Opportunistic species use what is called an R-strategy. By producing dozens of eggs and sperm they increase the chance of fertilization, species who perform to this are commonly first to colonize new environments with a with a short life cycle but large population. Once resources become scarce as they are consumed, the species often dies. Insects, corals, clams etc or other water spawning animals are good examples.
Equilibrium species use what's called a K-strategy. Such species often produce less offspring and nurture them more, the preference for such species is a stable environment. Mammals and large live-bearing species are the best examples of this.
