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Populations. Ecology 2007. Population definitions. The individuals of a species with a given area constitute a population . The distribution of the ages of individuals in a population and the way those individuals are distributed over the environment describe the population structure .

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Populations

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Populations

Ecology

2007


Population definitions

  • The individuals of a species with a given area constitute a population.

  • The distribution of the ages of individuals in a population and the way those individuals are distributed over the environment describe the population structure.

  • Ecologists study population structure at different spatial scales, ranging from local subpopulations to entire species.

  • The number of individuals of a species per unit of area (or volume) is its population density.


Population views

  • Ecologists study population structure at different spatial scales, ranging from local subpopulations to entire species.

  • The number of individuals of a species per unit of area (or volume) is its population density.


Population density

  • Ecologists are interested in population densities because dense populations often exert strong influences on their own members as well as on populations of other species.

  • Density of terrestrial organisms is measured as number of individuals per unit area.

  • Density of aquatic organisms is measured as individuals per unit volume.

  • For some species such as plants, the biomass or percentage of ground covered may be a more useful measure of density than the number of individuals.


Demography

  • The structure of a population changes continually because of demographic events—births, deaths, immigration, and emigration.

  • Population dynamics is the change in population density through time and space.

    • Demography is the study of birth, death, and movement rates that give rise to population dynamics.


Population numbers

  • Population dynamics can be represented by:

  • N1 = N0 + B – D + I – E

    • N1 = number of individuals at time 1

    • N0 = number of individuals at time 0

    • B = number of individuals born between time 0 and time 1

    • D = number of individuals that died between time 0 and time 1

    • I = number of individuals that immigrated

    • E = number of individuals that emigrated


Life Tables

  • Life table information can be used to predict future trends in populations.

  • A cohort is a group of individuals that were born at the same time.

  • A life table can be constructed by determining the number of individuals in a cohort that are still alive at specific times (the survivorship) and the number of offspring they produced in each time interval.


Life Table for Cactus Finch


Survivorship curves

  • Survivorship curves in many populations fall into one of three patterns.

  • In some populations (e.g., humans in the U.S.), most individuals survive for most of their potential life span and die at about the same age.

  • In some (e.g., songbirds), the probability of surviving over the life span is the same once individuals are a few months old.

  • In species that produce a large number of offspring and provide little parental care, high death rates for the young are followed by high survival rates during the middle of the life span.


Survivorship curves – man vs. bird


Mortality and survival


Baby Boomers and US statistics


Populations

  • The bay checkerspot butterfly provides an example of the dynamics of a divided population.

  • The larvae of this butterfly feed on only a few species of annual plants in a small area of California; the largest patch supports thousands of butterflies.

  • During drought years, most plants die early in the spring, and several subpopulations on small patches become extinct.

  • The largest patch then disperses individuals to recolonize the smaller patches.


Colonization


In experiments with springtails and mites, scientists created isolated patches of the animals’ habitat.

  • The number of species present declined 40% (rarer species declined more than common ones), showing that small, isolated populations are more likely to become extinct than larger ones.


Chorus forgs

  • Predators may eliminate their prey in some places but not in others.

  • In ponds on islands in Lake Superior, chorus frogs are found in only some of the habitats that seem suitable for them.

  • The tadpoles have three major predators: salamander larvae, dragonfly nymphs, and dytiscid beetles.

  • Experiments indicated that dragonfly nymphs were able to eat all sizes of tadpole and when these nymphs were present, the pond lacked tadpoles.


Extinction- Elimination or loss of a species

  • Loss of habitat

  • Disease( natural phenomenon)

  • Geothermal or Weather related calamity( natural phenomenon)

  • Immigration of new species into space encroaches on natural resources of original species

  • Hunting

  • Pollution – Acid Rain – Global warming


Genetic impacts

  • Isolation – Original populations are cut off from outlying groups

  • Allopatric speciation – As groups are separated they diverge in their characteristics

  • Founder effect- If the group separated from the rest possesses deleterious effects – they may occur in higher proportions in the population and be inherited by successive generations


The effect of predators on populations of chorus frogs


Bacteria and cell culture


Bacteria grown in culture


Growth curve


Growth Curve dynamics

  • Lag phase – bacteria are inoculated into a new growth media

  • Log phase – population doubles in a generation time – in E. coli that can be 20 minutes. This is also referred to as the exponential growth phase or the logarithmic phase

  • Stationary phase – nutrients and oxygen exist in lower quantities – waste products and toxins build up

  • Death phase – cells are starving – they undergo cytologic changes. Waste byproducts become toxic – population crashes


Logarithmic

  • Best phase for studying characteristics of the cells

  • Cell characteristics are uniform

  • All cells dividing at the same rate

  • Nutrients and oxygen enable cells to perform all life processes


Carrying capacity

  • The maximum number of organisms of one species an ecosystem can sustain without increasing the mortality rate


Managing Populations

  • A general principle of population dynamics is that the total number of births and the growth rates of individuals tend to be highest when a population is well below its carrying capacity.

  • If we wish to maximize the number of individuals that can be harvested from a population, that population should be managed so that its population is far below its carrying capacity.

  • Hunting seasons are established with this objective in mind.


Fish and population growth

  • Populations with high reproductive capacities can sustain their growth despite a high rate of harvest.

  • Fish are an example of a population with high reproductive capacity.

  • Many fish populations can be harvested heavily for many years because only a modest number of females must survive to reproductive age to produce the eggs needed to maintain the population.

  • However, any species—even those with high reproductive capacity—can be overharvested.


Survival


Muir Woods

  • West coast redwood trees dominate Muir Woods' forest. Douglas-fir, big-leaf maple, tanbark oak, and baylaurel grow along side the redwoods. At the lower end of the canyon, red alders line the stream and buckeyes cluster nearby. Baylaurels growing toward the light may assume contorted shapes or topple over.

  • http://www.virtuar.com/marin/Muir/muir_woods_virtual_tour.htm


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