28.4 : Energy Budgets

Organisms must balance energy intake with the energy required for growth, maintenance and reproduction. These trade-offs result in a variety of survivorship and reproductive strategies, including semelparity and iteroparity. Semelparous species, like annual plants, have only one reproductive episode in their lifetimes and consequently have short lifespans. Iteroparous species, by contrast, have many reproductive events during their lifetimes but have relatively few offspring. These two strategies are not mutually exclusive but represent two extremes on a continuum of possible reproductive strategies.

Semelparity and Iteroparity

During its lifetime, an organism has a limited amount of energy and resources available to it and must allocate the energy to growth, reproduction, and maintenance. Energy used for reproduction cannot be used for growth and vice versa. This creates a trade-off among fecundity, growth, and survivorship that is reflected by a variety of reproductive strategies. Two primary reproductive strategies are semelparity and iteroparity. However, rather than being strictly semelparous or iteroparous, many organisms lie somewhere on a continuum between the two reproductive strategies.

A truly semelparous species allocates all available resources to reproduction at the expense of lifespan, reproducing only once before death, but producing many offspring. Semelparous organisms include annual plants, which complete their entire life cycles in just one season.

On the other hand, iteroparous species have multiple reproductive events over the course of their lifespans. These organisms often produce fewer offspring per reproductive episode, but provide greater care for each offspring. Iteroparous species include birds; nearly all mammals; most perennial plants, reptiles, fish, and mollusks; and several insects.