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Semelparity
Figure 1: Semelparous plants
Like all grain crops, wheat is semelparous plant. Annual plants die after first reproduction within a year of germination. (Courtesy of Simon Howell: freedigitalphotos.net)
Examples of short-lived semelparous species include annual and biennial plants (including all grain crops, and many herbaceous vegetables), and certain invertebrate species, including many spiders (Figure 1). There are even a few semelparous marsupial mammals in Australia. It is important to note that while all annual plants are semelparous, not all perennial plants are iteroparous. There are a wide variety of plant and animal species that live for many years before a single, massive, fatal reproductive episode (certain species of salmon, bamboo, and century plants, Figure 2).
Dilemma and Cost of Reproduction
Natural selection maximizes total lifetime reproductive output. So how could evolution ever favor programmed death after first reproduction? A key clue lies in the observation that semelparous species typically produce more offspring in their single reproductive episode than closely related species do in any one of theirs. It appears that when an organism does not need to withhold some resources to ensure future survival and reproduction, it can mobilize virtually all available resources to put into a single, massive reproductive episode. For example, this fecundity advantage is two to fivefold in plants. So the essential question becomes, "Under what conditions does the increase in fecundity associated with semelparity more than compensate for the loss of potential subsequent reproductive episodes?"
Figure 2: Spawning salmon
Salmon at their spawning grounds, far from the ocean. Here, they will reproduce and die. (Courtesy of Friends of Butte Creek)
Theoretical Approaches
This question has been the subject of a rich theoretical literature. These models fall into three classes, all of which assume a tradeoff between reproduction and survival:
- Demographic models predict that when adult survival is low enough (relative to juvenile survival), evolution should abandon withholding resources for a future reproduction that is unlikely, and instead favor semelparity (Figure3).
- Bet-hedging models predict that when adult survival is highly variable, evolution should favor iteroparity, because it does not risk putting all reproductive effort into a single reproductive episode.
- Models incorporating non-linear patterns of reproductive costs and benefits predict that semelparity should be more likely to evolve when most of the costs of reproduction (reduction in future survival or reproduction caused by increases in current reproduction) happen even at low levels of reproductive effort, or conversely, when the benefits of reproduction accrue most rapidly at high levels of reproductive effort.
Figure 3: Schematic of the demographic model of the evolution of semelparity
Imagine semelparous and iteroparous populations. The semelparous (annual) individuals produce 2.5 times as many seeds as the iteroparous individuals (a reasonable estimate of relative fecundity, from natural systems). In the first year, the semelparous individuals out-produce the iteroparous, but the iteroparous have some probability of living to reproduce again. If the iteroparous population experiences 50% adult mortality each year, the average individual will produce 200 seeds in its lifetime, fewer than in the semelparous population. If the iteroparous population experiences 30% adult mortality each year, the average individual will produce 340 seeds in its lifetime, more than in the semelparous population. Therefore, in populations with sufficiently high mortality, semelparity will be favored over iteroparity. Demographic models of semelparity explore this more explicitly.
Empirical Evidence
Despite the abundance of theoretical models for the evolution of semelparity, direct empirical tests for each remain limited. For example, many species of annual plants are desert species or weedy species that are early arrivals after disturbance in many ecosystems. Both of these groups are subject to high variation in survivorship, in opposition to the predictions of the bet-hedging model.
In a particularly elegant test of the non-linearity model, it was demonstrated that the taller inflorescences of semelparous yucca species produced disproportionately more seeds than smaller inflorescences, but that this was not true in iteroparous yucca species. Similarly, pollinators preferred taller inflorescences of semelparous yucca species, but not iteroparous species. Although this observation fits the non-linear theory nicely, it has been pointed out that many of these species may not be pollinator-limited, and that inflorescence height patterns may be physiological consequences of life history differences, and not their evolutionary causes.
Synchronous Semelparity
There is an unusual pattern in semelparous plants characterized by single-aged populations that live for many years, then synchronously flower and die. Certain bamboos species are well known to exhibit this pattern, but other examples include certain palm species, shrubs in the family Acanthaceae, and even a tropical forest canopy tree. All of these grow in mesic forests that are climatically more moderate than the extreme environments characteristic of most semelparous species (e.g., deserts, alpine habitats, bogs, disturbed sites). Periodic cicadas in the eastern U.S. also exhibit this pattern. Predator satiation has been invoked to explain both synchrony and semelparity in these species, but there is no widespread consensus on its causes.
Species "Approaching" Semelparity
Semelparity and iteroparity have been represented here as a simple dichotomy. Nevertheless, the conceptual framework can be applied more generally. Several examples have been documented where high levels of adult mortality appear to be related to iteroparous life histories characterized by early reproduction that is more frequent and/or more intense. These include subalpine fir trees, patas monkeys, and several insect species.
Semelparity in Grain Crops
It is probably no coincidence that many herbaceous crops, including virtually all grain crops, are annuals. Their semelparity results in far higher yields than if they were iteroparous. It is likely that when selecting grain species for cultivation, early farmers deliberately chose those species with the highest yields, which were annuals, or selected strongly enough for high yields that iteroparous species evolved into semelparous species. The closest relatives of both rice and maize are iteroparous.
Given our dependence on semelparous species, it is important that we better understand the evolution and physiology of this curious life history.
Given our dependence on semelparous species, it is important that we better understand the evolution and physiology of this curious life history.
References and Recommended Reading
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