Bell Ringer 1.What critical function do nitrogen fixers serve? 2.What would happen to an ecosystem if the nitrogen fixers died out? 3.State whether primary or secondary succession will occur after: a.A glacier melts. b.A wildfire. c.Nature reclaims abandoned farmland. d.A volcanic eruption.

Population Growth Why do populations grow and shrink?

Populations Recall from chapter 1 that a population is the entire number of one organism. Populations do not stay static; instead, the size of populations changes with the availability of resources and the presence of predators. Because of this, the size of one population is often dependent on the health of other populations!

Populations For example, take the following food chain: Kelp →Sea Urchin → Sea Otter → Killer Whale At one point in time, otters were almost hunted to extinction. As a result, sea urchins grew out of control, and kelp was being overconsumed. As otters became a protected species, they managed to bring the urchin population under control. Kelp also was restored to healthy levels. However, killer whales are also protected, and as their numbers recover, they eat more and more otters. Urchin numbers are expanding as well.

Populations Populations have four important characteristics: Geographic Distribution Density Growth Rate Age Structure

Geographic Distribution A population’s geographic distribution is the relative area that a population inhabits. For instance, if we look at whales in the Pacific Ocean, the entire ocean would be the geographic distribution.

Population Density Population density is the total number of one kind of organism per unit area. For example, saguaro cactus in the Mojave desert has a low density, because there are very few saguaro cacti in that desert.

Growth Rate A population’s growth rate is the rate of change in the number of organisms that make up that population. It can be positive, if the population is growing, or negative, if the population is shrinking.

Growth Rate Growth rate is itself affected by three important factors: 1.The total number of births 2.The total number of deaths 3.The overall number of organisms that migrate in or out of the population.

Growth Rate Population Increase If more births than deaths occur, then the population will increase. If organisms immigrate – move into the area – then the population will increase. Population decrease If more deaths than births occur, then the population will decrease. If organisms emigrate – move out of the area – then the population will decrease.

Growth Rate Note that both immigration and emigration occur as organisms search for new resources, like food, water and mates. Also, those two events happen as organisms enter and leave to establish new territories.

Exponential Growth Exponential growth is a level of growth that occurs under ideal conditions. For exponential growth to occur, resources MUST be unlimited, and conditions MUST be ideal.

Exponential Growth Note the graph to the right. This is a good illustration of exponential growth. Growth happens at a constant rate. At first, growth occurs slowly, but over time, growth speeds up, until the population’s size approaches infinity.

Logistic Growth In reality, exponential growth is unsustainable. As growth increases, resources are used up at faster levels. Once resources begin to grow scarce, population growth slows or stops.

Logistic Growth Note the graph to the right. This is a good illustration of logistic growth. The population grows only up to a certain point. After reaching that point, growth tapers off.

Logistic Growth We call this point the carrying capacity. It is the maximum number of organisms that this ecosystem can viably support.

Exit Ticket Select the correct word to finish each sentence. 1.If the birth rate is greater than the death rate, the growth rate will be [positive, negative]. 2.[Immigration, Emigration] happens when organisms move out of their old habitat. This migration [raises, lowers] the growth rate. 3.Exponential growth occurs when resources are [limited, unlimited] and conditions are ideal. 4.Exponential growth is [sustainable, unsustainable].

Age Structure The age structure of a population refers to the relative age of all members in the population. This is important because age structure can predict future growth!

Age Structure General Notes: Each half of the graph relates to males and females. The bottom of the graph relates to young individuals, and the top to older individuals.

Age Structure General Notes: The wider the bottom, the greater the birth rate. The narrower the bottom, the lower the birth rate.

Age Structure General Notes: The wider the top, the lower the death rate (also called mortality). The narrower the top, the greater the death rate.

Age Structure If the curve is bottom-heavy, then the overall population is young. This pattern predicts population growth.

Age Structure If the curve is top heavy, then the overall population is old. This pattern predicts a shrinking population.

Exit Ticket 1.Is the birth rate high or low? 2.Is the mortality rate high or low? 3.Does this age curve predict positive population growth? Yes or no?

Limiting Factors Population growth is opposed by many factors, such as disease, predation and human activity. We call these limiting factors, because they limit the growth of other populations.

Limiting Factors Density-dependent Density-dependent limiting factors are more effective on larger populations. Predation Parasitism Competition Density-independent Density-independent limiting factors are equally effective on small and large populations. Human activity Drought and other climate changes