PDS APES 5th Period 2009-10

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Okay! So Friday in class we discussed population biology, or the characteristics of populations.  To review, a population is comprised of all individuals of one species. Populations make up communities.

How to describe populations

1) Age Distribution and Sex:

• Populations are typically  categorized into three age clusters: pre-reproductive, reproductive, and post-reproductive. Populations concentrated in the pre-reproductive category are likely to explode once their youths enter the reproductive stage of development, whereas populations with the majority of members in the post-reproductive stage experience relatively stagnant population growth. These patterns can be observed in age structure diagrams:

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Japan’s estimated population in 2055. An aging population, most of its members are in the post-reproductive stage, and therefore its future reproduction levels will be relatively stagnant.

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Pakistan’s population is very youthful. The country will see a significant increase in population as its youth enter into the reproductive stage of life.

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2) Geographic Distribution:

• Clumping (most common): (ex. schools of fish, elephant herds) Organisms clump together for protection, around a water or food source, and for mating and raising young. Some predator species clump to better catch their prey.
• ***The Burmese python clump to regulate body temperature at night. They are an invasive species in the Florida Everglades. In order to reduce their numbers, a few individuals are flown to Davidson College where Dr.Dorcas tags them with radio chips. The snakes are returned to Florida, where scientists track  the tagged individuals (who are by now clumped together with other pythons) and are thus able to exterminate entire snake clumps  efficiently.
• Random: few species are organized this way, but dandelions are a prime example of this since their seeds are spread by the whims of the wind, thus their distribution is completely random.
• Uniform: Desert plants like the creosote bush are uniformly distributed to better utilize scant resources like water.

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3)  Size:

Population change = (Births+ Immigration) – ( Deaths + Emigration)

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4) Density:

# of individuals/ unit of area or volume.

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5) Growth Rate:

Growth rates can be divided into two general patterns: J shaped growth (exponential) and S shaped growth (logistic).

J Curve

Populations with J shaped growth patters are typically short lived, and reproduce rapidly (think bacteria). Obviously, the exponential relationship displayed in the above graph cannot realistically continue as is. No population can perpetually reproduce at such lengths without eventually encountering a limiting factor (water, food, space), or a moment when the population outpaces available resources.

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Logistic Growth

Long lived, large species typically have an S shaped growth curve. While the population expands exponentially for a period of time, it eventually stabilizes at its carrying capacity, or the maximum population at which an environment’s given resources can support. These limiting factors, again, include water, food, and space. A population stabilizes at its carrying capacity (give or take some).

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We also watched a movie about a rare desert flower called the Santa Anna Wooly Star. Basically, the flower only grows around the sand deposits of rivers. However, with the creation of dams the flower is being striped of its habitat. The Wooly Star is a nice thing to have around since it’s an indicator of environmental heath and stabilizes sand deposits. To maintain the species, scientists are redistributing seeds and tracking its progression using line transits (tapes used to measure distance covered by plants).

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http://www.prb.org/images08/japan-age-sex-2.gif

http://www.populationaction.org/Publications/Working_Papers/Population_Fertility_and_Family_Planning_in_Pakistan/asset_upload_file732_7304.jpg

http://www.science.org.au/nova/087/087print.htm

http://www.design4effect.com/soc11/images/pg041.gif