Characteristics of Populations

Characteristics of Populations A population is a number of individuals belonging to the same species that reside in a given area/ecosystem. You know better know what a species is – you studied this as part of your evolution unit – review this concept if needed. The place where an organism normally lives is its habitat. The habitat is selected because it provides the organism with all of its required resources.

Population Size & Density Population density is the number of individuals of the same species that occur per unit of area or volume. The formula for population density is… D =N S In this… D is the population density. N is the number of organisms counted. S is the space occupied by the population.

Don’t Be Crude! Population density can be expressed as a crude density or an ecological density. Crude density is the number of organisms of the same species within the entire habitat. Ecological density is the number of individuals of the same species per unit area or volume that is used by the species. Ecological density is a more accurate descriptor of the population density of a species.

Population Dispersion Patterns The population dispersion for a species is the pattern in which individuals in a population are distributed through an area. Clumped dispersion – The population is highly concentrated in certain parts of the habitat. Uniform dispersion – The population is spaced evenly throughout the habitat. Random dispersion – The population is spread out in an unpredictable and patternless manner.

Measuring Population Characteristics You can’t just go out and count every single member of some populations – it could be next to impossible. So how do you get an accurate estimate of the population you are studying? Quadrat Sampling – Divide larger area up into smaller sections (quadrats)…count within a few of these smaller sections…average it out…multiply the average of the counted sections by the number of total sections in your area. Mark-recapture Method – capture as many organisms in the area as you can…mark them and leave…go back and capture more of the organisms…compare the number caught this time with the number that were marked.

Mark-recapture Sampling There is a ratio for the mark-recapture sampling procedure that allows you to estimate the population. M ═ m N n In this… M = Total # marked N = Total population m = # of recaptures n = Size of second sample Try the practice problems on pages 656 & 657!

Measuring & Modeling Population Change

Take it to the Limit How big can a population get? The answer involves the idea of a carrying capacity for the given area where the population resides – this is the maximum number of organisms that can be sustained by an ecosystem over time. The word “sustained” is used because the population must be able to live there year after year for the ecosystem to be considered healthy/successful. Organisms will multiply as long as there are resources available to support them. When one or more of these resources begins to run out – the population will stabilize or even decline.

Factors That Affect Population Growth There are a variety of factors that affect the growth of a population: Natality – Birth rate. Mortality – Death rate. Immigration – Move into area. Emigration – Move away from area. Fecundity – The potential number of offspring that could be produced in a lifetime. Survivorship - The ability to make it to sexual maturity (AKA – adulthood). Look at the survivorship curves on page 661. There are three types (I, II, III) which are all based on some of the factors above.

Calculating Population Changes The formula for population change is: Pop. Change = [(b + i) – (d + e)] x 100% n In this… “b” is the number of births. “i” is the number of immigrants. “d” is the number so deaths. “e” is the number of emigrants. “n” is the initial population size This formula will yield a percent change in the population for the given time in which the data was collected.

Open & Closed Populations An open population is one in which birth, death, immigration and emigration are all adding to or subtracting from the population numbers. An open population would resemble the population of people in Canada. A closed population is one in which only birth and death are contributing factors to the population growth. A closed population may include the populations of wild game within a closed reserve in Africa. Biotic potential – the maximum number a population can attain if resources were unlimited. (Ideal conditions)

Population Growth Models There are three different models which display population growth patterns. Geometric Growth – Population growth pattern where organisms reproduce at fixed intervals at a constant rate (seasonal – like deer). Exponential Growth – Population growth pattern where organisms reproduce continuously at a constant rate (all the time – like humans). Logistic Growth – Population growth pattern where population grows and then eventually levels off as the carrying capacity is reached. (All populations will follow this pattern eventually.)

FIN!