populations and communities

Ecology

Populations and Communities

Ecology

Ecology– Is the study of the interactions between

organisms and the environmentThese interactions

– Determine both the distribution of organisms and their abundance

39.1

The environment of any organism includesAbiotic, or nonliving components

– Temperature– Water– Sunlight– Wind– Rocks and soil

Biotic, or living components– All organisms in the environment

39.2

A population– Is a group of individuals of a single species

living in the same general area

Density– Is the number of individuals per unit area or

volumeDispersion

– Is the pattern of spacing among individuals within the boundaries of the population

39.3

A clumped dispersion– Is one in which individuals aggregate in

patches– May be influenced by resource availability

and behavior

(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

39.5

A uniform dispersion– Is one in which individuals are evenly

distributed– May be influenced by social interactions

such as territoriality

(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

39.5

A random dispersion– Is one in which the position of each

individual is independent of other individuals

(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

39.5

Life Tables

A life table– Is an age-specific summary of the survival

pattern of a population– Is best constructed by following the fate of

a cohort (group of individuals from a population)

39.6

The life table of Belding’s ground squirrels– Reveals many things about this population

39.6

Life Table for Batteries

Exponential GrowthThe J-shaped curve of exponential

growth– Is characteristic of some populations that are

rebounding

1900 1920 1940 1960 1980Year

0

2,000

4,000

6,000

8,000

Ele

phan

t pop

ulat

ion

39.7

The Logistic Growth Model

In the logistic population growth model– The per capita rate of increase declines

as carrying capacity is reached

39.7

800

600

400

200

0

Time (days)

0 5 10 15

(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.

1,000

Nu

mb

er

of

Pa

ram

eci

um

/ml

The Logistic Model and Real Populations

The growth of laboratory populations of paramecia– Fits an S-shaped

curve

39.7

Some populations overshoot K (carrying capacity)– Before settling down to a relatively stable

density180

150

0

120

90

60

30

Time (days)

0 16014012080 100604020

Nu

mb

er

of

Da

ph

nia

/50

ml

(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.

39.8

Some populations– Fluctuate greatly around K

0

80

60

40

20

1975 1980 1985 1990 1995 2000

Time (years)

Nu

mb

er

of

fem

ale

s

(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.39.8

Population Change and Population Density

In density-independent populations– Birth rate and death rate do not change with

population densityIn density-dependent populations

– Birth rates fall and death rates rise with population density

39.9

Competition for Resources– In crowded populations, increasing population

density intensifies competition for resources

100 100

100

0

1,000

10,000

Ave

rag

e n

um

be

r o

f se

ed

s p

er

rep

rod

uci

ng

ind

ivid

ua

l (lo

g s

cale

)

Ave

rag

e c

lutc

h s

ize

Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

3.0

3.2

3.4

3.6

3.8

4.0

(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

39.9

Abiotic and Biotic Factors Controlling Populations

– Many populationsUndergo regular boom-and-bust cycles

Year1850 1875 1900 1925

0

40

80

120

160

0

3

6

9

Lynx

pop

ulat

ion

siz

e (t

hous

and

s)

Har

e po

pula

tion

size

(t

hous

and

s)

Lynx

Snowshoe hare

Lynx populations are dependent on the snowshoe

hare population in a

community

39.10

The accumulation of toxic wastes can contribute to the regulation of population size

If snowshoe hares are exposed to toxic waste that reduces their reproductive rate, what will happen to the lynx?

Abiotic and Biotic Factors Controlling Populations

39.10

Human population growth has slowed after centuries of exponential increase

No population can grow indefinitely

The Global Human Population The human population increased relatively

slowly until about 1650 and then began to grow exponentially

8000 B.C.

4000 B.C.

3000 B.C.

2000 B.C.

1000 B.C.

1000 A.D.

0

The Plague Hum

an

pop

ulat

ion

(bill

ions

)

2000 A.D.

0

1

2

3

4

5

6

39.11

Though the global population is still growing

– The rate of growth began to slow approximately 40 years ago

1950 1975 2000 2025 2050Year

2003

Per

cent

incr

ease

2.2

2

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

1.8

39.11

50

40

20

0

30

10

1750 1800 1850 1900 1950 2000 2050

Birth rate

Death rate

Birth rate

Death rate

Year

Sweden Mexico

Birt

h or

dea

th r

ate

per

1,00

0 pe

ople

Age Structure

One important demographic factor in present and future growth trends– Is a country’s age structure, the relative

number of individuals at each age

Age structure– Is commonly represented in pyramids

Rapid growth Afghanistan

Slow growth United States

Decrease Italy

Male Female Male Female Male FemaleAge Age

8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

Global Carrying Capacity

Just how many humans can the biosphere support?

What Is a Community?

A biological community– Is an assemblage of populations of

various species living close enough for potential interaction

A community’s interactions include competition, predation, herbivory (plant/animal), symbiosis, and disease

Populations are linked by interspecific interactions– They affect the survival and reproduction of the

species engaged in the interaction

39.12

Interspecific interactions– Can have differing effects on the

populations involved

39.12

The Competitive Exclusion Principle

The competitive exclusion principle– States that two species competing for the

same limiting resources cannot coexist in the same place

39.13

Ecological Niches

The ecological niche– Is the total of an organism’s use of the

biotic and abiotic resources in its environment

39.13

Predation

Predation refers to an interaction– Where one species, the predator, kills

and eats the other, the preyFeeding adaptations of predators

include– Claws, teeth, fangs, stingers, and poison

Animals also display– A great variety of defensive adaptations

39.14

Cryptic coloration, or camouflage– Makes prey difficult to spot

39.14

Aposematic coloration– Warns predators to stay away from prey

39.14

In Batesian mimicry– A palatable or harmless species mimics

an unpalatable or harmful model

(a) Hawkmoth larva

(b) Green parrot snake

39.14

In Müllerian mimicry– Two or more unpalatable species

resemble each other

(a) Cuckoo bee

(b) Yellow jacket39.14

Parasitism

The parasite– Derives its nourishment from another

organism, its host, which is harmed in the process

39.15

MutualismIs an interspecific interaction that

benefits both species

39.15

CommensalismOne species benefits and the other is not

affected

39.15

Trophic Structure

Trophic structure– Is the feeding relationships between

organisms in a community– Is a key factor in community dynamics

39.16

Food chains

Quaternary consumers

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

Carnivore

Carnivore

Carnivore

Herbivore

Plant

Carnivore

Carnivore

Carnivore

Zooplankton

PhytoplanktonA terrestrial food chain A marine food chain

– Link the trophic levels from producers to top carnivores

39.16

Food Webs

A food web

Humans

Baleen whales

Crab-eater seals

Birds Fishes Squids

Leopardseals

Elephant seals

Smaller toothed

whales

Sperm whales

Carnivorous plankton

Euphausids (krill)

Copepods

Phyto-plankton

– Is a branching food chain with complex trophic interactions

39.16

Keystone Species

Keystone species– Are not necessarily abundant in a

community– Exert strong control on a community by

their ecological roles, or niches

39.17

Field studies of sea stars– Exhibit their role as a keystone species in

intertidal communities

(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.

With Pisaster (control)

Without Pisaster (experimental)

Num

ber

of s

peci

es

pres

ent

0

5

10

15

20

1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73

(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity.

39.17

What Is Disturbance?

A disturbance– Is an event that changes a community– Removes organisms from a community– Alters resource availability

Stability

39.18

The large-scale fire in Yellowstone National Park in 1988– Demonstrated that communities can

often respond very rapidly to a massive disturbance

(a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.

(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

39.18

Ecological Succession

Ecological succession– Is the sequence of community and

ecosystem changes after a disturbancePrimary succession

– Occurs where no soil exists when succession begins

Secondary succession– Begins in an area where soil remains after a

disturbance39.19

McBride glacier retreating

0 5 10

Miles

GlacierBay

Pleasant Is.

Johns HopkinsGl.

Reid Gl.

GrandPacific Gl.

Canada

Alaska

1940 1912

1899

1879

18791949

1879

1935

1760

17801830

1860

1913

1911

18921900

1879

1907 19481931

1941

1948

Cas

emen

t Gl.

McB

ride

Gl.

Plateau Gl.

Muir G

l.

Riggs G

l.

Retreating glaciers– Provide a valuable field-research

opportunity on succession

39.19

Succession on the moraines in Glacier Bay, Alaska– Follows a predictable pattern of change in

vegetation and soil characteristics

(b) Dryas stage

(c) Spruce stage

(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content.

Soi

l nitr

ogen

(g/

m2)

Successional stage

Pioneer Dryas Alder Spruce0

10

20

30

40

50

60

(a) Pioneer stage, with fireweed dominant

39.19