The first parental-investment strategy is to invest relatively little in each offspring, but to produce many offspring. This sort of strategy is typical of most insects and of most fishes. Typically, the female lays her eggs then promptly abandons them.
One disadvantage of this strategy is that while many offspring are produced, their
survivorship tends to be very low. Consequently, it’s typically the case that very few of the offspring manage to live long-enough to reproduce. On the other hand, in a good environment, this strategy can result in phenomenally-fast population growth.

Spawning salmon: Each female will lay thousands of eggs. Maybe one or
two of the young that hatch out of them will survive to adulthood.
In the field of
ecology, which studies how organisms interact with their environments, this sort of reproductive strategy is known as an
opportunistic strategy. This is because such a species can quickly colonize any suitable habitat that should appear. That is, it will quickly take advantage of any opportunity to colonize a newly-available habitat.
The thing is, any given habitat can support only so many individual organisms. The maximum number of individuals of a given species that can be supported by a particular habitat is known as the
carrying capacity for that habitat.
When environmental conditions are good, a population of organisms with an opportunistic reproductive strategy is likely to quickly grow to the point that there is not enough food or living space for all individuals in the population. That is, the population is likely to overshoot its habitat’s carrying capacity. When that happens, starvation and disease will cause a massive drop in the population’s size. Of course, after the “crash,” the amount of resources available per individual in the population is once-again relatively high. So the population begins to rise again.
The result is that populations of animals with opportunistic reproductive strategies often fluctuate (sometimes wildly) in size with time. In some species, these population fluctuations take the form of surprisingly-regular
population cycles.

Population cycling in Snowshoe Hares (Lepus americanus
). Population
cycling like this is relatively common in opportunistic species.