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Unit D: Population and Community Dynamics
Chapter 20: Population Growth and Interactions
Section 20.1 – Population Growth
A. Density and Distribution of Populations
a. All populations can be described in terms of two fundamental characteristics: density
and distribution.
i. _______________________________ (DP) is defined as the number of
individual organisms (N) in a given area (A) or volume (V).
𝐷𝑝 =
𝑁
𝑁
𝑜𝑟 𝐷𝑃 =
𝐴
𝑉
ii. There are three theoretical distribution patterns for populations that could
affect the accuracy of population density estimates – uniform,
_________________________ and clumped.
B. Factors That Affect Distribution Patterns
a. Distribution patterns are influenced by distribution of resources in a habitat and the
interactions among members of a population or of a community.
i. Example: Random distribution would occur when resources are abundant and
population members do not have to compete with one another. Generally,
random distribution in nature is ____________________.
ii. Example: Clumped distribution occurs when populations are found in close
proximity to each other in various groups within their habitat due to the
proximity of food, water, or shelter. Most populations, including humans,
exhibit a clumped pattern of distribution.
iii. Example: Uniform distribution often occurs in artificial populations such as
plants in orchards or agricultural fields as well as some species that behave
__________________________ to defend resources and protect their young.
b. Most populations do not perfectly fit any one pattern of distribution and often
represent different distribution patterns at different times.
C. Population Growth
a. A population’s size directly depends on how much and how fast it grows. There are four
processes that can change the size (number of individuals) in a population (ΔN):
i. Births (b) and immigration (i) cause _____________________ in population size
ii. Deaths (d) and emigration (e) cause ____________________ in population size.
b. The change in population size can be calculated with the following equation:
∆𝑁 = [𝑏 + 𝑖] − [𝑑 + 𝑒]
c. The Rate of Population Growth
i. Just as important as the change in the size of a population is the speed at which
the change occurs.
1. An increase in the size of a population that spreads before it can be
contained is called a ____________________________________.
2. A decrease that occurs very rapidly is referred to as a population crash.
ii. The change in the number of individuals in a population (ΔN) over a specific
time frame (Δt) is known as a population’s growth grate (gr):
∆𝑁
𝑔𝑟 =
∆𝑡
iii. To compare populations of the same species that are different sizes or live in
different habitats, the change in population size can be expressed as the rate of
change per individual. This measurement, the __________________________
__________________ (cgr), can be determined by calculating the change in the
number of individuals (ΔN) relative to N (the original number of individuals), and
then dividing this change in the number of individuals by the original number in
the population (N):
𝑁𝑓𝑖𝑛𝑎𝑙 − 𝑁
∆𝑁
𝑐𝑔𝑟 =
𝑜𝑟 𝑐𝑔𝑟 =
𝑁
𝑁
D. Factors That Affect Population Growth
a. Both biotic and abiotic factors limit the growth of a population.
i. Biotic Potential
1. Each species has an intrinsic rate of growth that is possible given
unlimited resources and ideal living conditions. The highest possible per
capita growth rate for a population is called its _________________ (r).
Factors that determine this are:
a. The number of offspring per reproductive cycle
b. The number of offspring that survive long enough to reproduce
c. The age of reproductive maturity and number of times
individuals reproduce in a life span
d. The life span of individuals
2. A population that is growing at its biotic potential would be expected to
grow exponentially. The brief lag phase, followed by a steep increase in
the growth curve – called an __________________________________
___________________ – can be described as a J-shaped curve.
ii. Carrying Capacity
1. The initial stage is called the lag phase since growth is small because
there are only a few individuals to reproduce.
2. The birth rate during the exponential growth phase is much greater
than the death rate, so population size will increase rapidly.
3. This rapid growth rate cannot continue indefinitely. Eventually,
competition for resources and other limiting factors will slow the rate of
growth until it reaches the _____________________________, when
the birth rate and death rate are equivalent.
4. This pattern of population growth is illustrated as an S-shaped
(sigmoidal) curve known as the logistic growth pattern. The population
growth levels off into the habitat’s carrying capacity (K) – the theoretical
maximum population size that the environment can sustain over an
extended period of time.
5. This carrying capacity may change from season to season or from year
to year but overall the population size will fluctuate around the carrying
capacity in a stable ___________________________.
6. Factors that limit a habitat’s carrying capacity can be categorized as
density-dependent or density-independent:
a. Density-dependent factors are biotic and include things like
parasites and diseases that spread more easily in dense
populations as well as predation.
b. Density-independent factors are abiotic and include harsh
weather, drought, floods, forest fires, etc.
7. The combined effects of various, interacting limiting factors is described
as the ___________________________________ to population growth.
iii. Life Strategies
1. Bacteria and other species that reproduce close to their biotic potential,
(r), represent one end of the life strategies continuum. They are said to
have _______________________________________ which include
short life spans, early reproductive ages, large broods of offspring that
receive little or no parental care. They take advantage of favourable
environmental conditions to reproduce quickly.
2. The other end of the continuum are populations that live close to the
carrying capacity (K) of their habitats so they are described as having Kselected strategies. They have few offspring per reproductive cycle, one
or both parents care for the offspring when young, take a relatively long
time to mature and reach reproductive age, and live a relatively long
time.
3. Recognizing the various K-selected and r-selected strategies of a species
can be used to predict the success of a population in a particular
habitat.