Ecology - the study of the relations between organisms and their
environment (both biological and physical)
Population Ecology - the study of the factors that influence the
numbers and distribution of a species
population - a group of individuals of the same species living
in an area
Some species have
limited
distributions
Population size
has an important
influence on
species persistence
Populations often change distributions through time due to
climate change
Some species have increased their range due to chance
introductions
Population Dispersion
Random distributions result when
individuals of a species do not interact
with each other positively or
negatively - rare in nature
Uniform distributions result from
competition for resources or
behavioral interactions like
territoriality
Clumped distributions often result
from an uneven distribution of
required resources or social
interactions like herding
Metapopulations - distinct populations that interact with each other
through dispersal - allow greater overall numbers and persistent
refuges that can be a source of species recolonization of an area
after local extinction
With increasing human impact on
habitats and the range of many
species - the study of the
dynamics of metapopulations has
important implications for the
long-term conservation of some
species
Demography - the study of population parameters and statistics
Can help to predict population change
Present distribution of ages, distribution of sexes, survival rates,
birth rates all have effects on the characteristics of the population
in the future
Factors that influence population change
sex ratio - birth rate is most closely related to number of
females in the population
generation time - the average amount of time that it takes for
a female to give birth to another female - shorter generation
times can lead to faster population growth
age structure - the number of individuals in each age class number of individuals of reproductive age can influence
population growth
Populations maintain stable numbers when the number of births
in each time period is matched by the number of deaths
Survivorship - the proportion of newborn individuals that
survive to a given age - if 50% survive to age 10, then age 10
survivorship is 0.5
Survivorship curve - shows
changes in mortality rate
through the life of a group
of individuals
Type I - most mortality
late in life
Type II - uniform
mortality throughout life
Type III - most mortality
early in life
Reproduction has a price - reduced survival and/or reduced future
reproduction - “Cost of reproduction”
There is a trade-off between the number of offspring produced
and the investment in each offspring
Investment each offspring receives influences its chance of
survival - large clutches can result in few surviving offspring,
small clutches also have few survivors, intermediate size
clutches may produce the greatest number of survivors - the
“optimal clutch size”
Number of reproductive attempts per lifetime
Iteroparous - having many attempts at reproduction - requires
low expenditure each time in order to ensure parental survival
Semelparous - having a single reproduction - maximal
reproductive expenditure - “big bang” reproduction - seen where
chance of parental survival is low regardless of expenditure annual plants in the desert, Pacific salmon
Age at first reproduction is earlier in species that have low
survival rates - high survival rates often allow species to delay
reproduction to gain experience and resources
The rate of population growth and the size of a population is
limited by the environment
The potential growth of populations is large - but most populations
maintain relatively stable numbers
Two models of population growth
Exponential growth - the rate of change in population size is
proportional to the number of individuals in the population
and the intrinsic rate of increase
Logistic growth - the rate of change in the population is
negatively impacted by an increase in the number of
individuals in the population - density dependent growth
Exponential growth - the rate of change in population size is
proportional to the number of individuals in the population and
intrinsic rate of increase
dN/dt = rN
dN/dt - change in numbers per unit time
N - population size
r - intrinsic rate of increase
r = birth rate - death rate
Logistic growth - the rate of change in the population is negatively
impacted by the number of individuals in the population - density
dependent growth
dN/dt = rN(K-N)/K
dN/dt - change in numbers per unit time
N - population size
r - intrinsic rate of increase
r = birth rate - death rate
K - the carrying capacity
K - the number of
individuals that a given
area can support
indefinitely
when N=K, dN/dt = 0
Examples of Logistic Growth
Logistic growth is also called
“density dependent” growth
High density populations have
fewer resources available
which leads to higher mortality
and lower birth rates
high mortality and lower birth
rates reduce dN/dt as the
population approaches its
carrying capacity
The growth rate of many populations is influenced by
environmental factors that act independent of density - floods,
freezes, storms, droughts, and other disasters - environmental
controls of population size often results in population “crashes.”
The effect of unchecked population growth
Other factors can lead to population cycles
Prey numbers can vary because of predator density
Predator numbers can vary because of prey density
Populations that are routinely grow and decline due to density
independent factors and those that grow in a density dependent
fashion have very different adaptations that promote growth and
reproduction under those conditions
Populations that routinely grow in a density independent fashion
have adaptations that favor their ability to reproduce rapidly in
size before the next population disaster. Such populations are
called “r-selected” because they have characteristics that result
in a high instrinsic rate of increase.
Populations that routinely grow in a density dependent fashion
have adaptations that favor their ability to survive and reproduce
when the population is at its carrying capacity. Such populations
are called “K-selected” because they have characteristics that
allow them to live in situations where resources are limited - at
their carrying capacity.
Adaptations to density independent and density dependent growth
The human species exhibits many K-selected traits - likely due an
ancestry of species that lived at or near K - limited by food
availability, predators, and disease
Technological advances have
reduced death rates and increased
birth rates
r is currently about .014 - a small
number, but large N produces a
large dN/dt
Agriculture and technology have
increased K but recent estimates
place K at about 6,000,000,000 about the current population size
In many developing countries the rate of growth ( “r” ) is larger
today than 50 years ago even though birth rates have fallen,
because death rates have declined more
Most growth in the human population is occurring in developing
countries - and this trend is expected to increase
The age structure of the human population in developing
countries results in a greater proportion of the population of
child-bearing age, and a greater proportion that will soon reach
reproductive age
The population of the U.S. represents 4% of the total human
population - and we consume 25% of the world’s resources