Transcript Populations

Populations
Ecology
2007
Population definitions
The individuals of a species with a given area
constitute a population.
The distribution of the ages of individuals in a
population and the way those individuals are
distributed over the environment describe
the population structure.
Ecologists study population structure at
different spatial scales, ranging from local
subpopulations to entire species.
The number of individuals of a species per
unit of area (or volume) is its population
density.
Population views
Ecologists study population structure at
different spatial scales, ranging from
local subpopulations to entire species.
The number of individuals of a species
per unit of area (or volume) is its
population density.
Population density
Ecologists are interested in population
densities because dense populations often
exert strong influences on their own members
as well as on populations of other species.
Density of terrestrial organisms is measured
as number of individuals per unit area.
Density of aquatic organisms is measured as
individuals per unit volume.
For some species such as plants, the biomass
or percentage of ground covered may be a
more useful measure of density than the
number of individuals.
Demography
The structure of a population changes
continually because of demographic
events—births, deaths, immigration, and
emigration.
Population dynamics is the change in
population density through time and
space.
– Demography is the study of birth, death,
and movement rates that give rise to
population dynamics.
Population numbers
Population dynamics can be represented by:
N1 = N 0 + B – D + I – E
– N1 = number of individuals at time 1
– N0 = number of individuals at time 0
– B = number of individuals born between time 0 and
time 1
– D = number of individuals that died between time 0
and time 1
– I = number of individuals that immigrated
– E = number of individuals that emigrated
Life Tables
Life table information can be used to predict
future trends in populations.
A cohort is a group of individuals that were
born at the same time.
A life table can be constructed by
determining the number of individuals in a
cohort that are still alive at specific times
(the survivorship) and the number of
offspring they produced in each time interval.
Life Table for Cactus Finch
Survivorship curves
Survivorship curves in many populations fall into one
of three patterns.
In some populations (e.g., humans in the U.S.), most
individuals survive for most of their potential life
span and die at about the same age.
In some (e.g., songbirds), the probability of surviving
over the life span is the same once individuals are a
few months old.
In species that produce a large number of offspring
and provide little parental care, high death rates for
the young are followed by high survival rates during
the middle of the life span.
Survivorship curves – man vs. bird
Mortality and survival
Baby Boomers and US statistics
Populations
The bay checkerspot butterfly provides an
example of the dynamics of a divided
population.
The larvae of this butterfly feed on only a
few species of annual plants in a small area of
California; the largest patch supports
thousands of butterflies.
During drought years, most plants die early in
the spring, and several subpopulations on small
patches become extinct.
The largest patch then disperses individuals
to recolonize the smaller patches.
Colonization
In experiments with springtails and mites, scientists
created isolated patches of the animals’ habitat.
The number of
species present
declined 40% (rarer
species declined
more than common
ones), showing that
small, isolated
populations are more
likely to become
extinct than larger
ones.
Chorus forgs
Predators may eliminate their prey in some
places but not in others.
In ponds on islands in Lake Superior, chorus
frogs are found in only some of the habitats
that seem suitable for them.
The tadpoles have three major predators:
salamander larvae, dragonfly nymphs, and
dytiscid beetles.
Experiments indicated that dragonfly nymphs
were able to eat all sizes of tadpole and when
these nymphs were present, the pond lacked
tadpoles.
Extinction- Elimination or loss of
a species
Loss of habitat
Disease( natural phenomenon)
Geothermal or Weather related
calamity( natural phenomenon)
Immigration of new species into space
encroaches on natural resources of
original species
Hunting
Pollution – Acid Rain – Global warming
Genetic impacts
Isolation – Original populations are cut off
from outlying groups
Allopatric speciation – As groups are
separated they diverge in their
characteristics
Founder effect- If the group separated from
the rest possesses deleterious effects –
they may occur in higher proportions in the
population and be inherited by successive
generations
The effect of predators on
populations of chorus frogs
Bacteria and cell culture
Bacteria grown in culture
Growth curve
Growth Curve dynamics
Lag phase – bacteria are inoculated into a new
growth media
Log phase – population doubles in a generation
time – in E. coli that can be 20 minutes. This
is also referred to as the exponential growth
phase or the logarithmic phase
Stationary phase – nutrients and oxygen exist
in lower quantities – waste products and
toxins build up
Death phase – cells are starving – they
undergo cytologic changes. Waste byproducts
become toxic – population crashes
Logarithmic
Best phase for studying characteristics
of the cells
Cell characteristics are uniform
All cells dividing at the same rate
Nutrients and oxygen enable cells to
perform all life processes
Carrying capacity
The maximum number of organisms of
one species an ecosystem can sustain
without increasing the mortality rate
Managing Populations
A general principle of population dynamics is
that the total number of births and the
growth rates of individuals tend to be highest
when a population is well below its carrying
capacity.
If we wish to maximize the number of
individuals that can be harvested from a
population, that population should be managed
so that its population is far below its carrying
capacity.
Hunting seasons are established with this
objective in mind.
Fish and population growth
Populations with high reproductive capacities
can sustain their growth despite a high rate
of harvest.
Fish are an example of a population with high
reproductive capacity.
Many fish populations can be harvested
heavily for many years because only a modest
number of females must survive to
reproductive age to produce the eggs needed
to maintain the population.
However, any species—even those with high
reproductive capacity—can be overharvested.
Survival
Muir Woods
West coast redwood trees dominate Muir
Woods' forest. Douglas-fir, big-leaf maple,
tanbark oak, and baylaurel grow along side the
redwoods. At the lower end of the canyon,
red alders line the stream and buckeyes
cluster nearby. Baylaurels growing toward the
light may assume contorted shapes or topple
over.
http://www.virtuar.com/marin/Muir/muir_wo
ods_virtual_tour.htm