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Chapter 52:Population Ecology
Population Ecology Study of the factors
that affect population size and composition.
Population Individuals of a single
species that occupy the same area.
Important Characteristics1. Density
Number of individuals per unit area or volume. Ex:
Diatoms - 5 million/m3
Trees - 5,000/km2
Deer - 4/km2
2. Dispersion Pattern of spacing among individuals. Types:
1. Clumped2. Uniform3. Random
Clumped Dispersion May result form a patchy environment. May increase chances for survival. Ex:
Schooling behavior Flocks of birds
Uniform Dispersion
Often the result of antagonistic interactions between individuals.
Ex: Territories Spacing between desert plants
Random Dispersion Often the result of the absence of strong
attractions or repulsions between individuals. Not a common pattern.
Demography The study of the vital statistics that affect
population size. Ex: Birth and Death rates Demographic Factors
Age structure of the population. Birth and Death rates. Generation time. Sex ratio and reproductive behavior.
Life Tables Mortality summary for a cohort of
individuals. First developed from life insurance studies.
Life Tables Show Mortality rate per year. Life span of the organism. Fecundity (birth rate).
Survivorship Curve Plot of the numbers of a cohort still alive over
time. Curve Types:
Type I Type II Type III
Survivorship Curves Type I
Low early deaths. High late deaths. Ex:
Humans Other large mammals
Type II Constant death rate. Ex:
Annual plants Many invertebrates
Type III High early deaths. Low late deaths. Ex:
Trees Oysters
Comment Curve type may change between young and
adults. Ex: Nestlings - Type III
Adult Birds- Type II
Life History Strategies1. "r" or Opportunistic species
2. "k" or Equilibrial species
"r" Species Increase fitness by producing as many offspring
as possible. Do this by:
Early maturation Many reproductive events Many offspring
Result Maximize reproduction so that at least a few offspring
survive to the next generation. Most offspring die (Type III curve).
"k" Species
Increase fitness by having most offspring survive. Do this by:
High parental care Late maturation Few reproduction events Few offspring.
Result Maximize survivorship of each offspring. Few offspring, but most survive (Type I curve).
What is the strategy For a weed? For an endangered species? For Garden Pests?
Population Growth N/t = b - d Where:
N= population size t = time b = birth rate d = death rate
Rate of Increase r = difference between birth rate and death
rate. r = b - d
Equation: N/t = rN N = population size t = time r = rate of increase
From Calculus
The equation N/t = rN becomes: dN/dt = rmax N
rmax = intrinsic rate of increase
Exponential Growth
dN/dt = rmax N Characteristic of "r" species. Produces a “J-shaped” growth curve. Only holds for ideal conditions and
unlimited resources.
Logistic Growth dN/dt = rmax N K-N
K K = carrying capacity
Result “S-shaped” growth curve. Characteristic of “k" species. Common when resources are limited.
Comment K is not a constant value. Populations often oscillate around “K” as the
environment changes.
Additional Comments Populations often overshoot “K”, then drop
back to or below “K”. AP Exam rarely asks you to work the
equations, but you should be able to give them.
Regulation of Population Size
1. Density- Dependent Factors Affect is related to N (pop. size). As N increases, mortality increases. Ex: Food, nesting space, disease
2. Density- Independent Factors Affect is not related to N. Mortality not related to population size. Ex: Weather and climate
Population Cycles Cyclic changes in N over time. Often seen in predator/prey cycles. Ex: Snowshoe Hare - Lynx
Causes Density dependent factors. Chemical cycles. Saturation strategy to confuse predators.
Age Structure Diagrams Show the percent of a population in different
age categories . Method to get data similar to a Life Table, but
at one point in time. Importances
Can be used to predict future population growth trends, especially for long lived species.
Exponential Growth Produces age structures that are a triangle or
pyramid shape. Logistic Growth
Produces age structures that have even sizes between most age categories.
Declining Populations Produce age structures with a narrow base and
wider middles.
Summary Know density and dispersion patterns. Know Life Tables and survivorship curves. Be able to contrast and compare “r” and
“k” strategies. Know exponential and logistic growth
curves and equations. Know density and density-independent
growth factors.