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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