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Let’s Eat!!
U.S.G.S.
Trophic
levels
What do the First and Second Laws of
Thermodynamics Tell us?
•
•
•
•
First Law: Energy in = Energy Out
Until humans:
Energy =sit by fire, or in the sun
Sun: 30% reflectd, 50% converted to heat,
the rest goes to the water cycle, except
<1% used by plants
• Second Law: No process is 100% efficient
– Energy In = Work + Heat
What is Ecological Efficiency?
• Plants absorb how much sunlight?
– 1-3%
• Herbivores use how much of the plant energy?
– 10%
• Where does the rest go?
– Heat and respiration
• What is the efficiency of a carnivore?
– 10%
• Example: Humans
– 0.02 x 0.1 x0.1 = 0.0002, 2% of the solar energy that
passed through the plant, cow, human
Species interaction tactics
• Unique niches
• Competition--competitive exclusion by
specialization vs. extinction
• Specialization
• Symbiosis--commensalism, mutualism,
parasitism
• Predation
• ==population ecology
Population Ecology
1. Density and
Distribution
2. Growth
a. Exponential
b. Logistic
3. Life Histories
4. Population
Limiting Factors
5. Human
population growth
(Modified from a WWW site that I have lost the reference to!
Examples of applications
•
•
•
•
Invasive species
Endangered species
Pest control (e.g., agriculture)
Human population growth
Population. Individuals of same species occupying same general
area.
Density: the number of organisms in a unit area
Distribution: how the organisms are spaced in the area
Fig. 52.2
Changes in population size
Northern Pintail Duck
Growing
Fig. 52.9
Shrinking
Fig. 52.16
Fluctuating
Fig. 52.19
Questions
• Why do populations change in size?
• What factors determine rates of population
growth or decline?
• How do these differ among species?
2. Population Growth
a. exponential growth
The change in population size (N) in an interval of time is
number of births – number of deaths, or
???
If b (birth rate) is the number of offspring produced
over a period of time by an average individual, and d
(death rate) is the average number of deaths per
individual, then
????
Population Growth: exponential growth
The difference between the birth rate and the death
rate is the per capita growth rate
???
The growth equation can be rewritten as
???
Exponential growth occurs when resources are
unlimited and the population is small (doesn’t
happen often). The r is maximal (rmax) and it is
called the intrinsic rate of increase.
Population Growth: exponential growth
Note that:
1. r is constant, but N grows
faster as time goes on.
2. What happens with
different r’s in terms of
total numbers and time to
reach those numbers?
Fig. 52.8
r can also be negative (population
decreasing)
if r is zero, the population does not
change in size
thus, the rate of increase (or decrease)
of a population can change over time.
Exponential growth does not happen often:
Fig. 52.9 – Whooping crane
Or indefinitely:
Reindeer on the Pribalof Islands, Bering Sea
reindeer slide
Unlimited growth, carrying
capacity, and limited growth
Models of population growth
Nt+1 = NtR
Or
Nt = N0Rt
• This is the simplest model of
population growth for
species with discrete
breeding seasons.
• In this model, there is no
competition, and population
dynamics are governed
solely by the net
reproductive rate, R.
• If R > 1, the population
increases indefinitely and
exponentially.
Models of population growth:
incorporating competition
• Graphically, we can see that
the population increases
exponentially when Nt is very
low.
• But the rate of increase
declines as population size
rises.
• At carrying capacity, the
growth rate is zero.
• Above carrying capacity, the
population will decline.
• K is therefore a stable
equilibrium.
St. Matthew Island, Alaska
Reindeer on St. Matthew Island, Alaska
• In 1944, 29 reindeer
introduced to St. Matthew
Island (300 km2)
• Approximate initial
density 0.1/km2
• 24 females, 5 males, all 2
years old
Reindeer on St. Matthew Island, Alaska
• R. Rausch visited the
island in 1954, and on the
basis of counts,
estimated the population
size at 400-500.
• C.J. Rhode visited the
island in 1955, and
estimated the population
size at 700-900.
Reindeer on St. Matthew Island, Alaska
• David Klein visited the island in
1957, and made a total count
of 1,350 animals.
• This implies an average
annual growth rate of 34
percent.
• Klein assumed that the
population growth rate earlier
in the explosion must have
been near the theoretical
maximum for the species.
Reindeer on St. Matthew Island, Alaska
• Population growth
during this period
looks like unlimited
growth.
• Klein recognized the
potential importance
of this study during
his 1957 visit.
Natural mortality was assessed from
skeletons
Physical condition was assessed from
animals shot during fieldwork
Physical condition was assessed from
animals shot during fieldwork
Physical condition was assessed from
animals shot during fieldwork
At a density of 4.5 inds./km2, the animals
were in excellent condition
• Noticeable, extensive fat deposition,
especially on large males
• Weights of all reindeer collected exceeded
the average weight range for other Alaskan
reindeer
• No external parasites noted
• Very large and uniform antler growth on
males and females
•
•
•
•
•
What contributed to the unlimited growth
and excellent condition of reindeer on St.
Matthew Island?
Abundant winter and summer forage
No competitors
No large predators
No large herbivores had been there
previously
But Klein sensed there was trouble on the
horizon
Signs that limits to population growth were
imminent in 1957
• Lichen beds were showing signs of fracturing
due to overgrazing and trampling (winter range)
• Prostrate willows were also showing signs of
heavy browsing (summer range)
• Calf percentage of 26% was “below the
indicated level of previous years”
• Klein concluded/warned that “the population
decline may be rapid after the peak is reached”.
What happened next:
• Klein revisited the island in 1963 and surveyed it with 2 Coast
Guard helicopters.
• “As their boots hit the shore, they saw reindeer tracks, reindeer
droppings, bent-over willows, and reindeer after reindeer.” – Ned
Rozell, Alaska Science Forum
• The survey revealed the population had increased to 6000
• Calf percentage was lower than in 1957
• Recruitment was down from 29% in 1957 to 17% in 1963
• “There was ample evidence of overpopulation, and the stage
was apparently set for wholesale die-off.”
What happened next:
• May 1964: an aerial survey of the island
located no reindeer. “We were unaware,
of course, that a die-off had already taken
place.”
The introduction, increase, and crash of reindeer
on St. Matthew Island
Klein, D.R. 1968. J. Wildl. Manage. 32:350-367.
• Upon returning in 1966,
Klein found only 42
reindeer;
• Of these, 1 male; the rest
were females 2yrs old
and older
• No calves or yearlings,
indicating the crash dieoff probably occurred in
late winter 1964.
What caused the crash die-off?
• Extremely high density (20/km2)
• Unusually harsh winter in 1963-64 (exceptionally
cold, with unusually deep snow)
• Long bones of examined skeletons contained no
marrow fat, indicating starvation
• Many skeletal remains were found in groups,
suggesting the animals died over a very short period.
• By the mid 1980s, there were 0 reindeer on the
island.
Sex and age composition of the die-off
Compare natural mortality (1957) with crash die-off
(1966)
• Physical characteristics of
the animals in 1957 and
1963:
• Avg body weight declined by
38% for adult females and
by 43% for adult males
• Not only were they smaller
just before the crash,
regressions between body
weight and skeletal
parameters indicated growth
rates were lower in 1963
• Lichens had been
completely eliminated as a
significant component of the
winter diet
Carrying capacity
• Klein (1968) suggested that forage
quantity primarily governs population size,
while quality determines the size of the
individual.
• The winter component governs the upper
limit of the population, and the summer
component determines the stature of the
individual.
Klein (1968) attributed the large-scale
die-off to the following factors:
• Overgrazing of lichens, with no possibility of the reindeer
expanding into alternative range;
• Excessive density of reindeer competing for a very
restricted winter resource;
• Relatively poor condition of reindeer going into the winter
of 1963, resulting from intense competition;
• Extreme weather conditions, primarily deep snow, during
the winter of 1963-64.
Intraspecific competition and carrying
capacity
• Competition may be defined as (Begon et
al. 1984):
An interaction between individuals, brought about
by a shared requirement for a resource in limited
supply, and leading to a reduction in the
survivorship, growth, and/or reproduction of
the competing individuals.
Effects of competition on individuals
• Increased energy expenditure (searching
for the unexploited resource), increased
risk of mortality, and decreased rate of
food intake may all decrease individual’s
chances of survival
Effects of competition on individuals
• Increased energy expenditure and
decreased food intake may leave less
energy available for development and
less available for reproduction.
• Increases in density will therefore
decrease the contribution made by each
individual to the next generation.
Common features of intraspecific
competition
• The ultimate effect of competition is a decreased
contribution to the next generation;
– Intraspecific competition leads to decreased rates of
resource intake per individual, decreased rates of
individual growth or development, or to decreases in
the amounts of stored reserves;
– These may lead to decreases in survival and/or
fecundity.
– Evidence from St. Matthew Island?
Common features of intraspecific
competition
• The resource for which individuals compete
must be in limited supply
– Competing individuals might or might not interact
directly;
– Exploitation competition occurs when individuals
remove an item needed by others;
– Interference competition occurs when individuals
interact directly and prevent others from occupying a
portion of habitat and exploiting its resources;
– Which type presumably occurred on St. Matthew
Island?
Common features of intraspecific
competition
• The competing individuals are in essence
equivalent, but in practice they are not
– “One-sided reciprocity” or “Asymmetric competition”;
– The effects of competition are not the same on all
individuals in the population;
– Evidence of asymmetry on St. Matthew Island?
Common features of intraspecific
competition
• The likely effect of competition on any individual
is greater the more competitors there are.
– The effects of intraspecific competition are thus said
to be density dependent.
Logistic Growth Equation: incorporates changes in
growth rate as population size approaches carrying
capacity.
dN = rmaxN (K - N)
dt
K
Fig. 52.10
At what point is the “effective” r the highest?
At what point are the most individuals added to the population?
Are these the same?
Logistic Model
Fig. 52.12
Fits some populations well, but for many there is not stable
carrying capacity and populations fluctuate around some
long-tem average density.
3. Life Histories
• How do we figure out r for different
populations?
• What accounts for different patterns or
rates of population growth among different
species?
– For example, different rmax
How do we figure out r?
a. Life History Tables : follow a cohort from
birth until all are dead.
life history table
Reproduction Tables : follow a cohort from
birth until all are dead.
b. Life history strategies
Life histories are determined by traits that
determine when and how much an
organism reproduces and how well it
survives.
b. Life history strategies
i. reproduction
“big-bang” reproduction
very high reproductive rates
per event
Vs. reproduction for
consecutive years
fewer young produced per
event but often more
parental care
b. Life history strategies
ii. mortality
Survivorship curves
Fig. 52.3
There are often trade-offs between reproduction and survival.
Fig. 52.6 - European kestrel
Reproduction has a cost when energy is limiting.
Fig. 52.5 – Red deer in Scotland
K-selection
3.b. Life history strategies
iii. r- and K-selection
Near carrying capacity natural selection will favor traits that
maximize reproductive success with few resources (high densities).
Density-dependent selection.
r-selection
Below carrying capacity natural selection will favor traits that
maximize reproductive success in uncrowded environments (low
densities).
Density-independent selection.
Density-dependent
Any characteristic that varies according to a
change in population density.
food availability, territories, water, nutrients,
predators/parasites/disease, waste accumulation
Density-independent
Any characteristic that does not vary as
population density changes.
weather events, salinity, temperature
Density dependent: decreased fecundity
Space-limited
Fig. 52.14
Food-limited
Density dependent: decreased survivorship
Fig. 52.15
Density-dependent changes
in birth and death rates slow
population increase.
They represent an example
of negative feedback.
They can stabilize a
population near carrying
capacity.
Fig. 52.13
4. Factors that limit population growth
• Density dependent birth and death rates
(as we just discussed). Many of these
reflect
– competition for resource (food/energy,
nutrients, space/territories).
– predation, parasites, disease
– waste accumulation (e.g., ethanol)
4. Factors that limit population growth
• Density independent survivorship or
mortality
– Extreme weather events
– Fluctuations in wind and water currents
Interactions among population-limiting factors
The dynamics of a population result from the interaction
between biotic and abiotic factors, making natural populations
unstable.
Water temperature,
Competition,
Cannibalism.
Fig. 52.18
Population-Limiting Factors
Some populations have regular boom-and-bust cycles.
Fig. 52.19
Predation
Food shortage in
winter
Prey availability
SUMMARY
Population. Individuals same species occupying same general area.
Have geographic boundaries and population size.
Key characteristics
Density. Individuals per unit of area or volume.
Distribution: uniform, clumped, random.
Demography. Studies changes in population size.
Additions (+) : Births and Immigration.
Subtractions (-) : Deaths and emigration.
Life histories. Affect reproductive output and survival rate and
thus population growth.
Life history strategies are trade-offs between survival and
reproduction.
Population Growth
Exponential. J-shaped. Idealized, occurs in certain conditions.
Logistic. S-shaped. A little more realistic. Carrying capacity.
K-selection. Density-dependent selection.
r-selection. Density independent selection.
Population growth is slowed by changes in birth and death
rates with density.
Interaction of biotic and abiotic factors often results in unstable
population sizes. In some populations they result in regular
cycles.
5. Human population growth
6,417,531,489 people
(as of 9:30, Feb. 8, 2005)
Questions
1. 1. Human growth
For example,
-
What factors are correlated with changes in human
population growth rate?
– How long has Earth’s population been similar to
what it is now?
– Over what time period has the human population
shown the greatest change in numbers?
2. How do the patterns compare with what we
have just studied about natural patterns of
population growth?
3. What new questions does this raise for you?
Human Population= 6,339,110,260 (this morning)
Exponential growth since Industrial Revolution: better nutrition,
medical care and sanitation.
http://www.ibiblio.org/lunarbin/worldpop
Growth rates ( r )
1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%
Growth will slow
down either due to
decreased births or
increased deaths.
Likely both as
suggested by agestructure pyramids:
relative number of
individuals in each
age-class.
Fig. 52.20
Age-structure pyramids
Fig. 52.22
BELLINGHAM
CensusScope
When and how will human
population growth stop?
• This question is likely to be answered one
way or another in your lifetime.
• What is Earth’s carrying capacity for
human’s?
• Have we already exceeded K?
• What are consequences of human
population growth for other species on this
planet?
Human impact
• Depends on
– Total human population
– Consumption by each individual
– Ecological impact of each unit of consumption
• I = PAT
– P = population
– A = affluence
– T = technology
(Ehrlich and Ehrlich)
Unknown what
the carrying
capacity of Earth
for humans is. A
useful concept is
the ecological
footprint: land
needed to
produce
resources and
absorb wastes
for a given
country.
World Wildlife Fund for Nature
Fig. 52.23 – Ecological footprints for various countries and the world
SUMMARY
Human population has been growing exponentially for a
long time.
A reduction is expected either through lower birth rates or
higher death rates. The age-structure suggest different
scenarios for individual countries.
Humans appear to be above Earth’s carrying capacity.