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Environmental Science
LECTURE NOTES FOR CHAPTER 8
UNDERSTANDING POPULATIONS
I. Population – all the member of a species living in the same place at the same time
A. Are a reproductive group because they breed with member of their own population
B. Also includes the number of the individuals it contains
C. Properties of a population – can be used to describe populations and to predict
changes within them
1. size
2. density – the number of individuals per unit area or volume
3. dispersion – the relative distribution or arrangement of its individuals within a
given amount of space
a. even
b. clumped
c. random
II. Population growth
A. determined by the number of births less the number of deaths
B. growth rate – a change in the size of a population over a given period of time
1. the birth rate minus the death rate
2. changes occur when there is a change in either birth rate or death rate
3. can be positive, negative, or zero
a. will be zero when the average number of births equals the average number
of deaths
b. will remain the same if each pair of adults produce exactly two offspring
and each of those offspring survive to reproduce
c. If adults are not replaced by new births, the growth rate will be negative and
the population will shrink
4. populations usually stay close to the same size because of various factors that
kill many individuals before they can reproduce
5. biotic potential – the fastest rate at which a species population can grow
6. reproductive potential – when the biotic potential is limited by the maximum
number of offspring that each member of the population can produce
a. some species have much higher reproductive potentials than others
b. Increases in reproductive potential caused by:
1) When individuals produce more offspring at a time
2) When individuals reproduce more often
3) When individuals reproduce earlier in life
a) has the greatest effect of the reproductive potential
b) shortens the generation time – the average time it takes a member
of the population to reach the age when it reproduces
c) small organisms have short generation times
d) large organisms have lower reproductive potentials
7. exponential growth – when populations grow faster and faster
a. occurs in nature only when populations have plenty of food, space and have
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no competition or predators
C. Limits to population growth – populations cannot grow forever nor grow at their
reproductive potential
1. restricted by:
a. natural conditions are neither ideal nor constant
b. resources are used up
c. the environment changes
d. deaths increase
e. births decrease
f. natural selection allows only some members to survive and reproduce
2. carrying capacity- the maximum population that the ecosystem can support
indefinitely
a. population may increase beyond this number but it cannot stay at the
increased size
b. difficult to predict exactly because ecosystems change
c. may be estimated by looking at average population sizes or by observing a
population crash after a certain size has been exceeded
3. limiting resource – a natural resource that a species consumes at the same rate
as the rate that the ecosystem produces the resource
4. Competition
a. members of a population use the same resources in the same ways
1) members will compete as the population approaches its carrying capacity
b. territory – an area defended by one or more individuals against other
individuals
1) species compete indirectly for social dominance or for a territory
2) valuable for space, shelter, food, or breeding sites
c. many organisms expend large amounts of time and energy competing for
mates, food, or homes
5. Two types of population regulation; causes of death may be one of two types
a. density dependent
1) deaths occur more quickly in a crowded population than in a sparse
population
2) happens when individuals of a population are densely packed together
b. density independent
1) certain proportion of a population dies regardless of the population’s
density
2) affects all populations in a general or uniform way
3) severe weather and natural disasters of examples of this type
III. Niche – the unique role of a species within an ecosystem
A. Includes physical home, the environmental factors for survival, and interactions
with other organisms
1. organism’s pattern of use of its habitat
B. Habitat – an organism’s location
IV. Species interactions
A. categorized at the level where one population interacts with another
B. based on whether each species causes benefit or harm to the other
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C.
D.
E.
F.
1. benefit or harm is in terms of total effects over time
2. other types of interactions are possible
Some interactions are indirect
Some interactions do not fit a category clearly
Some interactions are rarely found
1. may not be categorized or well studied
Types of species interactions
1. competition – a relationship in which different individuals or populations
attempt to use the same limited resource
a. each individual has less access to the resource
b. individual is harmed by the competition
c. may occur within and between species
d. niche overlap – when members of different species compete
e. indirect competition- when species compete although they ever come into
direct contact with each other
f. adaptations to competition
1) niche restriction – when each species uses less of the niche than
they are capable of using
2. Predation
a. predator – an organism that feeds on another organism
b. prey – the organism that is fed upon
c. most organisms have evolved some mechanisms to avoid or defend against
predators
3. Parasitism
a. parasite – an organism that lives in or on another organisms and feeds on
the other organism
b. host – the organism the parasite takes its nourishment from
c. parasites do not usually kill the host
4. Mutualism – a close relationship between two species in which each species
provides a benefit to the other
5. Commensalism – a relationship in which one species benefits and the other
species is neither harmed nor helped
6. symbiosis – a relationship in which two organisms live in close association
a. usually describes a relationship in which at least one species benefits
b. coevolve – when two species in close relationships adapt over time to either
improve the benefits or reduce the harm
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