaining a high density in a small area, are able to capture larger prey more reliably. For less social animals, the competition that comes with high population density is generally a negative effect on population.
Among both animals and plants, sufficient population density is needed for reliable sexual reproduction. Male plants require female plants to be close enough that wind or their animal pollinators can travel between them. Similarly, the further apart the two are, the less likely mating pairs happen upon one another.
High population density encourages an increase in predator numbers, and this can have a positive feedback effect where predator populations get too large and wipe out the prey population. Many types of insects take the strategy of reproducing in vast numbers, then dying off, thus not giving their predators enough time to build up their own populations.