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Limiting Factors and the Law of Tolerance
– limiting factor - a single component/resource that limits
population growth
Examples:
– black bear - territorial space
– bald eagle - large nesting trees near water
– predator populations (carnivorous fish, mammals) – prey
availability
– when we can identify one or more limiting factors for a
population we can control the size of the population; we can
increase a game population (quail) or decrease pests (rock
dove, house sparrows, rats and mice)
– Shelford’s law of tolerance - a species tolerance of
ecological minimums and maximums with regard to various
conditions; conditions (some feature of the habitat) are
optimal for a species in certain areas, and density decreases
the further removed individuals are from optimal conditions
Examples:
– snow cover and cold temps limit how far north quail can live
– oxygen levels determine which fish species can live in an
aquatic system
Dispersion patterns and dispersal
– dispersion - spatial pattern or distribution pattern of
individuals in a population; can have random, clumped, and
uniform patterns
– dispersal - outward movements of individuals away from
their established areas of activity; some animals must leave
natal areas (place of birth) and establish new territories;
resources may run low if all new individuals remained in the
same location
Dispersion Patterns of Populations
Ordered = Uniform
– migration - 2-way movement to and from an area on a
regular basis (seasonal) or corresponding to a life history
change (salmon migrations)
2 types:
– emigration - migration out of an area
– immigration - into an area
– we can endanger less mobile species by erecting barriers
that restrict movement; dams for migrating fish (sturgeon,
salmon), clearcuts for amphibians
Zoogeographic regions - continents or portions that are
relatively isolated from one another in terms of dispersal of
animals
– we’re in nearctic
– exotics - organisms introduced from another region
– transplant - organism moved outside its native region but
within same zoogeographic region; both transplants and
exotics are introduced organisms
Examples:
exotic - brown tree snake introduced to Guam and other
Pacific islands
transplant – several fish species taken from SE U.S. to Pacific
Coast (crappie, bass, sunfish)
– native range - geographic area in which the species is
originally found
– naturalized organisms - introduced species that reproduce
in new area
Examples:
- rainbow trout from Pacific coast to eastern U.S.
- ring-necked pheasants from SE Asia to Midwest
- European house sparrow and starling
Brown Tree Snake
(Boiga irregularis)
Introduced to Guam in 1950’s and
had spread over island by 1970
Caused extinction in 12 native
species of birds and several others
are barely hanging on
Over 1200 power outages due to
the snake since 1978
Introduced or seen on 9 other
Islands including Hawaii; also
Texas
– feral animals - reverted to wild state after being
domesticated; horses and pigs in U.S.
– restored species – eliminated from portions of native range
and then successfully released on these vacated portions of
native range; California condor and wild turkey in U.S.
Population Dynamics - study of changes in numbers or
weight of organisms in populations
– affected by 3 functions: natality (birth/new individuals),
growth (biomass), and mortality (individuals die)
– these 3 functions influence population structure and
population structure is assessed by measuring population
size, age distribution, and sex ratio
– density-dependent and density-independent factors:
dependent - environmental factor that affects a population
based on its density
Example: elk overgraze and then compete for food; this
stresses the herd and fewer cows produce young, body size
decreases, mortality increases
independent - factor that acts independently of population
density
Example: a harsh winter with cold temps and deep snow
affects individuals and is independent of what happens to
others in the population
– natality - birth or hatching; # of young born or hatched
within a specified period of time
Examples: all fawns born to deer herd in a year; measured as
# of offspring/1000 female
for fish we use recruitment; measured as # of fish hatched
that survive to reproductive size
– growth - gain in body size
— birds and mammals have determinate growth, fish have
indeterminate growth (never really quit growing)
— determinate – most large mammals reach adult size in 1
or 2 years; smaller body size can occur at high densities but
mortality is more likely to be affected
— indeterminate - growth patterns variable; growth is density
dependent and could have high density, slow growing
population (stunted population); yellow perch can get up to
260 g after 4 yrs at low densities, but about 20 g after 4 yrs
at high densities (about 7.5% body size of low density perch)
– mortality - death; % of pop that dies in one year; mortality
cause can be density dependent (DD) or independent (DI)
— DI - winter blizzard kills individual pheasants
— DD - winter shelter can only protect a certain number of
birds so the rest are left without cover and die
– natural mortality - caused by predation, starvation,
diseases, accidents, etc.
– harvest mortality - caused by human activities such as
trapping, hunting, and fishing; really another type of
predation
– harvestable surplus - if a certain % of the population will
die due to natural mortality and DD factors, then harvest
mortality can be negligible to an extent
Example: bobwhite quail have about 70% natural mortality/yr
so if you harvest 35% of pop then 35% of pop will die of
natural mortality; the part harvested is called compensatory
mortality; if harvest goes above natural mortality then it is
called additive mortality
– additive mortality can be avoided by hunting regulations
(seasons, bag limits); we must know population parameters;
additive mortality is not always negative to have, white-tailed
deer need it now in the SE U.S. and regulations are
becoming more liberal (a doe and buck/day during season)