ch. 26 population ecologycf.linnbenton.edu/mathsci/bio/klockj/upload/ch26 population ecology… ·...
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Ch. 26 Population Ecology
http://galen.metapath.org/popclk.html
Chapter 26 At a Glance
How Does Population Size Change?
How Is Population Growth Regulated?
How Are Populations Distributed in Space and
Time?
How Is the Human Population Changing?
26.1 How Does Population Size Change?
A population consists of all the members of a particular
species that live within an ecosystem
A community is group of interacting populations (Douglas
Fir forest community)
Communities exist within ecosystems (old growth forest
ecosystem in Pacific NW), which include all the living an
nonliving components of a defined geographical area
The biosphere is the enormous ecosystem that
encompasses all of Earth’s habitable surface
Ecology is the study of the interrelationships of organisms
with each other and with the nonliving environment
26.1 How Does Population Size Change?
Population size is the outcome of opposing
forces
– Four factors determine whether and how much
the size of a population changes
–Births
–Deaths
–Migration of individuals into the population
(immigration)
–Migration of individuals out of the population
(emigration)
26.1 How Does Population Size Change?
Population size is the outcome of opposing
forces (continued)
– Birth and immigration add individuals to a
population
– Death and emigration remove individuals from
the population
26.1 How Does Population Size Change?
Population size is the outcome of opposing
forces (continued)
– A simple equation describes the change in
population size within a given time period:
–Change in population size = (births – deaths)
+ (immigrants – emigrants)
births immigration
deaths
(births deaths) (immigrants emigrants)
change in population size
emigration
Population Change
Fig. 26-1
26.1 How Does Population Size Change?
Population size is the outcome of opposing forces
(continued)
– Two opposing forces that determine birth and death rates
are biotic potential and environmental resistance
– Biotic potential is the theoretical maximum rate at
which a population could increase, assuming a
maximum birth rate and minimal death rate
– Environmental resistance refers to the curbs on
population growth that are set by the living and
nonliving environment
26.1 How Does Population Size Change?
Population size is the outcome of opposing forces
– Examples of environmental resistance include:
– Interactions among species, such as competition, predation, and parasitism
– The always-limited availability of nutrients, energy, and space
– Natural events, such as storms, fires, freezing weather, floods, and droughts
26.1 How Does Population Size Change?
Biotic potential can produce exponential growth
– Evolutionarily successful organisms possess
traits that make them well adapted to their
environment
– These organisms pass these inherited traits on
to as many healthy offspring as possible
– If environmental resistance is reduced,
populations can grow extremely rapidly
26.1 How Does Population Size Change?
Population growth is a function of the birth rate,
the death rate, and population size
– The growth rate (r) of a population, also called
the rate of natural increase, is the change in the
population size per individual per unit of time
– Growth rate is expressed by the equation:
–r (growth rate) = b (birth rate) – d (death rate)
26.1 How Does Population Size Change?
The growth rate of a population
– Birth rate (b) is the number of births per individual during a specific unit of time, such as a month or a year
– For example, if there are 150 births among 1,000 individuals in a year, b = 0.15
– Death rate (d) is the number of deaths per individuals during a specific unit of time
– For example, if there are 50 deaths among 1,000 individuals in a year, d = 0.05 or (1000/50)
– If the birth rate exceeds the death rate, the population will grow
– If the death rate exceeds the birth rate, the population will decline
26.1 How Does Population Size Change?
The growth rate of a population (continued)
– The growth rate of this population of 1000 is
therefore:
–r (growth rate) = 0.15 (birth rate) – 0.05 (death
rate) = 0.1 = 10% per year
–population growth (rN) = 0.1 x 1,000 = 100, so
the population has grown by one hundred
individuals in the first year
26.1 How Does Population Size Change?
If births exceed deaths by a constant percentage, population growth produces a J-curve
– A patter of continuously accelerating increase in population size is called exponential growth
– When graphed against time, a shape called a J-curve is produced
TIME
PO
PU
LA
TIO
N S
IZE
45.3
26.1 How Does Population Size Change?
If births exceed deaths by a constant
percentage, population growth produces a J-
curve (continued)
– This high biotic potential evolved because it
helps ensure that, in a world filled with forces of
environmental resistance, some offspring survive
to reproduce
26.1 How Does Population Size Change?
Several factors influence biotic potential
– The age at which the organism first reproduces
– The frequency at which reproduction occurs
– The average number of offspring produced each
time
– The length of the organism’s reproductive life
span
– The death rate of individuals
– Increased death rates can slow the rate of
population growth significantly
26.1 How Does Population Size Change?
The age at which an organism first reproduces affects the size of the future population – For example, consider two populations of golden eagles
that are followed for 30 years
– Individuals in one population begin reproducing at the age of 4 years
– Individuals in the other population begin reproducing at 6 years
– At 30 years, the earlier reproducing population would be 10 times the size of the other population
1,764 17,314 30
392 2,504 24
86 362 18
18 52 12
4 8 6
2 2 0
Time
(years)
Number
of
eagles
(pop. 1)
Number
of
eagles
(pop. 2)
At 24 years,
this population
has 392 eagles
At 24 years,
this population
has 2,504 eagles
reproduce at 4 years (pop. 1) reproduce at 6 years (pop. 2)
Exponential Growth Curves are J-Shaped
Fig. 26-2
26.1 How Does Population Size Change?
The death rate affects population size
– As long as birth rate exceeds death rate,
population size will follow a J-shaped rate of
increase
26.2 How Is Population Growth Regulated?
Exponential growth only occurs under special
conditions
– Exponential growth cannot continue indefinitely
– All populations that exhibit exponential growth
must eventually stabilize or crash
– Exponential growth can be observed in
populations that undergo boom-and-bust
cycles, in which periods of rapid population
growth are followed by a sudden, massive die-off
26.2 How Is Population Growth Regulated?
Exponential growth only occurs under special
conditions (continued)
– Boom-and-bust cycles can be seen in short-lived, rapidly
reproducing species, such as microbes and insects
– Seasonal populations are linked to changes in rainfall,
temperature, or nutrient availability
– Ideal conditions encourage rapid growth; deteriorating
conditions encourage massive die-off
– Rabbits in Australia, algae in a pond
26.2 How Is Population Growth Regulated?
Boom-and-bust cycles can be seen in short
lived, rapidly reproducing species (continued)
– Complex factors produce four-year cycles for
small rodents, such as lemmings
–Lemming populations may grow until lack of
food, large migrations, and predators and
starvation cause sudden high mortality
Boom-and-Bust Population Cycles
Fig. 26-4b
(b) Boom-and-bust cycles in a lemming population in Alaska
26.2 How Is Population Growth Regulated?
Exponential growth occurs when environmental
resistance is reduced
– In populations that do not experience boom-and-bust
cycles, exponential growth may occur temporarily under
special circumstances such as:
– When food supply is increased
– When population-controlling factors, such as
predators, are reduced
– For example, the whooping crane population has grown
exponentially since they were first protected from hunting
and human disturbance in 1940
Exponential Growth of Wild Whooping Cranes
Fig. 26-5
26.2 How Is Population Growth Regulated?
Exponential growth can occur when individuals
invade a new habitat
– Invasive species are organisms with a high
biotic potential that are introduced into
ecosystems where they did not evolve and
where they encounter little environmental
resistance
Rubus armeniacus
Probably the most commonly eaten blackberry, the highly invasive also known as the
Himalayan . A shrubby weed native to Eurasia, the plant aggressively forms dense thickets .
26.2 How Is Population Growth Regulated?
Environmental resistance limits population
growth
– Many populations that exhibit exponential growth
eventually stabilize
–As resources become depleted, reproduction
slows and the growth rate would eventually
drop to zero, causing the population size to
remain constant
26.2 How Is Population Growth Regulated?
Environmental resistance limits population growth (continued)
– This growth pattern, where populations increase to the maximum number sustainable by their environment and then stabilize, is called logistic population growth
– The maximum population size that can be sustained by an ecosystem for an extended time without damage to the ecosystem is called its carrying capacity (K)
– When logistic growth is plotted, it results in an S-shaped growth curve, or S-curve
The S-Curve of Logistic Population Growth
Fig. 26-6a
(a) An S-shaped growth curve stabilizes at carrying capacity
Growth stops and the
population stabilizes close
to the carrying capacity
Population
grows rapidly
Growth
rate slows
26.2 How Is Population Growth Regulated?
Environmental resistance limits population
growth (continued)
– If a population far exceeds the carrying capacity
of its environment, excess demands placed on
the ecosystem are likely to destroy crucial
resources
– This can permanently and severely reduce
carrying capacity, causing the population to
decline to a fraction of its former size or
disappear entirely
The S-Curve of Logistic Population Growth
Fig. 26-6b
(b) Consequences of exceeding carrying capacity
High damage; the
carrying capacity is
permanently lowered
Low damage; resources
recover, and the
population fluctuates
Extreme
damage; the
population
dies out
The population
overshoots the
carrying
capacity; the
environment
is damaged
26.2 How Is Population Growth Regulated?
Environmental resistance limits population
growth (continued)
– For example, when reindeer were introduced
onto an island with no large predators, their
population increased rapidly, seriously
overgrazing the vegetation they relied on for food
– As a result, the reindeer population plummeted
http://galen.metapath.org/popclk.html
exponential
growth population
crash
The Effects of Exceeding Carrying Capacity
Fig. 26-7
26.2 How Is Population Growth Regulated?
Environmental resistance limits population
growth (continued)
– Logistic population growth can occur in nature
when a species moves into a new habitat
–For example, new barnacle settlers along a
rocky coast may find ideal conditions that
allow their population to grow exponentially
–As population density increases, however,
individuals begin to compete for space,
energy, and nutrients
A Logistic Curve in Nature
Fig. 26-8 time (weeks)
nu
mb
er
of
ba
rna
cle
s (
pe
r c
m2)
life span
population density
off
sp
rin
g p
er
da
y
da
ys
Density-Dependent Environmental Resistance
Fig. 26-9
26.2 How Is Population Growth Regulated?
Environmental resistance limits population
growth (continued)
– As environmental resistance increases,
population growth slows and eventually stops
– In nature, conditions are never completely
stable, so both carrying capacity and the
population size will vary somewhat from year to
year
– However, environmental resistance ideally
maintains populations at or below the carrying
capacity of their environment
26.2 How Is Population Growth Regulated?
Environmental resistance can be classified into
two broad categories
– Density-independent factors, which limit
population size regardless of the population
density
– Density-dependent factors, which increase in
effectiveness as the population density increases
–Nutrients, energy, and space are all density-
dependent regulators of population size
26.2 How Is Population Growth Regulated?
Density-independent factors limit populations
regardless of their density
– The most important natural density-independent factors
are climate and weather, which are responsible for most
boom-and-bust population cycles
– For example, most insects and annual plant
populations are limited in size by the number of
individuals that can be produced before the first hard
freeze
– Hurricanes, droughts, floods, and fire can have
profound effects on local populations, particularly
small, short-lived species
26.2 How Is Population Growth Regulated?
Density-independent factors limit populations
regardless of their density (continued)
– Human activities can also limit the growth of
natural populations
–Pesticides and pollutants can cause drastic
declines in natural populations
–Overhunting has driven some species to
extinction
26.2 How Is Population Growth Regulated?
Density-dependent factors become more effective as
population density increases
– Populations of organisms with a life span of more than a
year have evolved adaptations that allow them survive
seasonal changes, such as the onset of winter
– Many mammals develop thick coats and store fat;
some hibernate
– Many birds migrate long distances to find food and a
hospitable climate
– Tree and bushes enter dormancy, dropping leaves
and slowing their metabolic activities
26.2 How Is Population Growth Regulated?
Important density-dependent factors limiting
population growth are:
– Predation
– Parasitism
– Competition
26.2 How Is Population Growth Regulated?
Predators exert density-dependent controls on populations
– Predators are organisms that eat other organisms, called their prey
– Predation becomes important as prey populations grow because predators eat a variety of prey, depending upon what is most abundant and easiest to find
– In this way, predators exert density-dependent population control over more than one prey population
Predators Help Control Prey Populations
Fig. 26-10a
26.2 How Is Population Growth Regulated?
Predator populations often grow as their prey
become more abundant
– The number offspring produced is determined by
the abundance of prey
–For example, snowy owls hatch up to 12
chicks when lemmings (their prey) are
abundant, but may not reproduce at all in
years when the lemming population has
crashed
Predators Help Control Prey Populations
Fig. 26-10b
26.2 How Is Population Growth Regulated?
An example of predator-prey population cycles are the fluctuations seen in laboratory populations of the bean weevil (prey) and its braconid wasp predator
– Wasps lay their eggs on the bean weevil larvae, which provides food for the newly hatched wasp larvae
– A large weevil population ensures a high survival rate for wasp offspring, increasing the predator population
– Under intense predation, the weevil population plummets, reducing food for the next generation of wasps
– Reduced wasp predation then allows the weevil population to increase
– The cycle thus begins again
Experimental Predator-Prey Cycles
Fig. 26-11
bean weevils (prey)
A high predator
population
reduces the prey
population
The prey population
peaks when the
predator population
is low
braconid wasp (predator)
26.2 How Is Population Growth Regulated?
Predation may maintain prey populations near a
density that can maintain carrying capacity, or at well
below carrying capacity
– For example, the prickly pear cactus (from South
America) was introduced into Australia in the late 1800s
and spread uncontrollably
– In the 1920s, cactus moths (predators of the prickly pear)
were introduced from Argentina to feed on the cacti
– Within a few years, the cacti were virtually eliminated
– Today, the predatory moth maintains its prey cacti at low
population densities, well below carrying capacity
26.2 How Is Population Growth Regulated?
Parasites spread more rapidly among dense populations
– Parasitism involves a parasite living on or in a host organism, harming it but not generally killing it because many parasites benefit by having their host remain alive
– Most parasites cannot travel long distances, so they spread more readily among hosts in dense populations
– Examples of parasites include the bacterium that causes Lyme disease, some fungi, intestinal worms, ticks, and some protists
26.2 How Is Population Growth Regulated?
Competition for resources helps control
populations
– Competition is defined as the interaction among
individuals who attempt to use the same limited
resource, and this interaction limits population
size in a density-dependent manner
26.2 How Is Population Growth Regulated?
There are two major forms of competition:
– Interspecific competition, between individuals
of different species
– Intraspecific competition, between individuals
of the same species
–Because the needs of members of the same
species for resources are almost identical,
intraspecific competition is an important
density-dependent mechanism of population
control
26.2 How Is Population Growth Regulated?
Organisms have evolved ways to deal with intraspecific
competition (continued)
– Many animals have evolved contest competitions,
where social or chemical interactions determine access
to important resources
– Territorial species—such as wolves, fish, rabbits, and
songbirds—defend areas that contain important
resources
– Only the best adapted individuals are able to defend
their territories, whereas those without territories may
not reproduce or may become easy prey
26.2 How Is Population Growth Regulated?
Organisms have evolved ways to deal with
intraspecific competition (continued)
– As population densities increase and competition
becomes more intense, some animals react by
emigrating
– Large numbers leave their homes to colonize
new areas; many die in the quest
–For example, locusts emigrate, consuming
vegetation in their path
Emigration
Fig. 26-13
26.2 How Is Population Growth Regulated?
Density-independent and density-dependent factors interact to regulate population size
–For example, a caribou weakened by hunger (density-dependent) and attacked by parasites (density-dependent) is more likely to be killed by an exceptionally cold winter (density-independent)
26.3 How Are Populations Distributed in Space and
Time?
Populations exhibit different spatial distributions
– There are three major types of spatial
distributions
–Clumped
–Uniform
–Random
Clumped Distribution
Fig. 26-14a
Uniform Distribution
Fig. 26-14b
Random Distribution
Fig. 26-14c
26.3 How Are Populations Distributed in Space and
Time?
Survivorship in populations follows three basic
patterns
– Late-loss populations
– Constant-loss populations
– Early-loss populations
26.3 How Are Populations Distributed in Space and
Time?
Survivorship in populations follows three basic
patterns (continued)
– Survivorship describes the pattern of survival in
a population
– Survivorship tables track groups of organisms
born at the same time throughout their life span,
and record how many continue to survive in each
succeeding year
26.3 How Are Populations Distributed in Space and
Time?
Survivorship in populations follows three basic
patterns (continued)
– A survivorship curve for a population can be
produced by graphing survivorship table data
–The Y-axis logs the number of individuals
surviving to a particular age out of an initial
population size born at a specific time
–The X-axis plots increasing age categories
after a specific birth date
Survivorship Tables and Survivorship Curves
Fig. 26-15
late loss
(human)
constant loss
(American robin)
early loss
(dandelion)
100,000 0 (birth)
99,124 10
98,713 20
97,754 30
96,489 40
93,698 50
87,967 60
76,241 70
54,117 80
22,312 90
2,523 100
Age Number
of
survivors
(a) A survivorship
table
(b) Survivorship curves
Fig. 45-10a, p.805
Fig. 45-10b, p.805
Fig. 45-10c, p.805
26.4 How Is the Human Population Changing?
Demographers track changes in human populations
– Demography is the study of the changing human
population
– Demographers measure human populations, track
population changes in different countries and regions,
and make comparisons between developed and
developing countries
– Demographic data are used to formulate policies in
public health, housing, education, employment,
immigration, and environmental protection
26.4 How Is the Human Population Changing?
The human population continues to grow rapidly
– In the last few centuries, the human population has grown at nearly an exponential rate following a J-shaped growth curve
– Over the last decade, however, the human population has been growing at a relatively constant rate, suggesting that it may not longer be growing exponentially
– However, Earth’s human population grows by 75 million people each year
– Are we entering the final bend of the S-shaped growth curve?
Human Population Growth
Fig. 26-16
Technical advances Agricultural advances Industrial and
medical
advances
Date
1804
1927
1960
1975
1987
1999
2012
Time to add
each billion
(years)
All of human
history
123
33
14
13
12
13
Billions
1
2
3
4
5
6
7*
*projected
1804
1975
1960
1927
1987
1999
2008
2012*
Bubonic
plague
year
26.4 How Is the Human Population Changing?
A series of advances have increased Earth’s
carrying capacity to support people
– Humans have manipulated the environment to
increase the Earth’s carrying capacity
– Several technological advances have greatly
influenced the human ability to make resources
available
–Technical advances
–Agricultural advances
– Industrial and medical advances
26.4 How Is the Human Population Changing?
Technical advances
– Early humans discovered fire, invented tools and
weapons, built shelters, and designed protective
clothing
–Tools and weapons allowed them to hunt
more effectively to increase their food supply
–Shelter and clothing increased the habitable
areas of the globe
26.4 How Is the Human Population Changing?
Agricultural advances
– Around 8000 B.C., animals and plants were
domesticated, providing a larger and more stable
food supply
– This resulted in a longer life span and more
childbearing years, although disease continued
to restrain population growth
26.5 How Is the Human Population Changing?
Industrial and medical advances
– Beginning in England in the mid-eighteenth
century, medical and industrial advances
permitted a population explosion
– Industrial advances allowed fewer people to
produce more food
–Medical advances decreased the death rate
from infectious disease
26.4 How Is the Human Population Changing?
The demographic transition explains trends in
population size
– In developed countries, people benefit from a relatively
high standard of living, with access to modern technology
and medical care, including contraception
– Examples include Australia, New Zealand, Japan, and
countries in North America and Europe
Most of Earth’s people, however, live in developing
countries, which lack these advantages
– Examples include countries in Central and South
America, Africa, and much of Asia
26.4 How Is the Human Population Changing?
The rate of population growth in countries that are now developed has changed over time in predictable stages, in a pattern called demographic transition – Pre-industrial stage: The population was relatively small
and stable, with high birth rates and high death rates
– Transitional stage: Food production increased and health care improved, which caused death rates to fall; because birth rates remained high, there was an explosive population increase
– Industrial stage: Birth rates fell as contraceptives were more available, and as people moved from farms to cities, where children were less important as a source of labor
– Post-industrial stage: Populations are relatively stable, with low birth and death rates
The Demographic Transition
Fig. 26-17
birth rate
death rate
population size
natural rate
of population
increase
Pre-industrial
Stage Transitional
Stage Industrial
Stage Post-industrial
Stage
Birth rate
declines
Population
grows rapidly
Death rate
declines
Population
remains low
Birth rate
remains high
Population
growth slows
Population
stabilizes
Birth and death
rates are low
Birth and death
rates are high
26.4 How Is the Human Population Changing?
The demographic transition explains trends in
population size (continued)
– A population will eventually stabilize if parents
have just the number of children to replace
themselves, called replacement-level fertility
(RLF)
–Because not all children survive to maturity,
RLF is 2.1 per woman
26.4 How Is the Human Population Changing?
World population growth is unevenly distributed
– Many developing countries still have rapidly
growing populations, as birth rates vastly exceed
death rates
–This results from low incomes and the need
for many children to raise family income or
produce food
– In these countries, knowledge of, and access
to, contraception are limited
26.4 How Is the Human Population Changing?
World population growth is unevenly distributed
(continued)
– In spite of the population reduction of some
developing countries, zero population growth will
not be achieved globally
–The U.N. predicts a global human population
of over 9 billion, and growing, by the year
2050
–7.9 billion of those people will live in
developing countries
http://galen.metapath.org/popclk.html
Historical and Projected World Population
Fig. 26-18
developing countries
developed countries
2009: 6.8 billion
26.4 How Is the Human Population Changing?
The current age structure of a population
predicts its future growth
– Age structure diagrams show the distribution of
human populations by age and gender
– Age structure can be shown graphically
–Age is shown on the vertical axis
–The number of individuals in each age group
is shown on the horizontal axis, with males
and females placed on opposite sides
26.4 How Is the Human Population Changing?
The current age structure of a population
predicts its future growth (continued)
– All age-structure diagrams peak at the maximum
life span, but the shape below the peak reveals if
the population follows one of three basic age-
structure patterns
–A growing population
–A stable population
–A shrinking population
26.4 How Is the Human Population Changing?
A growing population
– If adults of reproductive age (15 to 44 years) are
having more children than are needed to replace
themselves, the population is above RLF and is
expanding
–The age-structure diagram will be roughly
triangular
–An example of a country with a growing
population is Mexico
female male
Mexico 2009
(a) Population pyramid for Mexico
Age-Structure Diagram
Fig. 26-19a
26.4 How Is the Human Population Changing?
A stable population
– If adults of reproductive age have just the
number of children needed to replace
themselves, the population is at RLF and is
stable
–The age-structure diagram will have relatively
straight sides
–An example of a country with a stable
population is Sweden
female male
Sweden 2009
(b) Population pyramid for Sweden
Age-Structure Diagram
Fig. 26-19b
26.4 How Is the Human Population Changing?
A shrinking population
– If adults of reproductive age have fewer children
than are needed to replace themselves, the
population is below RLF and is shrinking
–The age-structure diagram will be narrow a
the base
–An example of a country with a shrinking
population is Italy
Age-Structure Diagram
Fig. 26-19c
female male
Italy 2009
(c) Population pyramid for Italy
26.4 How Is the Human Population Changing?
Average-age structure diagrams have been
plotted for developed and developing countries
for 2009, with predictions for 2050
– These diagrams reveal that even if developing
countries were to achieve RLF immediately, their
population increases would continue for decades
–A large population of children today create a
momentum for future growth as they enter
their reproductive years
26.4 How Is the Human Population Changing?
Fertility in Europe is below replacement level
– A comparison of growth rates for various world regions shows Europe as the only one with an average rate of change in population that is negative
–The average fertility rate is 1.5, which is substantially below RLF
–Concerns about the availability of future workers and taxpayers have prompted several countries to offer incentives for couples to have children at an earlier age
Europe: 0.0%
Latin America/Caribbean: 1.5%
Asia (excluding China): 1.5%
Developing countries average: 1.5%
Africa: 2.4%
N. America: 0.6%
World average: 1.2%
Developed
countries average: 0.2%
China: 0.5%
Population Change by World Regions
Fig. 26-21
26.4 How Is the Human Population Changing?
The U.S. population is growing rapidly
– With a population of over 307 million and a
growth rate of about 1% per year, the U.S.
population is the fastest growing of all developed
countries
–The U.S. fertility rate is about 2.0—actually
below RLF
–However, immigration is adding rapidly to the
population, because the fertility rate of new
immigrants is above RLF
0
25
50
75
100
125
150
175
200
225
250
275
300
325
1850 1950 year
1800 1900 2000
U.S
. p
op
ula
tio
n (
in m
illi
on
s)
U.S. Population Growth
Fig. 26-22
26.4 How Is the Human Population Changing?
Rapid population growth in the U.S. may have serious implications for the environment of the U.S. and the Earth
– Americans consume far more resources and produce far more pollution than the global average
– The spread of housing, commercial establishments, and energy-extracting enterprises degrades and destroys natural habitats, reducing the carrying capacity for non-human life of ecosystems in the United states
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