POPULATIONS

A population consists of all the individuals of a species that live together in one place.

The area inhabited by a population is called its geographic range. Small geographic range Large geographic range

Population density refers to the number of individuals per unit area. Two ducks Hundreds of fish Low population density High population density

Distribution refers to how individuals in a population are spaced out across the range of the population.

Population size- the number of individuals in a population.

Population growth – the increase in the size of a population over time.

A population grows when exceed . births deaths More births than death…population grows. 100 +20 120 -15 105 Initial population=100 Births=20 Deaths=15 +5 105 Change in population = ____ Final population= _____

More deaths than births…population decreases. 100 +20 120 -25 95 Initial population=100 Births=20 Deaths=25 - 5 95 Change in population = ____ Final population= _____

The factors that can affect population size are the birthrate, death rate, and rate at which individuals enter or leave the population (immigration and emigration). emigration - exit immigration - in Population decreased Population increased

If more individuals come in (immigration) than exit (emigration), then population increases.

Births and immigration add individuals to a population The population increases The population decreases Deaths and emigration remove individuals to a population

What does a population need to grow? food water space few predators less disease

Example: A single bacteria divides to produce two bacteria every 20 min. 1 2 Time (min) # of bacteria   20 40 60 80 100 120 140 160 180 4 1 2 8 4 8 16 16 32 64 32 128 64 256 512

Compare the growth rate of bacteria: Time (min) # of bacteria   20 40 60 80 100 120 140 160 180 1 2 1st hr Very slowly 4 8 16 Slow-medium 2nd hr 32 64 128 3rd hr Rapidly 256 512

Let’s graph the data. Exponential Growth Curve Check out the slopes Time (min) # of bacteria   20 40 60 80 100 120 140 160 180 Check out the slopes 1 2 4 8 16 32 64 128 256 512

Under ideal conditions with unlimited resources, a population will grow exponentially. Exponential growth means that as a population gets larger, it also grows faster.

Can a population of organisms grow indefinitely?

Can a population of organisms grow indefinitely? Population growth is limited by food water predators disease space

any biotic or abiotic factor that controls the growth of a population. Limiting factors- any biotic or abiotic factor that controls the growth of a population. Limiting factors are environmental factors that affect an organism’s ability to survive in its environment.

Some limiting factors may be: water food predators disease competition temperature natural disasters

Limiting factors prevent a population from growing exponentially forever. Therefore, most organisms’ populations follow a logistic growth curve, an S-shaped curve. Check out the slopes Number of Organisms Time

Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth.

Phases of Growth: Phase 1: Exponential Growth In the beginning, the population grows slowly. After a short time, the population grows exponentially. Resources are unlimited, so individuals grow and reproduce rapidly. Population becomes larger and larger.

Phases of Growth: Phase 2: Growth Slows Down As a result of limiting factors, the growth of the population begins to slow down. The population size increases more slowly.

Phases of Growth: Phase 3: Growth Stops The population stops growing, the population will remain at or near this size. The population reaches its carrying capacity.

Logistic Growth (S-shape) Growth slows. Carrying capacity Growth stops. Population grows rapidly.

Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support. 1000 bison is the carrying capacity of this preserve. 1000 Carrying capacity

What determines the carrying capacity of an area? 1000 bison is the carrying capacity of this preserve. 1000 Limiting factors determines the carrying capacity of an environment for a species.

When populations are under the carrying capacity, births exceed deaths and the population grows until the carrying capacity is reached. If the population grows beyond the carrying capacity, deaths exceed births and the population decreases to below the carrying capacity. Number of Organisms Time (yrs)

Growth of a Sheep Population Population overshoots carrying capacity Environmental resistance 2.0 Carrying capacity 1.5 Population recovers and stabilizes Number of sheep (millions) Population runs out of resources and crashes 1.0 Exponential growth Animated Figure 5.16 Growth of a sheep population on the island of Tasmania between 1800 and 1925. After sheep were introduced in 1800, their population grew exponentially thanks to an ample food supply and few predators. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep. .5 1800 1825 1850 1875 1900 1925 Year Fig. 5-16, p. 115

the number of individuals in the population. Density-dependent factor- Any factor limiting the size of a population whose effect is dependent on the number of individuals in the population. Examples: food, disease, parasites, predation, and competition

Density-independent factors control population growth regardless of the population's density. Example: fire, flood, hurricane, severe freeze, habitat destruction

How do communities and ecosystems respond to changing environmental conditions? The structure and species composition of communities and ecosystems change in response to changing environmental conditions through a process called ecological succession.

Mount St Helen, WA (before 1980)

Mount St Helen, WA (May 18, 1980)

One of the most violent natural disasters of our time, the colossal eruption of Mt. St. Helens in 1980 blasted away an entire mountainside. Over 200 square miles of pristine forest were buried under millions of tons of lava, ash, mud, and avalanche debris. How could life ever return to this barren moonscape?  May 1980

https://www.youtube.com/watch?v=R3R0eQOWmPE

Today

Communities and Ecosystems Change over Time: Succession Gradual change in species composition in an area Two types of succession: Primary succession Secondary succession https://www.youtube.com/watch?v=V49IovRSJDs

- occurs in lifeless area….no soil…no life Primary succession - occurs in lifeless area….no soil…no life Examples – bare rock, lava flow, new volcanic island, land exposed after retreating glacier, an abandoned parking lot.

https://www.youtube.com/watch?v=CjB6zmEO5BY

- occurs in lifeless area….no soil…no life Primary succession - occurs in lifeless area….no soil…no life A series of changes occurs over tens to hundred of years ….as different organisms inhabit the area Break down rock making soil

- occurs in lifeless area….no soil…no life Primary succession - occurs in lifeless area….no soil…no life Break down rock making soil

Perennial weeds and grasses Secondary succession Occurs in an area where the natural community has been disturbed or destroyed, but the soil remains. Clear-cut forest, burned forest, flooded land, hurricane Mature oak and hickory forest Young pine forest with developing understory of oak and hickory trees Shrubs and small pine seedlings Perennial weeds and grasses Annual weeds Time Fig. 5-12, p. 110

Perennial weeds and grasses Secondary succession Occurs in an area where the natural community has been disturbed or destroyed, but the soil remains. Clear-cut forest, burned forest, flooded land, hurricane https://www.youtube.com/watch?v=SoQA6gGEyLg Mature oak and hickory forest Young pine forest with developing understory of oak and hickory trees Shrubs and small pine seedlings Perennial weeds and grasses Annual weeds Time Fig. 5-12, p. 110