Download Lecture 30

Population dynamics: eruptive and logistic
growth
The main difference
between J and S is the
pattern of “adjustment”
to system carrying
capacity
eruptive
(malthusian)
Eruptive
(fastest)
Logistic
(slowest)
#
logistic
time
1
Fates of eruptive “crashes”
2
#
3
1
time
Fig 3-21 (I&II)/3-22
(III&VI)
1. Ecosystem not
seriously impacted:
population recovers
and repeats the J-crash
sequence
2. Post-crash, population
comes into a lower Stype equilibrium
3. Ecosystem seriously
impacted by the
overshoot: population
is extirpated or persists
only at a very low level
2
Take-home message: populations are
never static!
In the language
of KR#8
n “overshoot” =
biotic potential
n “dieback” =
environmental
resistance
Each population in the system may be a different place in
the population cycle, while the carrying capacities of the
system for different species may also vary!
3
Are there differences between populations that
grow eruptively and those that grow logistically?
Eruptively growing populations (more J-curvish)
tend to be:
• Generalists with broader environmental niches
• Do well in unstable/rapidly fluctuating environments
• Tend to be short lived species that reach sexual
maturity early in life
• Produce numerous offspring
• Provide little parental care
• Populations predominately regulated
by extrinsic or (density independent)
factors
4
Logistically growing populations
(more S-curvish) tend to be:
• Specialists with narrower environmental niches
• Do well in stable environments, rarely tolerate
rapidly fluctuating abiotic situations
• Have long life spans reaching sexual maturity
relatively later in life
• Produce few offspring
• Provide extensive parental care
• Populations primarily regulated
by intrinsic or density dependent
factors
5
While the shape of the curve differs, growth of all
populations follows the same “rules”
r
dN/dt =rN(1-N/K)
r= intrinsic rate of increase
K = carrying capacity
6
From the basic model of population growth,
ecologists have taken the labels r and K for
these 2 patterns or “life history strategies”
n
n
Eruptive (generalist or density independent or
r-selected) strategies work for species that
can tolerate a broad range of environmental
conditions, move quickly into disturbed
environments, grow rapidly, mature early,
produce numerous young to which they
provide little parental care.
Logistic (specialist or density dependent or Kselected) strategies work for species adapted
to specialized, stable environmental
conditions, grow slowly, are late maturing,
have few off-spring for which they provide
extensive parental care.
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Why are there two “strategies” for population
regulation, r and K, or why do some species opt for the
eruptive pattern while others follow a logistic pattern?
Answer derives from the environmental conditions in a
particular habitat:
n In habitats with extremes of environmental conditions or
conditions that fluctuate rapidly, density independent
factors (which are generally abiotic) are the stronger
regulating factor
n Conditions do not remain the same for sufficiently long
periods of time to favour specializations – successful
species (those leaving the most offspring) are genetically
equipped to reproduce early, often
n In more stable habitats, density dependent factors which
are generally biotic are more important.
n “success” is favoured by traits that lead to specialization,
habitat partitioning, co-existence, symbiotic relationships
8
Time plays an important role
n
n
n
n
n
Density dependent factors act over successive generations
and long periods of time (evolutionary time)
We have a very poor understanding of factors controlling
the distribution of K strategists and hence poor predictive
power
However it is not surprising that increasing the disturbance
regime of an environment leads to losses of the more
specialized K-selected species in that environment or at a
minimum, reductions in their abundances.
With a particular species loss, symbionic partners may also
be lost
Those losses open up carrying capacity (light, nutrients,
water, space, etc.) for more r-selected (generalist) species
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Current patterns of anthropogenic land use, e.g.
forestry, agriculture, urbanization - not to mention
climatic warming - change environments:
• making them less “predictable”
• better suited to opportunistic r-strategists (rats,
cockroaches, flies, pigeons, starlings, dandelions, tree-ofheaven)
• less hospitable to K-strategists (oak and maple trees,
hawks and owls, butterflies and a host of species many
urban dwellers have never seen)
• We may think we’re “ok” but what are the implications for
ecosystems’ abilities to adapt to an unknown future?
• Humans appear to more K than r-selected – will our own actions
make our environment too unpredictable for human persistence?
• When might stress push ecosystems to new regions of attraction?
• What are the implications for production of the ecosystem services
we require to survive as a species?
homogenization of landscapes
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Ecological Succession
Often subtle, on-going changes that result in
transitions from one type of biological
community to another
The experimental approach:
Suppose you stopped
mowing your lawn or in this
case, a plowed field is
simply left to see what will
happen
http://natl.ifas.ufl.edu/oldfgall.html
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The nature of disturbance (and response):
Scales of space and time
1.
2.
3.
Totally unpredictable to species, rapid, rare,
short-lived: tsunami, earthquake, landslide
Uncommon but within the lifetimes of the
system’s longest lived species: ENSO, fire,
floods
Unpredictable but unremittingly constant or
permanent: climate change or other human
interventions such as agriculture/urbanization
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Mt. St. Helens
n
n
n
Mud slides and the
largest debris avalanche
in recorded history.
Blast waves leveled
trees 11 km from the
explosion.
Succession (primary?
secondary?)
13
n
The eruption occurred on an early
morning in May
• survivors included nocturnal animals
that were in their burrows,
• plants that had not yet broken winter
dormancy under the snow,
• migratory bird populations that had
not yet returned.
1984
Timing is everything!
“Gophers came up through the ash,
and succeeded in mixing the soil with
the ash layer, with seeds and parts of
plants."
n In areas with less than 25
centimeters of ash, gophers "were
really important to the recovery."
n 25 years post-eruption, scientists
have counted more than 150 species
of wildflowers, shrubs and trees, with
an average of ten new plant species
re-gaining a foothold every year.
n
- Virginia Dale (Oak Ridge National
Laboratory)
2004
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Fire
n
n
n
n
n
Before european contact, large fires burned
through a variety of NA montaine landscapes (like
Yellowstone) every 250 - 400 years.
Large fires also burned central NA prairie
grassland systems every 25 - 60 years and
southeastern pine systems perhaps every 100
years
Lightning starts an average of 22 fires each year.
80% of which go out by themselves.
Plants in these systems are adapted to fire.
Yet until the Yellowstone fires, park managers
believed they had to extinguish fires to preserve
park resources (Smokey the Bear and all that).
See the disturbance and resilience section
15
Over time, fire suppression
n
n
n
Led to the replacement of grasses by woody
shrubs (remember the Australian example from
KR # 4E?)
Forests were increasingly “cluttered” with woody
debris that became a haven for wood-boring
insects that then attacked living trees
“Valuable” commercial conifer species (e.g.
redwood) failed to regenerate as various species
of deciduous trees increasingly dominated the
landscape
16
Yellowstone
n
n
n
n
n
Yellowstone had implemented a natural fire
policy in 1972
From ‘72 to 1987, 235 fires were allowed to
burn 33,759 acres (13,354 ha) ~2250 acres.y-1
All fires were naturally extinguished
Public response was good, and the program
was considered a success.
1988 was radically different
17
n
n
n
n
n
n
1988 turned out to be the driest in the park’s
recorded history
By July 15, 8,500 acres had burned
Due to continued dry conditions, the decision
was made to suppress all fires – it didn’t work
Within a week, fires had encompassed nearly
99,000 acres
By the end of the month, the extant fuel load
and high winds made the fires uncontrollable
On the worst single day, Aug 20, winds
pushed the fire across 150,000+ acres.
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More than 25,000 firefighters, as many as
9000 at one time, worked the Yellowstone
fires at a total cost of about $120 million.
n Yellowstone’s fire management policy was the
topic of heated debate, from the bars in park
border towns to the US Congress.
n By September snow dampened the fires that
the US’s largest fire-fighting effort could not.
n Hence the joke:
n
• “How do you put out the Yellowstone fires?”
• “Pour on a hundred million $ and wait for snow”
19
Ecosystem wide . . .
36% of the park burned
n 345 elk (of an estimated 40,000-50,000), 36
deer, 12 moose, 6 black bears, and 9 bison
died (most were trapped as fire quickly swept
down two specific valleys)
n Some small fish-kills occurred as a result of
heated water and/or fire retardant in the
streams
n Less than 1% of soils were heated enough to
burn below-ground plant seeds and roots.
n
20
What we learned from Yellowstone:
n
n
n
n
n
n
n
Yellowstone’s grasslands returned to pre-fire appearance
within 8-10 years.
Forest ecosystems continue to recover.
The grasses elk feed on were more nutritious after the
fire – as evidenced by increased fecundity of elk.
Bears were seen grazing more frequently at burned vs.
unburned sites.
The fires had no observable impact on the number of
grizzly bears in greater Yellowstone.
Cavity-nesting birds, such as bluebirds, had more dead
trees for their nests; birds dependent on mature forests,
such as boreal owls, lost habitat.
No fire-related effects have been observed in the fish
populations or the angling experience in the six rivers
that have been monitored regularly since 1988.
21
Compare Yellowstone with the “stump
barrens” of Michigan
ƒ In late 19th and early 20th century
Michigan, selective logging for pine
and hemlock left enormous debris
piles in the what was left of the
forest
ƒ When fires inevitably hit, they were
extremely hot due to the
combustion of the resinous slash
ƒ They burned deeply destroying the
organic matter in the soil and
incinerating residual seeds
ƒ These areas have proved extremely
resistant to rehabilitation/restoration
for over 100 years
22
or the Sonoran Desert fires of 2007
n
n
n
n
n
n
n
Desert plants have evolved to deal with heat and
drought
Fire has never been sufficiently frequent or wideranging to force adaptation
So, unlike plants that evolved with fire, desert
plants have not developed thick bark or cork
layers to protect delicate tissue and are easily
killed
An invasive weed known as Sahara mustard has
moved in to desert areas formerly dominated by
creosote bushes
Mustard is not grazed by indigenous herbivores so
dead plant material is leading to high fuel loads . .
. . . and fire
Plant ecologists fear the formerly saguarodominated landscapes may morph into something
that more closely resembles savannah as native
plants with no defense to fire are killed
23
Consider flooding . . . and flood
dependent ecosystems
Furbish's Lousewort is an endangered
species.
n The only known place in the world that this
plant grows is the northern St. John River
Valley (Maine and New Brunswick)
n
n An endemic species restricted to just a few
locations on Earth is important as a unique
reservoirs of genes – Lousewort is called a
“Lazarus” species as it was thought to be
extinct
Lousewort does not compete well with other plants. It would likely
soon be crowded out by taller, more aggressive plants without the
spring floods along the river. Ice jams and subsequent flooding mean
shards of ice scour the shore, removing most of the vegetation on the
shore each spring – resetting early successional conditions
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Interrupting flooding with dams
ƒ Sediment build-up behind the
dam changes downstream
morphology
ƒ Changes in stream
temperatures
ƒ Losses of fisheries
ƒ Failure of downstream riparian
vegetation to regenerate
The Dickey-Lincoln dam, a $227 million
hydroelectric project proposed for the
upper St. John River in 1974 was blocked
by the U.S .Congress in 1986 after years of
study, because the dam would have
flooded 360 km2 of Maine forest and
severely reduced the habitat for Furbish’s
Lousewort (only “re-discovered” during the
environmental assessment for said dam)
The Eau Claire River as it enters
Lake Altoona, a reservoir in
Wisconsin
The 70 m high Matilija dam in Ventura Co
Calif has virtually completely filled with
sediment over its 62 year “life”
n
n
The capacity of the
Matilija Reservoir
has been reduced by
over 90%
Estimates are
somewhere around
300 million $ to
remove the dam and
associated
sediments
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What we “think” we know on disturbance
1.
Natural environmental variability (disturbance) is
important in structuring systems
ƒ The more frequent the disturbance, the more rselected species we expect to find (compare the plant
communities of prairie ecosystems that experience fire
on a decadal scale with those for forests that burn on a
scale of centuries)
2.
3.
As disturbance becomes more infrequent,
biological interactions and K-selected species
become the more significant element structuring
ecosystems
There are more types of biological interactions
than we have covered so far – what do we know
about how they operate?
Our topic for Thursday!
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