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

Life History

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).

Reproduction

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

  • Disadvantages:  necessity of finding a mate, only provide 50% of offspring's genes
  • Advantages:  offspring are genetically diverse

    Animals

    • Dioecious - individual organisms are single-sexed
    • Monecious - Hermaphrodite - individual organisms are, at some time during their lives, male and female
      • Simultaneous Hermaphrodite - adults produce both sperm and eggs.  Some hermaphrodites are self-fertile and can reproduce without mating but some are not self-fertile and must mate with another hermaphrodite with which it will exchange sperm
      • Sequential Hermaphrodite - adults are both sexes but not a the same time.  Male function may come first or last.  This strategy usually associated with a strong advantage for one sex to be large.  Where males fight to keep a harem, larger males are favored and young (therefore small) individuals are female.  Where females produce many more eggs when they are larger, young (small) individuals have more offspring if they are male (sperm are produced in excess)

    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!

    • Dioecious - all flowers on a plant have just one sex (are imperfect) and the sex is the same for all flowers on a plant.
    • Monecious -same as Hermaphrodite - both sexes on the same plant - as in animals, these may be self-fertile or may not be. In addition, plants may be simultaneously or sequentially hermaphroditic (sometimes to prevent self-fertilization)
      • Perfect - bisexual flowers producing both eggs and pollen
      • Imperfect - male flowers separate from female flowers

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.

  • Advantages:  no need to find a mate, all genes transmitted to all offspring; offspring phenotype already prove to be successful in that environment
  • Disadvantages:  all offspring vulnerable to same enemies and all offspring respond in same fashion to change in environment (physical or biological) - no "spreading of risk"

    Parthenogenesis - egg develops into offspring without fertilization.

    • The genetic mechanism for parthenogenesis varies and so do the details.  Two rare but interesting variations are:
      • In Gynogenesis, the egg will not develop without being fertilized by a sperm, although the sperm makes no genetic contribution to the offspring that result.  This results on all female populations, so Gynogens rely on matings from closely-related species. 
      • In Hybridogenesis, egg and sperm unite to form a zygote that grows into a male or female.  When a female produces eggs, they will contain only chromosomes derived from the females mother (thus, a female's egg is genetically identical to the egg that produced her).  This is sometimes called Hemiclonal reproduction
    • In plants, parthenogenesis is called Apomixis ("without mixing) and is a common reproductive strategy.  Because each lineage (parent-offspring line) is separate, apomixis can lead to a "species" containing lots of variation.  An example are the blackberries (brambles), where hundreds of  varieties are found in nature.
    • An unusual case of apomixis occurs in a cypress tree species found in the Sahara.  Here, it is the pollen that develops into the next generation, not the egg.

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 Systems

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)

  • Monogamy - formation of a pair bond during a mating season (or, less commonly, for life) so that females produces offspring from a single male and males can identify their offspring as all of the offspring from a female with which they have mated
    • Monogamy is often valuable for females because they choose males with access to resources needed for nesting and rearing young.  This access to resources is often secured by the male by successfully defending a Territory rich in the needed resources.
      • Some males do not defend territories with resources but defend territories within an competitive arena called a Lek. The lek is a gathering place for the males, who battle for choice positions.  Occupants of these choice positions are the males with which female who visit the lek will mate. 
    • Sometimes monogamy is forced on females when females will have multiple matings
    • Multiple matings by females can lead to a loss of male success because
      • later males can dislodge the sperm from earlier matings
      • Sperm Precedence - it is often the sperm from the last mating before the female fertilizes her eggs that are utilized. Sperm from earlier matings often fertilize no eggs
    • Males have strategies to maximizes the contribution of their sperm to the female's brood under these circumstances
      • Mate Guarding - the denial by a male of other male's access to the female once the male has mated with the female
        • ensures that their sperm are not displaced and are the most recently deposited
      • Mating Reactions - when sperm from a male cause a reaction in the female that interferes with subsequent matings
        • can be a swelling of the mating apparatus so that the female can not mate again
        • may be a sperm plug that caps the female reproductive tract or her sperm storage organ
  • Polygamy - mating and pair-bonding with more than one mate in a single reproductive season.  The other sex typically has only one mate (a result of the pair bonding).  There are two possibilities:
    • Polygyny - male mates with multiple females - far more common than polygyny - males often control access to resources or to other males and the number of females depends on this ability, which varies among species and sometimes from population to population within species - often linked to mate choice in that females will only mate with males who either command resources or dominate other males
    • Polyandry - female mates with multiple males.  This is uncommon and occurs when a female can produce multiple broods per reproductive season and males must contribute to rearing offspring.  Females compete for males, who are in limited supply and can choose the female they will mate with.  Males have no interest in multiple matings as they can only rear a single clutch of eggs at a time.  Females maximize the number of offspring by courting and mating with as many males as possible until no more clutches of eggs can be produced. 
      • Jacanas (the family Jacanidae), about 8 species of wading birds found in the tropics, are a well known example of Simultaneous Polyandry.  Males of polyandrous species mate with a female controlling the territory in which they live and rear the young in the clutch she produces after mating.  The females are larger than the males, contribute no parental care to their hatchlings, and fight with other females for territory.  When they manage to take territory from another female, they kill the prior female's young and mate with the males that were caring for the broods.  Two factors are thought to contribute to this mating system:  an environment rich enough in resources to allow production of multiple clutches of eggs and a high rate of egg predation.
      • Other shorebirds (some Sandpipers and Phalaropes) practice Sequential Polyandry in which the female mates with a male, lays a clutch that is cared for by the male, and then leaves to find another male.  No territory is maintained.
      • Some primates (e. g. some marmosets) practice Cooperative Polyandry in which a female forms a social group with more than one male.  When young are born, all of the males and the female cooperate in rearing the young
  • Promiscuity - this is a situation in which each sex mates with one or more members of the opposite sex but no pair bonds are formed.

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.

  • Intrasexual - Here, mate choice is indirect at most.  The traits under sexual selection are those that promote the ability of a male to win contests with other males for the opportunity to mate.  These may be size or armament.  This sort of sexual selection can still be contrary to natural selection if the trait is costly outside of its use in winning contests.  Mate choice is a part of the system only indirectly because females must choose the contest winner.  Were they not to do this, winning the contest would have no value and traits associated with the contest would not grant advantage to their possessors.  In some cases, contest winning males prevent other males from mating and there is no role for mate choice, even indirectly.
  • Intersexual - Mate choice is key to intersexual selection.  Here, one sex chooses based on the traits displayed by the other sex.  Intersexual sexual selection may explain why males are often more conspicuously colored (females choose the brightly colored males so color is sexually selected) even when the colors make them conspicuous to predators (which means natural selection favors the less conspicuous males). Females may benefit because the brightly colored males that survive may do so because of a host of good traits.  Choosing a brightly colored male means the female's eggs get the benefit of these superior traits.

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.")

Life History Trade-Offs

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. 

  • Reproductive Effort - the energy and resource used to produce gametes, find a mate, and rear young until they are independent of the parent.   Species differ with regard to the effort spent in these areas.  Asexuals may have no cost associated with finding a mate and many organisms provide little post-fertilization care.
  • If there is a trade off between growth and reproduction, we should be able to see a negative relationship when the number of offspring per female is graphed against growth of the mother.  The book has several examples of such a relationship.

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 is used to indicate the potential growth rate for a population in a particular environment
    • here it is associated with populations in disturbed environments that are often growing populations
  • K is used to indicate the maximum number of individuals that can exist and reproduce in an environment (called the carrying capacity of the environment).
    • here K is associated with stable populations in constant environments

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:

  • r- and K-selected life histories are extreme ends of a continuum
    • many environments are neither completely stable or constantly disturbed
  • idea is most useful when organisms are closely related to one another
    • difficult to tell the reason for differences among distantly-related organisms
  • r- and K-selected life histories depend on the existence of trade-offs
    • a trade-off is situation in which progress in one area must be offset by regression in another
    • example - if a species increases it ability to retain heat in its body, it will make progress in adapting to cold environments but will regress in its adaptation to hot environments
    • applying this idea to r- and K-selection implies that no organism can be a good competitor and a good disperser, etc.
    • much of life history theory depends on trade-offs

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.

  • Ruderals - (weeds)
    • good dispersers, poor competitors adapted to disturbed or highly variable environments
  • Competitors
    • do best in more stable environments where population sizes are large enough to lead to scarce resources and, thus, to competition for the resources
  • Stress Tolerators
    • live in environments that present extremes of one or more physical factors (high salt, low or high temperature, low moisture)

Terms

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