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Population Dynamics
Ch. 35
Measuring Populations
• Population density = # of individuals of a
particular species per unit area
Measuring Populations, cont.
• Sampling – used to estimate size of
– Quadrats – count all organisms in a block and
use this to estimate population size
– Indirect counting – count nests, burrows, etc
instead of organisms
– Mark recapture – trap animals, mark them,
release, recapture and count marked vs.
Limiting population growth
• Limiting factor – a condition that can limit
population growth
– Ex: space, food, disease
• Carrying capacity – the number of
organisms in a population that the
environment can maintain
– Birth rate and death rate are about equal
Limiting population growth
• Density-dependent factors – a factor that
limits population growth more as the
population density increases
– Ex: food, disease that spreads by contact
• Density-independent factors – a factor
that limits population and is unrelated to
population density
– Ex: extreme weather
Types of population growth
• Exponential – population multiplies by a
constant factor at constant time intervals
– there are few factors that stop overall growth
– Also known as J-curve
Types of population growth
• S-curve – population
begins growing
exponentially, but
environmental factors
begin to limit growth;
population stops
growing or may begin
to decrease
Types of population growth
• Boom and bust – increase rapidly (boom)
and then decrease rapidly (bust)
Population interactions
• Interspecific competition – two or more
species rely on the same limited resource
– Competitive exclusion – when one species
succeeds over the other due to limited
– Niche – each organism’s unique living
arrangement, such as habitat, food, time
when active, etc
Population interactions
• Predation – an interaction in which one
organism eats another
– Predator – doing the eating
– Prey – being eaten
– Both predator and prey have evolved with
adaptations to enhance survival
• Ex: camouflage, hunting in packs, warning
Prey adaptations
Population interactions
• Symbiotic relationships – a close
relationship between species in which one
of the species lives in or on the other
– Parasitism – the parasite obtains food at the
expense of the other organism (host)
– Mutualism – both organisms benefit
– Commensalism – one organisms benefits and
the other is neither helped nor harmed
Parasitism – blowfly larvae kill purple martin chick
Mutualism – sea anemone protects
clownfish from predators; clownfish is
territorial and protects anemone from
predators and shares food
Commensalism – grey whale carries
barnacles; whale not harmed nor helped,
barnacles have more access to food
Disturbances in communities
Ecological succession – community change
when new species colonize disturbed area
– Primary succession – community arises in
lifeless area without soil
• Ex: volcano creating new island
– Secondary succession – community arises in
disturbed area with soil
• Ex: growth in plowed farm field
Primary succession – growth on lava rock
Secondary succession after forest fire
Disturbances in communities
• Pioneer species – predominant species in
the early stages of succession; tend to be
small, fast-growing, and fast-producing
• Succession proceeds through many stages
until it reaches a stable end point called
the climax community
Pioneer species – can grown on rock and will
help form soil