By Melissa Murphy,2014-04-11 16:46
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    A population is a group of organisms of the same species occupying a particular space at the same time.

    ;Same species

    ;Same space

    ;Same time

    Populations have traits that are different from those of the individual composing the population.

    Individuals are born once, die once, are either male or

    female, young or old.

    Populations have birth rates, death rates, and growth

    rates, have sex ratios, age structure, density, and distribution

    in time and space.



    An individual is derived from one zygote. What is an individual is not always clear.

    Plants derived from one zygote can produce new plants or modules a sexually by means of buds on hallow horizontal roots or by stems that touch the ground.

    ;Individual plants produced by sexual reproduction are


    ;Asexually produced individuals derived from the

    generic parent are called ramets.

    ;A group of ramets originating from the same genet

    makes a clone.

    Ramets may remain linked to the parent or exist individually.Examples:

    a) Plants as grass clumps, blackberries, strawberries, that

    reproduce asexually by

     runners or stolons. Often never clear whether its a single

    modular organism or a


    b) Animals such as corals, hydroids, byrozoans, and

    tunicates that reproduce asexually

     by budding. Many primitive metazoans are loosely

    organized and form modular


    c) Fungal mycelia are in the same category, Each hypha is

    potentially an organism, but

     the mycelium as a genet can be enormous in size.All ramets are derived from the same zygote and may be considered as one individual or genet.

    For practical reasons, in plant population studies individual ramets are counted as and function as individual members of the population.


    Partially isolated groups of a population that occasionally exchange individuals through immigration form a metapopulation.

    The habitat is fragmented by unsuitable areas.

    The area can be easily re-colonized in case of a


    The subgroups or subpopulations have their own dynamics independent of other subpopulations: birth and death rates.Limited dispersal links the subpopulations.

    Individual subpopulations have finite lifetime.

    Islands may maintain populations derived from the mainland. If extinction occurs, the island could be repopulated from the mainland population.

    Local extinction and recolonization allow metapopulations to exist.


    A population is a genetic unit.

    Gene pool ;种群基因种;is the sum of all the genes in a


    Evolution from a genetic point of view is the change in gene frequencies in a gene pool from one time to a later time.Natural selection acts on the less favored genes and decreases their frequency in the next generation.

    These genes have low fitness.

    An outcome of this selection of genes may be changes in the physical expression of organisms.



    Density is the number of individuals per unit of space (area, volume). This called crude density.

    Populations are often discussed in terms of density.Habitats are uniformly habitable because of microdifferences in light, moisture, temperature, or exposure, to mention a few conditions.

    Each organism occupies only areas that can adequately meet its requirements, resulting in patch distribution.

    Density measure in terms of the amount of area available as living space is ecological density.

    Crude density includes all the land within the organism's range whereas ecological density includes only that portion of land that can actually be colonized by the species.

    Ecological density is rarely used because of the difficulty in determining what is a livable habitat, and organisms require different habitats during the year or during their developmental changes.

    Usually smaller organisms are more abundant than larger ones.The tropic level the organism occupies helps to determine its density.


    Spacial dispersion:

    Spacing refers to the position of members of a population in reference to its neighbors

    1.Clumped or aggregate spacing is the most common in nature.

    Caused by microhabitat preference (e.g. shaded or

    moist places), dispersal pattern (e.g. root shoots), social

    behavior of animals (e.g. flocking).

    2.Even distribution is rare; organisms are equidistant from one


    Caused by competition (e.g. territoriality of animals,

    lack of soil water in the desert).

    3.Random spacing is rare in nature; spacing varies between the


    The environment is uniform, resources are evenly

    distributed and available throughout the year, and

    interaction between the members of the population does

    not cause attraction or avoidance.

    A population may show one pattern at one scale and another pattern at other scale, e.g. large herds of antelope, where the animals are clumped together over the grassland but evenly spaced from one another.

6. Temporal dispersion ;种种尺度上的种散;

    Organisms are also dispersed in time due to circadian rhythms, changes in humidity and temperature, seasons, lunar cycles, and tidal cycles.

    e. g. bats dispersing in the night and regrouping in the

    cave during the day; the return and departure of migrating


    7. Dispersal movements ;种散种形式;

    Emigration is movement out of one habitat and into another, immigration.

    Dispersal with a return to the place of origin is called migration.

    Plants depend on passive means of dispersal: wind, water, animals, and gravity.

    Some are better means of dispersal than others.

    Some animals also depend on passive transport like stream and sea currents, wind disperses the young of some spiders. Most animals disperse actively by walking, crawling, flying, and swimming.

    Natal dispersal occurs when the young disperse, like

    in the case of many birds.

    Breeding dispersal occurs when adults disperse to

    find better reproductive habitats.

    The pre-reproductive period is the usual time of dispersal.Rodents disperse when the population has increased and reached a peak. Dispersal declines when the population declines.Animals will settle in first empty and suitable site. They usually travel in straight lines and then settle.

    Some animal species make exploratory trips around the natal territory before settling on their territory.

     Migratory movements

    Migration is a periodic or seasonal movement of an organism or population from one habitat, climate, or stratum to another.It usually involves long distance and affects the range of distribution of the individual or population.

    There are three types of migration:

    1) Two-way migration either daily or seasonal.


    Zooplankton moves down to greater depth during the

    day, and moves up closer to the surface by night.

    Bats move out of the cave at night and return to roost

    during the day.

    Other migrations are seasonal:

    Earthworm move deeper into soil during the winter and

    move closer to the surface in the spring.

    Elk moves down the mountain in winter and return to

    higher altitudes in the spring.

     2) Some migrations involve only one return trip, e. g. salmon species.

    3) In another type of migration, the fall migrants do not return north but their offspring do. Insects that migrate in this way are the monarch butterfly, two species of leafhoppers, harlequin bug, and the milkweed bug.

    8.AGE STRUCTURE ;种群年种种;

    Populations often have individuals of different ages. The grouping of members of a population by age is called aged distribution or

    age structure.

    Age distribution is typically presented in a modified bar chart called a large pyramid.

    Age distribution contributes in part to the reproductive rate, death rate, vigor, survival, and other demographic attributes.Several categories can be used to analyze the age structure of a population: years, months, etc,, life-history stages (pre-reproductive, reproductive and post-reproductive; size classes in herbaceous plants, tree diameter, etc. In plants, size is a good predictor of reproduction.


    Stable Age Distribution: The age distribution, which the

    population will reach if allowed to progress until there is no longer a change in the distribution.

    Age distribution can be disrupted by a natural catastrophe, disease, starvation, or emigration.

    Stationary or near-stationary population pyramids display

    somewhat equal numbers or percentages for almost all age groups.

    Births replace deaths and the population is not growing.Of course, smaller figures are still to be expected at the oldest

    age groups.

    The age-sex distributions of some European countries,

    especially Scandinavian ones, will tend to fall into this category.Population can increase, decrease or remain stable.

    Growing population in general are characterized by a large number of young, giving the pyramid a broad base.

    Older individuals dominate narrow pyramids.

    The loss of age classes can have a profound influence on a population's future.


    Asexual reproduction and modular structure in plants present a problem in analyzing age structure.

    Age in plants is determined by following a cohort of individuals over a period of time, or by determining age by growth rings, bud scars, or other indicators.

    In even-aged stand of trees, individuals fall into very few age classes because young age classes are excluded.

    Competition may cause a size difference between trees of the same age. In this case, size may be more useful than age.Seed banks in the soil present another problem. Seed may remain viable in the soil for many years. Seeds that germinate in a given year may be of different ages. What is the age of the plants?10. SEX RATIOS ;性比;

    Most population of sexually reproducing organisms tends to have a 1:1 sex ratio.

    The primary sex ratio (at conception) tends to be 1:1.

    The secondary sex ratio (at birth) among mammals is often

    weighed in favor of males, but the population shifts toward females in older age groups.

    Tertiary sex ratio is calculated at a later age.

    "Some recent studies, however, indicate that, within species, the sex ratio varies with the costs or benefits of producing male or female offspring... Sex differences in energy requirements or viability during early growth, differences in the relative fitness of male and female offspring, and competition or cooperation between siblings or between siblings and parents might all be expected to affect the sex ratio. Although few trends have yet been shown to be consistent, growing numbers of studies have demonstrated significant variation in birth sex ratios in non-human mammals."

    Basic tendency in mammalian, avian and freshwater fish patterns:

    ;mammals: sex ratio shifts toward females in later age classes;birds: sex ratio shifts toward more males in later age classes;freshwater fishes: more males in young-of-the-year and a shift

    toward females in older fish.

    Some possible explanations


    ;The heterozygous condition of males in mammals and females

    in birds, e.g. XY is a male in mammals, and ZW is a female in


    ;In the homozygous condition, deleterious genes can be masked

    by a dominant in the homologous chromosome; in the

    heterozygous condition, all genes will be expressed including

    deleterious genes in the Y or W chromosome.

    External: environmentally induce mortality related to behavior and/or life history of the sexes.

    Potential mechanisms for sex ratio adjustment in mammals and birds. Krackow S.

    "Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which

    gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well known to be influenced by many parameters, which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined."


    Natality: number of individuals added to the population by birth, hatching, cloning or germination.

    Crude birthrate: number of births in a year per thousand, e.g. 50 births 1000 population.

    Specific birth rate is the number of offspring produced per unit time by females in different age classes.

    Mortality or death rate: the number of organism that die in a certain time period divided by the number of organisms that were alive at the beginning of the time period.

    Crude death rate: number of deaths per thousand persons in a year.

    Wealthier countries generally have mortality rates

    around 10/1000.

    E.g. Costa Rica 4/1000; E.g. Denmark 12/1000.

    There are proportionately more young than elderly

    people in rapidly growing countries.

    Mortality typically concentrates on the very young, reducing a potential abundance of new individuals entering perhaps an already crowded population, and on the old, removing senescent individuals to make room for more vigorous young.

    The probability of dying is the number that died during a given time interval divided by the number alive at the beginning of the period, e.g. 400/1000 or 0.40.

    The number of survivors is more important to a population than the number dying. Mortality is best expressed as life expectancy.

    Life expectancy is the average number of years to be lived in the future by members of a given age in the population.

    ;The average length of life remaining at a given age.

    ;It represents the average longevity of the whole

    population and does not necessarily reflect the longevity of

    an individual.

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