Factors Affecting
Population Change
Exponential Vs. Logistic Growth
Two Strategies for Growth
1. “r-strategists”: Spawners!
Characterized by exponential growth, which results
in temporarily large populations, followed by
sudden crashes in population size.
Ex. Insects, bacteria, some plants
live in unpredictable and rapidly changing environments
 Reproduce quickly when conditions are favorable
 External Fertilization
 Many offspring: small, mature rapidly, no parental care
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Two Strategies for Growth
2. “K-strategists”: Brooders!
Characterized by a high degree of specialization.
Ex. Trees, whales, tigers, etc.
Live in stable and predictable environments
 Can compete effectively
 Reproduce late in life
 Internal Fertilization
 Few offspring: large, mature slowly, often much parental
care

Density Dependent Factors
 Factors that influence population regulation,
having greater impact as population density
increases or decreases

Struggle for survival includes factors like competition,
predation, disease, and other biological effects
Density Dependent Factors
 Intraspecific
Competition –
Ecological interaction
where individuals of the
same species/population
compete for resources in
their habitat.
Density Dependent Factors
 Predation -
Consumption of prey
by carnivores
-Regulation can occur due to
preferred species of prey
Density Dependent Factors
 Disease –
Pathogens able to pass
from host to host in
overcrowded populations
with greater ease
Density Dependent Factors
 Allee effect –
Occurs when population
cannot survive or fails to
reproduce enough to
offset mortality once the
population density is too
low; such populations
usually do not survive.
Eg. The extinct
passenger pigeon.
Density Dependent Factors
 Minimum viable
population size –
Smallest number of
individual needed for
a population to
continue for a given
period of time

Small population size
can result in inbreeding
and loss of genetic
variation
Density Independent Factors
 Factors influencing population regulation
regardless of population density
Human intervention – Pesticides…Climate change
 Environment - reproductive success based on
temperatures, natural disasters etc

Limiting Factors
 Any essential resource
that is in short supply
or unavailable. These
factors determine how
much the individual or
population can
reproduce.

Eg. light, space, water,
nutrients
Think-Pair-Share
 Classify each of these as either
density-dependent or densityindependent. Share with a
classmate – do you agree or
diagree? Why? [I][C]

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A drought decreases the water level
of a lake, thus decreasing its
carrying capacity.
Lynx prey on snowshoe hares. An
increase in the snowshoe hare
population caused an increase in the
lynx population.
Due to the introduction of a new
species of fish, a lake becomes
crowded and some species do not
survive.
Increased water temperature causes
many aquatic organisms to die.
 Why might the birth and death
rates of a white-tailed deer
population in the Carolinian
forest of southern Ontario be a
density-dependent factor?