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BIOL 4120 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782 |
This Bristlecone
Pine is over a thousand years old |
Lecture 8 Life History
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Overview - Link to Course Objectives
Lectures 4 to 7 have focused on the individual and the environment experienced by individuals. This chapter is a transition between ecology considered from the individual perspective to ecology considered from the population perspective. We are moving a step up the organizational hierarchy.
A Life History is a description of the pattern of mortality, growth, development, and reproduction characteristic of a population. Each species has a characteristic mode of reproduction, of development, and individual growth. We will examine general patterns of each aspect of life history. Remember that populations of a species may vary in life history. An example of such variation comes from many temperate insects. It is not uncommon for southern populations to complete two or more generations each calendar year (a Multivoltine life history) while the northernmost populations complete only one (Univoltine).
Reproductive patters vary widely and we will not cover the subject exhaustively by any means. We will look at some general patters within plants, fungi and animals.
For animals, Adults are reproductive and Juveniles are not, whether or not the reproductive mode is sexual or asexual.
Sexual - biparental reproduction, zygote formed as a fusion of two gametes, male function is to produce small motile gametes and female function is to produce larger, immobile gametes
Animals
Plants - animals usually have only a pair of gonads, where germ cells (gametes) are produced. Plants often have germ tissue scattered all over. This complicates the possibilities as we will see below. Not all varieties of plant sexuality are covered here!
Asexual - unisexual reproduction producing genetically identical offspring. It should be mentioned that some organisms undergo meiosis but always mate with themselves. Thus, there is some recombination but, in this case, it leads to greater homozygosity for the lineage.
Parthenogenesis - egg develops into offspring without fertilization.
Clonal Growth - growth of a new individual through mitotic growth alone. This is what happens when a plant produces plantlets (often called "suckers") that easily detach from the parent plant or when a Hydra simply grows an offspring as an outgrowth of its body wall
- clonal growth can result in colonial animals or plants where offspring remain connected to their parents
- Budding - clonal growth where a single offspring is grown as an outgrowth from a parent - usually applied to clonal growth that does not lead to colony formation
Fungi - fungi have both sexual and asexual reproduction. Many species incorporate both into their life history in that the complete life cycle might involve more than one environment and sexual reproduction in one environment and asexual reproduction in another.
Some fungi grow as single cells (called Yeast) and can reproduce asexually through mitosis. Most yeast also undergo meiosis and produce Sexual Spores, which are gametes and can fuse with another sexual spore to produce a zygote. Although the sexual spores are Isogametic (zygote produced from two gametes of equal size), not Heterogametic (gametes of unequal size, like sperm and egg or pollen and egg), there are sexes determined by the presence or absence of proteins on the surface of the gamete and sexes are designated as either a and alpha or as + and -. Sometimes the zygote produces the next generation of diploid yeast cells by mitotic cell division (called Budding in yeast). Sexual spores are produced when one of the diploid cells undergoes meiosis. Sometimes the zygote immediately undergoes meiosis and produces 4 haploid cells which then divide mitotically. Sexual spores arise from haploid cells in these yeast.
Multicellular Fungi grow as Hyphae, chains of cells thinner than a hair. The entire network of hyphae that comprise the somatic tissue of the fungus is called a mycelium. Special structures are produced for reproduction (sexual or asexual). Some fungi (especially those that parasitize plants) can have complex life histories that involve growth in different hosts and transmission between hosts by spores that can be either sexual or asexual.
Multicellular fungal reproduction has many variations and separates the fusion of cells (plasmogamy) from the fusion of nuclei (karyogamy) and life histories can involve stages with vegetative growth of hyphae containing nuclei from two different individuals. However, the ultimate rules are still obeyed and sexual fungi must, at some point in the life history, undergo meiosis and subsequent fusion of gametes to produce a zygote from which the next generation develops
Some Fungi can alternate between yeast-like and hyphal growth.
Mating System is the pattern of mating found in a population or species. The basic division among mating systems is between those in which Pair Bonds are formed and in those without pair bonds. Pair bonds are cooperative relationships between a mating pair that lasts for at least a reproductive season and sometimes for the entire reproductive lives of the pair. Pair bonds allow for males to identify their offspring and for females to elicit parental care from males, either in direct form (feeding of young by the female) or indirect form (males may defend a feeding territory to which the female has access)
NOTE - not all biologists make the distinction among systems based on pair bonds. To those that do not, for example, promiscuous females that mate with more than one male, like many frog species, are examples of polyandry because more than one male may contribute to fertilizing a clutch of eggs.
Finally, plants (and presumably fungi) have mating systems too. We have considered some of the topic (when we considered apomixis) but there is much more to the topic, which we haven't the time to consider here.
Mate Choice and Sexual Selection
Mate Choice - In many animal species, one sex is much more choosy about with which of the opposite sex it will mate, no matter what type of mating system the species employs.
Mate choice is usually exercised by the sex which invests more resource and effort into each offspring. This is usually females. Females produce the larger gamete, eggs, and the number of offspring they produce is limited by the number of eggs they produce. This is not so for males as there are many more sperm produced in a population than eggs. The number of offspring a male will produces is usually limited by the number of females with which it can mate.
Females are more choosy because, if they mate with a poor-quality male and then meet a higher quality male, no more eggs are available and the female loses access to the high quality male and the quality of her offspring suffer. If a male mates with a low quality female and subsequently has a chance to mate with a high quality female, sperm are available for the second mating and there is not cost (and lots of potential benefit) to the first mating.
Sexual Selection - when one sex chooses a mate it may do so based on some aspect of the prospective mates phenotype, including its behavior. Presumably the feature is some kind of indicator of the male's quality as a parent. However, once that feature becomes the basis of choice, those males with the extreme in that phenotype have an advantage over other males simply based on that one feature. This advantage is not due to natural selection as the advantage is not based on the ecological success of the phenotype. The advantage comes from mate choice and is an advantage in sexual reproduction only. Traits with this sexual advantage are under Sexual Selection.
Thus natural selection and sexual selection may seem at odds. The sexually selected trait may seem to be a handicap for males. Think of the famous Peacock's tail. Males fly poorly and are prone to higher levels of predation than peahens. However, females choose based on the males display of the tail and males with small tails may survive but they have no offspring. Why choose the gaudier males? If the gaudier males are gaudier because their general health is better, they may represent successful genotyes (This explanation of sexual selection is known as the "Good Genes Hypothesis") or, if the gaudier males have demonstrated that they can bear the cost of the flashy ornamentation in spite of higher predation rates, they have demonstrated traits that may have general value (This is called the "Handicap Hypothesis.")
An organism has a finite amount of energy and resource to spend. Some must be spent for basic metabolic needs. Any resource left over can be spent in two general ways: growth or reproduction. Allocation of resources to one function must, then, reduce the amount available for the other. This relationship is known as a Trade-Off.
There are many aspects to the trade-off. Populations may vary as to the nature of the trade-off. The trade-off may affect such population parameters as the size of first reproduction and the timing of reproduction.
Growth and Survivorship versus Reproduction
There are two basic life histories with respect to the trade-off between growth and reproduction. In both cases, organisms must balance current reproduction against future growth and reproduction. In general, as species is either one or the other but cases are known (especially among plants) where some populations are iteroparous and other populations of the same species are semelparous.
- Iteroparity - iteroparous organisms reproduce more than once, which means that there are periods of growth possible between bouts of reproduction. They must trade-off between the benefits of growth (presumably, larger organisms produce more gametes or are more likely to survive until the next reproductive opportunity) and reproduction. Less reproduction may lead to more growth and becoming a larger individual for the next reproductive opportunity. However, there is a chance that a next opportunity for reproduction will not come and the resources reserved for growth will have been wasted. The best strategy maximizes both the total offspring produced and minimizes the time it takes to produce them (we will see why having young early is advantageous in the lecture about population growth).
- Semelparity - semelparous organisms grow until, at some point, they devote all of their resources to reproduction and reproduce only once. The trade-off here involves the point at which it is best to stop growth and begin reproduction. Many annual plants do so using environmental clues. If they wait too long to make the transition from growth to reproduction, they run the risk of the weather becoming unsuitable or of producing flowers when their pollinator is no longer present and incurring reproductive failure.
Adult Growth and Fecundity - Organisms have two general patterns of growth once they become adults:
- Determinate Growth - adults are all about the same size and do not grow once they attain adult size. Insects have determinant growth. Once they molt into the adult state, they do not molt again and do not grow (they can't inside their exoskeletons). Many annual plants also have determinant growth and, once the critical size is reached, they begin to reproduce. If the critical size is not attained, they may never flower.
- This does not mean that there is no variation in adult size. The juveniles of insects (called Larvae in Holometabolous insects and Nymphs in Hemimetabolous insects) are the growth life stage and larger larvae produce larger adults. Annual plants may grow past the critical size when growth is rapid and produce larger plants when reproduction begins.
- Most organisms with Determinant Growth show little variation in reproductive effort among individuals, especially those species that are semelparous.
- Indeterminate Growth - growth continues throughout the adult stage. Many perennial plants and fish show this sort of growth, although the rate of growth may decline in adults until the it has slowed to near zero.
As mentioned above, the expected relationship between adult size and Fecundity (number of offspring) is positive. Larger adults often produce more offspring.
- Often, this is a simple result of larger organisms having more resources to devote to reproduction and, when semelparous, no reason to lower their reproductive effort.
- In other cases, larger adults may have more successful offspring because they are more likely to survive, they may be better able to care for their offspring, and/or they may be able to produce larger, healthier offspring
Offspring Success and Birth Size
In addition to a trade-off in reproductive effort versus growth, a second important trade-off exists. Reproductive effort must be partitioned among offspring. In general, organisms seek to maximize the number of offspring but having fewer offspring allows for more resource to be allocated for each offspring. If smaller offspring are less likely to grow up and reproduce, the long-term success may mean that there is a trade-off between the number and size of offspring.
There are also differences in when the effort is expended on offspring. In general,
- Altricial - Less effort is spent prior to birth and the young are born in need of extensive parental care, usually a long period when the young are fed by their parent or parents. We are altricial, as are all marsupials (offspring are tiny at birth and can only crawl to the pouch were they feed on their mother's milk for a long time). Many bird species have altricial young (the American Robin is a good example).
- Precocial - More effort is spent in either stocking the egg with resources or in a longer gestation period, and the offspring are able to move about and forage soon after birth
Reproductive Effort and Latitude -
When similar species are compared or northern populations are compared to southern populations, a general trend in reproductive effort can often be seen. Species or populations from the northern latitudes expend greater reproductive effort.
- No one knows for sure what the cause is for this trend
- Lack proposed that number of offspring depends on food supply for many organisms. The latitudinal trend could be an outcome of greater food availability in the north.
- This is reasonable if you consider that growing seasons may be longer there (day length is longer during summer, the reproductive season) compared to tropical latitudes with constant 12 hour day length.
- Cody and Ashmole have similar ideas tied to the observation that temperate and northern populations are prone to greater environmental stress from periodic disasters (bad storms, very cold winters) and the populations are smaller in the northern areas than the resources present could sustain. Individuals maximize reproductive effort in an expanding population. In the tropics, the stable environment allows populations to grow to their maximum and effort has to be diverted from reproduction into competition.
General Patterns of Life History and the Environment
A relationship between the environment of a population and its life history has been demonstrated for many populations. The variation in life histories is great but there is still a desire to find large-scale patters and some general explanations for the patterns that can be applied to many species. Below are two attempts to generalize.
r- and K-selection are opposed patterns of life histories
r and K refer to aspects of population growth
r- and K-selected life histories are responses to two contrasting kinds of environments
r-selected organisms live in disturbed, variable environments
- r-selected species have high growth potentials but have poor competitive ability and can be displaced from a habitat by more efficient competitors
- Weeds are often r-selected (and are often semelparous, although not always)
- often live in variable environments (highly variable environments are called Disturbed)
- individuals often have high investment of resources in reproduction
- often experience high, variable levels of mortality during at least part of life history
- population size often varies greatly from year-to-year
K-selected organisms live in stable environments
- K-selected populations are slow growing but good competitors because their environment eventually gets crowded
- trees and man are good examples (K-selected species are often iteroparous)
- often live in stable environments
- individuals often have low proportion of resources invested in reproduction
- often experience low, stable levels of mortality during at least part of life history
- population size often relatively constant from year-to-year
r- and K-life histories often lead to organisms with different sets of characteristics
r-selected |
K-selected |
|
First
Reproduction |
early |
late |
Population
Growth Rate |
fast |
slow |
Developmental
Rate |
fast |
slow |
Adult
Size |
small |
large |
Life
History |
semelparity |
iteroparity |
Life
Span |
short |
long |
NOTE:
Grime's 3-cornered continuum of life histories
r- and K- selection is based on the idea that habitats can be separated into two general types: those that are constant and lead to population sizes that promote competition versus those that vary and usually have uncrowded populations. Grime has separated habitat variability and physical stress to give three different environmental influences on life history (stable environments, variable environments, and stressful environments), which leads to three different general life history strategies.
Life History, Multivoltine, Univoltine, Adults, Juveniles, Sexual Reproduction, Male, Female, Dioecious, Monecious, Hermaphrodite, Simultaneous Hermaphrodite, Sequential Hermaphrodite, Perfect, Imperfect, Asexual Reproduction, Parthenogenesis, Gynogenesis, Hybridogenesis, Hemiclonal, Apomixis, Clonal Growth, Budding, Yeast, Sexual Spores, , Isogametic, Heterogametic, Mating System, Pair Bonds, Monogamy, Territory, Sperm Precedence, Mate Guarding, Mating Reactions, Polygamy, Polygyny, Polyandry, Simultaneous Polyandry, Sequential Polyandry, Cooperative Polyandry, Promiscuity, Mate Choice, Sexual Selection, Intrasexual Selection, Intersexual Selection, Trade-Off, Reproductive Effort, Iteroparity, Semelparity, Determinant Growth, Larvae, Holometabolous, Nymphs, Hemimetabolous, Indeterminant Growth, Fecundity, Altricial, Precocial, r- and K-selection, Weeds, Disturbed, Ruderals, Competitors, Stress Tolerators
Reference for Pollen Asexuality - Pichot, C., Fady, B., & Hochu, I. (2000). Lack of mother tree alleles in zymograms of Cupressus dupreziana A. Camus embryos. Ann. For. Sci. 57: 17–2
Last updated January 29, 2007