Reproductive and Survivorship Strategies
Many fish and insects experience this type of reproduction, including salmon and some species of spiders and butterflies. On the other end of the spectrum, some organisms have only a few, or even a single, offspring at a time, but will reproduce multiple times throughout their lives. Most large mammals, including humans, and many insects utilize this reproductive strategy, called iteroparous reproduction. Reproductive strategies are not an either/or sort of affair; some organisms fall somewhere between semelparity and iteroparity reproduction.
In addition to various reproductive strategies, organisms differ in their survivorship strategies. Some organisms are at high risk of dying early in life, but if they can survive long enough, they have a decreasing probability of dying as the years go on. Well, to a point anyway. Other organisms have a steadily increasing probability of dying while still others live for a long time with the probability of dying only increasing dramatically after a certain (often old) age. These survivorship types are actually called Type I, II, and III and are related to reproductive strategies.
- Type I organisms have a low probability of dying early and are often—you guessed it—iteroparous.
- Type II organisms have a constant probability of dying throughout life and are often somewhere between semelparous and iteroparous.
- Type III organisms have a high probability of dying early and are often semelparous, reproducing once early in life and dying shortly thereafter.
The question you should be asking yourself right now is, "Which reproductive and survivorship strategy is best?"
The answer, of course, is, "It depends."
All organisms strive to pass as many genes onto the next generation as possible. But, each species’ evolutionary environment was, or is, different from the next. Some species evolved in environments where the likelihood of survival was low, and the risk of reproduction was high. In these situations, natural selection favored organisms that could reproduce early and in large numbers. Over time—a long, long time—these species acquired the semelparous reproductive strategy and the Type I survivorship strategy that comes with it…or precedes it, however you think about it.
In other cases, some species evolved in environments where the likelihood of survival was high and the risk of reproduction was low. In these situations, natural selection favored organisms that produced many offspring over a long period. These are the iteroparous species.
Of course, it is not as clean and neat as all that, with natural selection favoring organisms all along the spectrum between complete semelparity and complete iteroparity (if those bookends even exist), depending on the various evolutionary contexts. The key point here is that life histories are shaped by natural selection and, through the evolutionary process, nature finds multiple ways to solve the same problem…in this case, when and how often to reproduce.
Related to reproductive and survivorship strategies is the concept of life history trade-offs.
Organisms can partition resources among three main aspects of their lives:
Some animals, like the cicada, reproduce on 13- or 17-year cycles and spend most of their lives dormant and under ground.