Populations & Trophic CascadesOooooohhhh…sounds exciting!

You’ve read the two articles about wolves in Yellowstone National Park…

Bottom-up vs. top-down control

•Bottom-up…organisms on each level are limited by the resources available from the level below

•Plants are key in biotic systems because no plants means no consumers to make a trophic structure

•Bottoms-up forces include level of primary productivity and availability of light & nutrients

Bottom-up vs. top-down control

•Top-down forces include predation and herbivory

•Predators affect herbivores which affect primary productivity

•Examples are Isle Royale, Yellowstone NP, and Kaibab Plateau

Trophic (or ecological) cascade:•The progression of indirect effects of

predators across successively lower trophic levels (Estes et al, 2001)

•When the presence of top trophic-level predators significantly affects herbivores & this interaction affects vegetation (e.g., species composition, age structure, or spatial distribution), a trophic cascade occurs

•May be crucial for the maintenance of biodiversity

Population

•The number of individuals of the same species in a given area whose members share resources and can breed with one another

population growth (or not)

•A change in population size—can be positive or negative▫Additions to the population

Births Immigration

▫Subtractions from the population Deaths Emigration

Birth rate (b) – death rate (d) = rate of natural increase (r)

Thomas Malthus (1766-1834)“The Principle of Population” essay 1798

Famous essay—often cited …suggested that unchecked population growth always exceeds the growth of food production. Actual (checked) population growth is kept in line with food supply growth by positive checks (i.e., starvation, disease, etc.) and preventive checks (i.e., postponement of marriage, etc. that keep down birthrate). Malthus’s hypothesis implied that actual population always has a tendency to push above the food supply.

Population growth, US

•During the 20th century, what contributed the most to the US population growth?

3 patterns of population growth

• Ideal conditions with unlimited resources▫Lots of food▫No predators▫No disease, &c.

• J-shaped curve• r = rmax =biotic

potential

Exponential Growth

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 240

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3 patterns of population growth

• Population growth slows down or stops after exponential growth▫ Resources less

available• S-shaped curve

▫ Grows to carrying capacity

Logistical Growth

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

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3 patterns of population growth

• Limit to growth• Wide swings of

population• Predator-prey

effects▫ One population

often follows the other in terms of size/pattern

Saw-tooth curve

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/environment/populationsandpyramidsrev5.shtml

density independent factors

•The population is regulated but the size of the population does not change the effects.▫weather/climate▫natural disasters▫drought▫development

density dependent factors

•Regulates the population size based upon the size of the group.▫habitat size▫disease▫parasites (like winter ticks)▫predator-prey

Population distribution—possibilities

uniform distribution

random distribution

clumped distribution

So…which of these is most common? WHY???

Methods for determining population size1. Direct count—count the number of individuals.

The direct count of humans is called a census.2. Capture-recapture (or mark-recapture)—

capture, tag, release, and recapture. Lincoln Index Formula. For species that move around.

3. Quadrat—sample the number of individuals in a small area to determine overall population. Used for species that don’t move quickly.

4. Transect—sample along a sampling line. Does not give a population number but indicates trends in the population or is used for comparison or to see transitions between habitats.

Quadrats1 km

1 k

m

1 m

1 mFound 3 S. rigida

Capture-recapture and the Lincoln Index Formula