Semelparity and iteroparity

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Semelparity and Iteroparity refer to the reproductive strategy of an organism. A species is considered semelparous if it reproduces a single time before it dies, and iteroparous if it has many reproductive cycles over the course of its lifetime. In plants, the term monocarpy is equivalent to semelparity, and polycarpy is equivalent to iteroparity.

Horticulturists and plant ecologists use the related terms annual and perennial. An annual is a plant that completes its life cycle in a single season, and is usually semelparous, while perennials live for more than one season and are usually iteroparous.^[1] However, there are many exceptions.

[edit] Overview

[edit] Semelparity

The Pacific salmon is an example of a semelparous organism

The word semelparity comes from the Latin semel, once, and pario, to beget. It is often known as "big bang" reproduction, since semelparous organisms reproduce only once before death.^[2] A classic example of a semelparous organism is Pacific salmon (Oncorhynchus spp.), which lives for many years in the ocean before swimming to the freshwater stream of its birth, laying eggs, and dying. Other semelparous animals include many insects such as butterflies, cicadas, and mayflies, some mollusks such as squid and octopus, and many arachnids. Most annual weeds, flowers, and vegetables may be considered semelparous, as well as the longer-lived century plant (agave), and bamboo.

[edit] Iteroparity

The pig is an example of an iteroparous organism

The term iteroparity comes from the Latin itero, to repeat, and pario, to beget. An example of an iteroparous organism is a human—though many people may choose only to have one child, humans are biologically capable of having offspring many times over the course of their lives. Other iteroparous organisms include all mammals, birds, and reptiles, most fish, most mollusks, and some insects, for example mosquitoes and cockroaches. Among plants, ferns and most perenial seed bearing plants, trees, and shrubs have adopted an iteroparous strategy.

[edit] Semelparity vs. Iteroparity

[edit] Trade-offs

An organism has a limited amount of energy available, and must "choose" how to use it. It faces a trade-off between fecundity, growth, and survivorship in its life history strategy. For example, one species might devote a good deal of energy into escaping from predators, so it has relatively few offspring. Another species might be extremely small and short-lived, but it has many offspring. These trade-offs come into play in the evolution of iteroparity and semelparity in particular species.

When thinking about semelparity and iteroparity, the question is the trade-off between offspring produced and offspring forgone. In economic terms, offspring produced is equivalent to a benefit function, while offspring foregone is comparable to a cost function. The reproductive effort of an organism—the proportion of energy that it puts into reproducing, as opposed to growth or fecundity—occurs at the point where the distance between offspring produced and offspring forgone is the greatest.^[3] The graph below shows the offspring-produced and offspring-forgone curves for an iteroparous organism:

In the graph above, the marginal cost of offspring produced is increasing (each additional offspring is less "expensive" than the average of all previous offspring) and the marginal cost of offspring foregone is decreasing. In this situation, the organism only devotes a portion of its resources to reproduction, and uses the rest of its resources on growth and survivorship so that it can reproduce again in the future.^[4] However, it is also possible for the marginal cost of offspring produced to decrease, and for the marginal cost of offspring forgone to decrease. When this is the case, it is favorable for the organism to reproduce a single time. The organism devotes all of its resources to that one episode of reproduction, so it then dies. The graph appears as below:

[edit] Cole's Paradox

The question of whether semelparity or iteroparity is a more successful strategy is an important one in ecology. In Lamont Cole's classic 1954 paper, he examined this question, and came to an interesting conclusion:

For an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equivalent to adding one individual to the average litter size.

—Lamont C. Cole, The Population Consequences of Life History Phenomena^[5]

In other words, suppose you are observing two species—one is iteroparous, and the other is semelparous. The iteroparous species has annual litters averaging three offspring each, while the semelparous species has one litter of four, and then dies. However, these two species have the same rate of population growth! This phenomenon is known as Cole's Paradox.

Cole made two important assumptions in his analysis:

• All individuals of the semelparous species survive to age 1, then reproduce with a litter of size b

• All individuals of the iteroparous species survive forever

One interesting thing to notice is that if it always takes one extra offspring for a semelparous organism to "keep up" with an iteroparous organism, the amount of extra effort that requires is much greater when litter sizes are small than when litter sizes are large. For example, to go from 2 to 3 offspring is an increase of 50%, but going from 100 to 101 only requires an increase of 1%. For this reason, for smaller litters, iteroparity is advantageous, whereas for larger litters, semelparity is preferred.^[6]

[edit] r/K selection

In general, semelparity is characteristic of r strategists, while iteroparity is characteristic of K strategists.^[7]

[edit] See also

• annual plants
• perennial plants
• r/K selection theory
• life history theory
• ecology

[edit] Further reading

• De Wreede, R.E, and T. Klinger. "Reproductive Stategies in Algae." Plant Reproductive Ecology: Patterns and. 267-76.

• Fritz, R. S., N. E. Stamp, and T. G. Halverson. "Iteroparity and Semelparity in Insects." The American Naturalist 120 (1982): 264-68.

• Rant, Esa, David Tesar, and Veijo Kaitala. "Environmental Variability and Semelparity vs. Iteroparity as Life Histories." (2002).

• Tesar, David. Evolution of life-histories in stochastic environments: Cole’s paradox revisited. Diss. University of Helsinki, 2000.

[edit] References