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Life
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Ecology: Population Growth & Regulation
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What is Ecology?
From the Greek meaning “a place to live”, it refers to the study of interrelationships between living things and their environment (including the nonliving things)
The environment includes:
An abiotic component – nonliving things such as soil, water, and weather and
A biotic component – all forms of life
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What is an Ecosystem?
All the organisms and their environment (nonliving) within a defined area
What is a Community?
All the interacting populations of organisms within an ecosystem
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How Does Population Size Change?
A population is the members of a single species, capable of interbreeding, that live in a specific area
The size of the population changes depending on the number of Births and Deaths, the number leaving (Emigration), and the number coming in (Immigration).
(B - D) + (I - E) = change in population size
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If life in the ecosystem is ideal, the population will increase according to its biotic potential, that is, its maximum rate
However, resources are limited and organisms interact with one another for these resources
Therefore, the population's size is limited according to …
environmental resistance
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The rate at which a population size changes, the rate of growth, is determined by: b - d = r (birth rate – death rate = growth rate)
Furthermore, the number of individuals that are new to a population within a certain time period is the growth rate (r) multiplied by the number of members in the population at the beginning of the time period (N): rN = population growth within a given time period
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If a population is growing at an ever-accelerating rate, then the population is experiencing exponential growth (typically a population will double during the same unit of time for EVERY increment of time)
Exponential growth is graphed as a J-curve
Figure 39-1
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Life
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(i) Reproduction begins at 4 years
(ii) Reproduction begins at 6 years
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What Influences Biotic Potential?The age at which the organism first
reproducesThe frequency with which reproduction
occursThe average # of offspring produced each
timeThe length of the organism’s reproductive
life spanThe death rate of individuals under ideal
conditions
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How Is Population Growth Regulated?
Any given area can support only a certain population size indefinitely
This size is the carrying capacity of the ecosystem. Population numbers that have reached carrying capacity can be graphed as an S-curve
Figure 39-5
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carrying capacity
time
exponentialgrowth: J-curve
(environmental resistance)
equilibrium(bioticpotential)
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Population numbers are affected by the course of nature in ways that may or may not be due to the size of the population
Populations that get too crowded or dense may be adversely affected by density dependent factors such as predation, parasitism, disease, or intense competition
On the other hand, density independent factors, such as weather, fire events, or human activities, impact a population regardless of its size
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When an animal kills and eats another organism, predation has occurred, and the animal doing the killing and eating is the predator
Predation is an important mechanism in natural population control
However, predation not only controls the size of the prey populations, but it also serves to control the size of the predator populations
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As predators reduce the number of prey available, they are, in effect, reducing their own food resource
This results in a reduction in the predator population
When predator numbers are reduced, the prey population will increase again
Thus, predator populations and prey populations undergo population cycles
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When an animal feeds on another organism without killing it, the animal is a parasite, and the organism on which it is feeding is its host
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When population numbers increase, competition for the resources on which the organisms depend becomes more intense
If the competition occurs among members of different species, interspecific competition is occurring
However, if the competition is among members of the same species, the more intense intraspecific competition is occurring
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How Is The Human Population Changing?
The human population grew slowly for over one million years
During that time, fire was discovered, tools and weapons were fashioned, shelters were built, and clothing was made to protect individuals
Each of these "inventions" led to a cultural revolution as the populations adapted to these innovations
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The domestication of crops and animals led to an agricultural revolution, providing a more dependable food supply
Once advances in medicine and health care occurred the human death rate was reduced dramatically
This industrial-medical revolution led to an increase in population
How is the Human Population Currently Growing? (Figure 39-10)
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Technical and cultural advances Agricultural advances Industrial andmedicaladvances
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20031999
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12,000 11,000 10,000 9000 8000 7000 6000 5000 4000 3000 2000 1000 B.C./A.D. 1000 2000 B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. A.D. A.D.
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In developed countries, the industrial-medical revolution also led …
to reduced birth rates, stabilizing their population growth
In developing countries, however, reduced birth rates …
have not occurred, primarily due to social traditions and a lack of access to education and contraceptives
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The United States is experiencing the most rapid growth among developed countries (Figure 39-13)
U.S. fertility rate is only 2.03, actually below Replacement Level Fertility (RLF) which is 2.1
Continued immigration to the United States is a significant source of this population growth
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U.S. population (in millions)(1790–2003)
year
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• This migration represents a redistribution of Earth's human population
– it may have serious implications for the environment of both the U.S. and the Earth, because …
– Americans consume far more resources and produce far more pollution than the global average
• One measure of this effect is the "ecological footprint“– It is more than four times greater for U.S. residents than the
global average
• There is compelling evidence that, because of our consumption of nonrenewable fossil fuels and depletion of groundwater reserves, humans have already exceeded the Earth's carrying capacity
• Furthermore, our overgrazing of grasslands, deforestation, etc., may actually be decreasing Earth's carrying capacity (Figure E39-3)
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Summary: Major PointsThis chapter has covered:1. Factors that control the size and rate of
growth of populations 2. How the environment plays a role in
controlling populations and how individual interactions among members of the same species, as well as among members of different species, influence population size
3. How a population grows may depend on how its members are distributed within a given area, or it may depend on the number of offspring that survive to reach maturity
4. These factors apply to the human population as well
