Populations Chapter 19

19-1 Understanding Populations The human population size has increases from just under 2 billion in 1900 to over 7.3 billion in May 3, 2015 www. Worldometer.info/world-population

I. Properties of Populations Population – all of the organisms of the same species living together within the same given area.

A. POPULATION SIZE The number of individuals that the population contains. Changes based on the following: Immigration (increases) Births (increases) Emigration (decreases) Deaths (decreases)

Often a population size is often estimated. This is done by taking a SAMPLE (take a count of all of the individuals of the population in a much smaller area). The sample is used to estimate the population of the entire area. So if there are 100 individuals in 1 km2, we can assume there will also be this number in every 1km2. So, in 100 km2 we can estimate there are 10,000 individuals. Key to remember here is this is an estimate and there is potential for error. What if we used the population of rats in NYC to estimate the population of rats in NYS. Would this work? Why/why not?

B. POPULATION DENSITY Population density is the number of individuals in a population per unit area or volume. (could be water) People per mile2 See table 19-1 on p382 Based on the information in the chart – Which country is the most densely populated? Which country is the least densely populated? What is the mean population density for the countries listed?

C. DISPERSION describes how the individuals of a population are spread out in the given area Clumped – individuals are clustered together (like a flock of geese or a pack of wolves) Uniform – the individuals of the population are spread out evenly (the same distance apart from each other) think of nesting penguins – close together but out of pecking range. Random – individuals of the population are spread out over the area without regard to one another.

II. POPULATION DYNAMICS Populations are constantly changing in size. Factors that impact population size: Birth rate (BR) – the number of births per year Death rate (DR) – the number of deaths per year Life Expectancy – is how long an organism lives (survives)

A. AGE STRUCTURE usually a depiction, a chart or graph that shows the relative number of individuals of a population at specific age ranges. Age structure of a population is important in determining a populations potential for growth or decrease over time. Populations that have more individuals in their reproductive years have greater potential for population growth than a population with very few younger individuals.

B. PATTERNS OF MORTALITY Mortality = state of being alive A survivorship curve is a line on a graph that shows the mortality of individuals of a population over time. The mortality of different species tend to match one of the three following survivorship curves:

Survivorship Curves: Type I Type II Type III there is a low mortality rate relative to the number of individuals. Individuals tend have a strong chance to survive to old age. Type II There is a steady death rate regardless of age Half of the population dies by mid life expectancy for random reasons. Type III Many more are born than can survive Many die very early in life Most die before reproducing

19-2 Measuring Populations

I. POPULATION GROWTH RATE an indication of change in population size over time Calculated using the following equation: (# births + # immingrants) – (#deaths + # emigrations) = change in population If the number is positive the population is growing If the number is negative the population is decreasing If the number is zero, there is no change.

B. POPULATION SIZE because populations, especially human population tends to be so large, we divide the population size by 1000 ( to make the numbers easier to deal with) we indicate we have done this by using term “per capita”. Example: for a population of 1000 there are 20 births – so 20/1000 = .02 (births/percapita/year) this is our birth rate per capita. If within that same population there are 35 deaths, the death rate would be 35/1000 = .035 (deaths/per capita/ year) death rate per capita.

What is the population growth rate? Example: for a population of 1000 there are 20 births – so 20/1000 = .02 (births/percapita/year) this is our birth rate per capita. If within that same population there are 35 deaths, the death rate would be 35/1000 = .035 (deaths/per capita/ year) death rate per capita What is the population growth rate? (.02 - .035) = -.015 (a negative number so the population is decreasing). If the number were positive, the population would be growing. So the population is decreasing by 1.5 % per capita per year.

II. EXPONENTIAL MODEL Describes a population in rapid population growth. Even in the number is small, overtime the population grows rapidly as more and more individuals reproduce. The exponential model for growth is represented on a graph as a “J” curve. Rapid population growth means that resources plentiful and there is no competition.

Applying the exponential model rapid population growth until resources become scarce. At this point the population may overshoot the carrying capacity (maximum number of individuals the ecosystem can provide for) population size will level off and fluctuate very little after that. Exponential growth cannot continue indefinetly, just until carrying capacity is reached.

III. Logistic Model Describes a population that grows rapidly until resources become limited (Carrying Capacity is reached). This leads to competition and struggle for resources and survival. This causes population growth to slow and eventually level out. Logistic model is sometimes called an “S” curve.

IV. Population Regulation Population size is controlled in two mechanisms: Density Dependent Factors - these are factors that are caused by the size of the population : Disease due to proximity and sanitation, overcrowding. Competition for limited resources – Food Shelter Nesting materials and sites Density Independent Factors – these are factors that decrease population size that are not related to the size of the population: Natural disasters Flood Drought Fire Volcanic eruption

B. Population Fluctuations Often population size is related to natural cycles in the ecosystem A plentiful summer may lead to more food supply for herbivores. Herbivores as a result have more offspring. More herbivores means more food for predators, which means higher birth rate for predators. Now, more predators, eat more herbivores which means higher death rate, decline in population of herbivores. This eventually leads to less food and starvation for predators without prey. So cyclic, trends in populations effect trends of other populations within the community.

(Skip Perils of small populations – bottlenecking ) we did this in chapter 17.