Download Factors Affecting Population Change

How did the introduction of zebra
mussels affect the health of the Illinois
River?
 What is a density-dependant factor?

› ___________ and ____________ recognized the
struggle for available resources within a
growing population would inherently
_____________ population size.
› Examples: ___________________,
_________________, _____________________.

What is intraspecific competition?
› As density increases, there is more
______________ among individuals.
› At carrying capacity, _____________
individuals will survive whereas the
______________ individuals will starve or risk
death by moving areas.
› Building roads?

Less resources?
› Decrease in individual’s growth/reproductive
success.
› Harp seals: reach sexual maturity at 87% body
weight  if not enough food, slower sexual
maturity  decrease fecundity.

Predation?
› Many prey go for larger populations  easier to
catch.
› Predators regulate population. How?
› How have caribou populations been disturbed
by humans?  need to share habitat with
wolves.

Disease?
› Why?  pathogens able to pass from host to
host with greater ease
› Overcrowding.

Low density?
› All plant and animal species suffer decrease
in per capita rate of increase as populations
reach small sizes or low densities.
ALLEE EFFECT.
Finding mates, social interactions requiring high density
Ex// passenger pigeon: hunted. Declined in numbers. 
could only lay one egg per nest  last one died in
1914.
Therefore, harder for pop. To recover with species with
low fecundity.

Small population
› Results in inbreeding and loss of genetic
variation.
› Minimum viable population size?
 Smallest number of individuals that ensures the
population can persist for a determined
interval of time.
 So pop can cope with variations in natality and
mortality, environmental changes and disasters.
 Used to figure out which species at risk.

What are density- independent factors?
› Changes in size not related to population
density

Human intervention, weather changes,
disasters.
› Ex// thrips: small insect: consumes many
different plant species that food supply is not
a limiting factor.
 Do no breed in cooler temperatures
 Regardless of population density

Increase of a substance throughout a
food-chain.
› One predator consumes 100 prey which
consume 10,000 blades of grass EACH which
was treated with DDT.

Prevent populations from achieving their
biotic potential.
› What was this again?
Biotic potential: the maximum rate a population
can increase under ideal conditions.
› Limiting Factor: any essential resource that is in
short supply or unavailable.
 Determines how much an individual or population
can grow.
 Ex// a plant requires nitrogen, CO2, and sunlight for
growth. If all other factors are available, but nitrogen is
used up, N is the limiting factor.

Too many births/immigration may result
in population overshooting its carrying
capacity.
› When pop. Surpasses carrying capacity of
habitat, d> b = population decrease.

All populations in a given ecosystem at a
given time
› Interact with each other directly or indirectly.
› Some organisms cannot exist independantly
of one another
 Ex// flowers/insects.
Ecological Niche: an organism’s biological characteristics,
including use of and interaction with abiotic and biotic
resources in its environment.
 Fundamental Niche:
› Biological characteristics of an organism and the set of resources
that individuals in the populations are theoretically capable of
using under ideal conditions.

However, organisms face interspecific competition with
other organisms which use the same resources.
› Interspecific competition: BETWEEN SPECIES.

Realized Niche:
› (ex// a lion may compete for food with vultures and hyenas)
› The biological charatceristics of the organism and the resources
individuals in a population actually use under the prevailing
environmental conditions.
Intraspecific: b/n individuals of same
species
 Interspecific: b/n individuals of different
species.

Symbiosis: various interactions in which two species
maintain a close, usually physical, association;
includes parasitism, mutualism, and commensalism.

Between individuals of different
populations.
› Restricts population growth.

Interference competition  actual fighting
over resources
› Tree swallows + bluebirds for birdhouse

Exploitive competition  consumption or
use of shared resources.
› Two predators rely on same prey (arctic foxes
and snowy owls both prey on same pop. Of
arctic hares)



1934
“The Struggle for Existence”
Two species with similar requirements could
not coexist in the same community.
› The more niches that overlap, the greater the
competition.
› One species consumes most of the resources,
reproduces more efficiently  inevitably drive
other species to extinction.
› CONCLUSION: if resources are limited, no two
species can remain in competition for the same
niche indefinetely.
› COMPETITIVE EXCLUSION

Resource Partitioning (Animals)
› Individuals of the same species avoid competition for similar
resource
› Ex// birds or lizards (of same species) occupy different
perching/nesting/eating sites.
› Reduces interspecific competition
 Increases likelihood of species surviving.

Resource Partitioning (Plants)
› Annual plants: foxtail, mallow, smartweed
› All require similar nutrients from the same habitat
› Each evolved unique ways to survive in the presence of each
other
 Foxtail: shallow, fibrous roots  absorbs water from surface of soil
 Mallow: deep taproot system  grow deeply down into
soil/obtain moisture later in growing season
 Smartweed: branches both in the topsoil and soil below the roots
of other plants: “best of both worlds”
Interspecific interaction  why?
 When prey population increases,
predator population can increase 
why?

› Reduction of prey results in a decline in
predator population... Unless it has access to
another food source.

Two species cycle slightly out of
synchronization
› Predator lags behind prey
Describe what is happening at Time:
A)
B)
C)
D)
E)

Prey have adapted unique ways to
detour predators
› Thorns, hooks, spines, needles, chemicals
(plants)
 Mustard family (pungent odour)
 Balsam fir (chemicals that mimic an insect
growth hormone  when linden bugs feed on
balsam fir, remain in juvenile stage and
eventually die)
 How would this help the balsam fir?

Monarch Butterfly
› Consumes toxins (not toxic to itself)  is distasteful
to its predators.
 Butterflies start out as caterpillars (ie. larvae)  feed
on milkweed  through metabolism milkweed
becomes unpalatable.
 Blue jays don’t ‘finish’ eating monarch butterflies.

Passive or Active
› Passive: hiding
 Camouflage (cryptic colouration)
 Bright colours: warn predators of chemical
defence mechanisms (poisons)
 Monarch butterfly
 Wasp (hover fly?)
 Lethal octopus, Haplochlaena maculosa: pattern
(rings) contract and expand as warning signal.
› Active: fleeting
 Alarm calls  to mob the predator
Different types
of defense
mechanisms
(Are the
animals
‘meaning’ to
do this?)

Mimicry
› Mimic resembles distasteful/harmful species
› Batesian
 Palatable/harmless species mimics
unpalatable/harmful organism
 Ex// Viceroy butterfly  monarch
› Mullerian
 Several unrelated species that resemble one
another and are all poisonous/dangerous.
 “pooling of numbers` causes predators to learn
more quickly to avoid these species.

Read pages 684-687 (Symbiosis &
Disruption of Community Equilibrium)
› Define/understand all of the definitions:
 Mutualism, commensalism... Macroparasites,
etc.
› Relate definitions to example species (in
text)

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