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 the single reproductive event of semelparous organisms is usually large, as well as fatal.^[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, including some species of butterflies, cicadas, and mayflies, some molluscs such as squid and octopus, and many arachnids. Semelparity is much rarer in vertebrates, but in addition to salmon, examples include smelt, capelin, and a few lizards, amphibians, and marsupial mammals. Annual plants, including grain crops and most domestic vegetables, are semelparous. Long-lived semelparous plants include century plant (agave), and some species of bamboo.

[edit] Iteroparity

An iteroparous organism is one that can undergo many reproductive events throughout its lifetime. 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. Iteroparous vertebrates include all birds and reptiles, virtually all mammals, and most fish. Among invertebrates, most molluscs and many insects (for example, mosquitoes and cockroaches) are iteroparous. Most perennial plants are iteroparous.

[edit] Semelparity vs. Iteroparity

[edit] Trade-offs

Iteroparous reproductive effort.

An organism has a limited amount of energy available, and must partition it among various functions. Of relevance here is the trade-off between fecundity, growth and survivorship in its life history strategy. These trade-offs come into play in the evolution of iteroparity and semelparity in particular species. It has been repeatedly demonstrated that semelparous species produce more offspring in their single fatal reproductive episode than do closely related iteroparous species in any one of theirs.

[edit] Models based on non-linear trade-offs

One class of models that tries to explain the differential evolution of semelparity and iteroparity, examines the shape of 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 survivorship'occurs at the point where the distance between offspring produced and offspring forgone is the greatest.^[3] The accompanying graph shows the offspring-produced and offspring-forgone curves for an iteroparous organism:

Semelparous reproductive effort.

In the first graph, 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. This intriguing mathematical/graphical model has not yet found solid quantitative support from nature.

[edit] Bet-hedging models

A second set of models examines the possibility that iteroparity is a hedge against unpredictble juvenile survivorship (avoiding putting all your eggs in one basket). Again, intriguing mathematical models have not found empirical support from real-world systems. In fact, many semelparopus species live in habitats characterized by high (not low) environmental unpredictability, such as deserts and early successional habitats.

[edit] Cole's Paradox and demographic models

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 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.

In his analysis, Cole assumed that there was no mortality of individuals of the iteroparous species, even seedlings. Twenty years later Charnov and Schaffer ^[6] showed that reasonable differences in adult and juvenile mortality yield much more reasonable costs of semelparity, essentially solving Cole's paradox. An even more general demographic model was produced by Young ^[7]. Such demographic models have been more successfully tested with real-world systems, where it has been shown that semelparous species have higher expected adult mortality, making it more economical to put all reproductive effort in to the first (and therefore final) reproductive episode. ^{[8] [9]}.

[edit] r/K selection

Semelparity can be a characteristic of r strategists, and iteroparity a characteristic of K strategists.^[10]

[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