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Ecology – Population Growth and Regulation
Ecology – the study of interrelationships
among living things and their nonliving
environment
Environment – 2 components:
•Abiotic – nonliving (soil, water, weather,
pH)
•Biotic – living (all forms of life)
Levels of Organization:
• Population – all members of a particular
species who live within an ecosystem, interact
with one another and can potentially interbreed
• Community – all the interacting populations in
an ecosystem
• Ecosystem – all the organisms and their
nonliving environment within a defined area
(prairie, forest)
• Biosphere – the part of the earth inhabited by
living organisms; includes both living and
nonliving components
Populations
• Population density – number of
individuals/unit area
• Measuring density
– In rare cases, population size and density can
be determined by counting the actual number
of organisms
– Usually done by random sampling
– Common method used is the mark-recapture
method (read in textbook)
•
Population distribution – the spatial pattern in
which members of a population are dispersed
within a given area
1. Clumped – members of a population live in groups
(herds, flocks, schools)


Advantage includes more individuals to find food, large groups
confuse predators
May clump due to resource availability (cotton trees cluster
along streams, animals around water holes)
2. Uniform – organisms maintain a relatively constant
distance between individuals


Common in animals that are territorial
Helps ensure adequate resources for each individual
3. Random – patternless and unpredictable, occurs in the
absence of strong attractions or repulsions among
individuals, resources equally available (more rare) ex.
Trees and plants in rainforest
Dispersion Patterns
Clumped
dispersion
Uniform
dispersion
Random
dispersion
Clumped Dispersion
Uniform Dispersion
Random Dispersion
• Population growth – determined by three factors:
births, deaths, and migration (immigration –
migration into a population; emigration – migration
out of a population)
• (births – deaths) + (immigrants – emigrants) = change in
population size
• Ultimate size of a population results from a balance
between opposing factors:
 Biotic potential – maximum rate at which the population
could increase, assuming ideal conditions that allow a
maximum birth rate and minimum death rate
 Environmental resistance – limits set by the environment
such as availability of food, space, competition, predation,
and parasitism
Biotic Potential
• Growth rate (r) – measure of the change in
population size per individual per unit of time
b
d
=
r
(birth rate)
(death rate)
(growth rate)
• To determine number of individuals added
to a population in a given time period,
growth rate (r) is multiplied by original
population size (N)
population growth = rN
Biotic Potential
• Exponential growth – pattern of continuously accelerating
increase in population size – forms a J-curve
– Example: bacteria under ideal lab conditions could produce enough
bacteria to form a foot deep layer over the entire Earth starting with
just one cell dividing every 20 min.
•
Biotic potential is influenced by:
1. Age at which the organism first reproduces
2. Frequency with which reproduction occurs
3. Average number of offspring produced each
time
4. Length of organism’s reproductive life span
5. Death rate of individuals under ideal
conditions
• Biotic potential helps ensure that at least one
offspring survives to bear it’s own young
• In nature, exponential growth occurs only for a
limited time
Regulation of Population Growth
• Population size is limited by environmental resistance (such
as available resources)
• Boom-and-Bust cycles – populations characterized by rapid
growth followed by a sudden massive die-off
ex. Algae, insects –
have seasonal cycles
linked to rainfall,
temperature, nutrient
availability
Insects grow during
spring and summer and
die with the frost
• S-curves – after a period of growth, populations tend to stabilize
at or below the max number the environment can sustain
– Reaches a state of equilibrium with growth rate of zero
– Carrying capacity – max population size that an ecosystem
can support indefinitely
– Typical of long-lived organisms
Limiting Factors – 2 Major Types
1. Density independent – limit size of
population regardless of population density
• Weather (causes boom and bust cycles),
human activity (pesticides, pollution,
habitat destruction), natural catastrophes
(flood, fire, hurricane)
2.
Density dependent – limit size of population as population
density increases
• Major limiting factor for long-lived species
• Examples include:
 predation and parasitism – control prey/host
populations
 Competition for limited resources
• Intraspecific competition – among individuals of
the same species (very intense, impt in natural
selection)
• Interspecific competition – among individuals of
different species
• Usually, if two species niches overlap too
much, will lead to Competitive Exclusion
 Overcrowding may lead to physiological and
behavioral changes that increase emigration
Competitive Exclusion Principle
• Proposed by G.F. Gause (Gause’s Principle)
• If two species niches overlap too much, one species will be
better at getting the resources and the other will be
eliminated
Mortality and Survivorship Curves
• Populations show characteristics patterns of deaths or
survivorship over time
1. Late Loss – show a convex shaped curve, relatively low
infant death rates, most individuals survive to old age
Red curve
Ex. Humans,
large animals
Mortality and Survivorship Curves
2. Constant Loss – individuals have a fairly constant
death rate, have an equal chance of dying at any
time during their life span
Black line
Ex. annual
plants, hydra,
some
invertebrates,
some rodents
Mortality and Survivorship Curves
3. Early Loss – show a concave curve, produce large numbers
of offspring that receive little parental care, death rate is very
high among offspring, those that become adults have a good
chance of surviving to old age
Blue curve
Ex. Most
invertebrates,
plants, many
fish
Life History
• Traits that affect an organism’s schedule of
reproduction and death
• K- selected populations (species or
strategists) and r-selected populations
(species or strategists) represent
hypothetical models
r-selected populations
• Usually do not reach carrying capacity
• Tend to live in unstable, temporary
environments
• Generally limited by density independent
factors
• Produce large numbers of offspring
• Early loss survivorship curve
K-selected populations
•
•
•
•
•
Usually have slow or no population growth
Usually at or near carrying capacity
Live in stable predictable environments
Produce few young and provide parental care
Competition for limited resources limits
population size
• Late loss survivorship curve