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8 July 2019

Intro to Ecology

•  Today�s topics: •  Global climate affects species

distributions •  Limiting factors for aquatic and terrestrial

biomes

•  Population Growth is multiplicative •  Exponential growth •  What causes density dependence? •  Density independent regulation

Species Diversity

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Ecological Diversity

Organismal ecology and behavior

Population ecology

Community ecology

Ecosystem ecology

Where do Saguaro cacti grow? Why?

Hypothesis: Cold Temperature Limits Distribution

Saguaro Distribution Freeze Free Desert

How strong is this evidence?

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How can you test that hypothesis?

http://www.nps.gov/history/history/online_books/symposia/1/chap5.htm

Hypothesis: Rainfall Limits Distribution

Saguaro Distribution Arizona rainfall

Why would too much rain limit cacti?

Why is species X absent from an area?

Does dispersal limit its

distribution? Does behavior limit its

distribution?

Area inaccessible or insufficient time

Yes

No

No

No

Yes

Yes Habitat selection

Do biotic factors (other species)

limit its distribution?

Predation, parasitism, competition, disease

Do abiotic factors limit its

distribution?

Chemical factors

Physical factors

Water Oxygen Salinity pH Soil nutrients, etc.

Temperature Light Soil structure Fire Moisture, etc.

What limits species distributions? Terrestrial Biomes Reflect Average Annual Temperature

and Precipitation

Why is it warmer near the equator? Global air circulation affects climate

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Global Atmospheric Circulation and Prevailing Winds Ocean Currents affect climate

Biomes

30ºN Tropic of Cancer

Equator

Tropic of Capricorn

30ºS

Tropical forest Savanna Desert Chaparral

Temperate grassland Temperate broadleaf forest Northern coniferous forest

Tundra High mountains Polar ice

Terrestrial Biomes

Ann

ual P

reci

pita

tion

Average Temperature

Fig. 53-UN1

Patterns of dispersion

Clumped Uniform Random

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Dispersion – the spacing among individuals within the population boundaries Births

Births and immigration add individuals to a population.

Immigration

Deaths and emigration remove individuals from a population.

Deaths

Emigration

Changes in population size

How long can the world population continue growing like this?

Exponential Growth

•  N = the population size •  r = the intrinsic rate of natural increase. •  = the per capita birth rate minus the death

rate •  r is constant per individual •  dN/dT = the rate of change in population size

Growth without limits

€

dNdT

= rN

Elephants in Kruger National Park

Year

8,000

6,000

4,000

2,000

0 1900

Pop

ulat

ion

size

1910 1920 1930 1940 1950 1960 1970

Growth with Limited Resources

•  r is the (maximum) per capita growth rate when density is low and resources are abundant

•  K is the carrying capacity.

dN dt = (K - N)

K r N

The Logistic Equation:

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Density dependent Population Growth Limits to Exponential Growth

Drosophila expts.

Birt

h or

dea

th ra

te

per c

apita

Equilibrium density

Density-independent death rate

Density-dependent birth rate

Population density

K is where b=d How well do actual populations fit the logistic model?

This laboratory population of Paramecium grew according a logistic model.

This laboratory population of Daphnia overshot carrying capacity. And this natural

population of song sparrows fluctuated considerably in response to environ- mental stresses.

Fig. 53-18

2,100

1,900

1,700

1,500

1,300

1,100

900

700

500

0 1955 1965 1975 1985 1995 2005

Year

Num

ber o

f she

ep

Density INDEPENDENT Factors also affect population size

Age Structured Populations

Belding�s Ground Squirrel

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Life history (the timing of birth and death) affects population growth.

Age

Age (years) 2 0 4 8 6

10

10 1

1,000

100

Num

ber o

f sur

vivo

rs (l

og s

cale

)

Males

Females

Survivorship Curve: Belding�s Ground Squirels

Survivorship curves differ among species

1,000

100

10

1 0 50 100

II

III

Percentage of maximum life span

Num

ber o

f sur

vivo

rs (l

og s

cale

)

I

Why do these species have different survivorship patterns?

Reproductive schedules also affect population growth

(a) Semelparity, one-time reproducer

(b) Iteroparity, repeat reproducer

Some species make many tiny offspring Others have just a few large ones

(a) Dandelion

(b) Brazil nut tree (right) and seeds in pod (above)

Table 53-2

The timing of reproduction also affects population growth

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Lifetime Reproductive Success

Ro = the lifetime reproductive success measured as the number of Daughters produced per Female.

Ro < 1 population will decline = 1 population will stabilize > 1 population will grow

Why?

Age Structure Pyramids. Rapid growth Afghanistan

Male Female Age

85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14

5–9 0–4

10 8 6 4 2 0 2 4 Percent of population

6 8 10 5 4

Slow growth United States

Male Female Age

85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4

Percent of population 4 5 5 4

No growth Italy

Male Female

Percent of population 4 5 3 2 1 0 1 3 2 3 2 1 0 1 3 2

Resource allocation Resource Acquisition Increases with Resource Availability—Up to a Point

Male Female

100

Tradeoffs: survival vs. reproduction

80

60

40

20

0 Reduced brood size

Normal brood size

Enlarged brood size

Pare

nts

surv

ivin

g th

e fo

llow

ing

win

ter (

%) Tradeoffs: growth vs. reproduction