Why don't elephants take over the world

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Why don't elephants take over the
world? Stability in nature and its causes
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If all their offspring survived, any species
would soon take over the world Even the
slowest reproducing ones like elephants.
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If elephants managed to double their numbers
each generation...
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The stability of nature, despite the capacity of
species to increase, wasnoticed from the
earliest days of ecology
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Gilbert White's classic book (the first book on
ecology?)
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The village of Selborne, Hampshire, England
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Gilbert White noted the stability of swallow
populations in Selborne
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(No Transcript)
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In the 1970s, ecologists returned and found
hardly any change about 12 pairs from
year-to-year nowadays
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Heron populations in England 1928-1970 Stability
despite some knocks
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Despite some knocks, populationkeeps coming back
up towards a stable level
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What enforces this stability (so a species
neither takes over the world, nor crashes down to
extinction)??
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What prevents a species from taking over the
world is that most offspring die young, and/or
dont reproduce
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It is all about something called density
dependence. How likely an individual is to die,
or how fertile it is, depends on how many others
of its species there are round-about it.
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Too many in the population and individuals
start dyingToo few, and the death rate eases off
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Too many in the population and individuals
become infertile due to poor nutritionToo few,
and fertility improves
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Whether a population grows, stays stable or
shrinks depends on both death rate and birth
ratethe balance between them
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When death rate 19
When death rate birth ratePopulation shrinks
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When death rate birth ratePopulation stays
steady
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Density dependent processes stop numbers from
getting too largeby killing young, killing
adults or preventing breedingLack of
foodPredation (getting eaten)Disease
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Density dependent deaths increase when population
gets too high, but ease off as it gets too
small.Tending to bring about stability, and
moderate levels
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But is nature really like that? 1930s-1950s some
ecologists questioned the idea
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Suggested that really species are whacked down by
disasters so much, that they almost never see
density-dependent controls
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Thrips populations in an Australian rose garden
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Density independence depends how many you start
from, where you end up. e.g. 99 mortality,
from 100, or from 1 million
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Density independent factors.. Storms, floods,
frosts, heatwaves
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Not a very satisfying explanation surely after a
run of good luck some species would eventually
bump upto levels at which density dependence
operates
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A matter of common sense density dependence must
be in there even if we hardly ever see
it.Just once in a while, the population bumps
up against it
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So. If you have density dependence, what should
you find if you plot initial population density,
against death rate??
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Diverse examples of density dependence, from
four very different organisms.
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Density dependent depression of birth rates
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Most large mammals, birds, some insects are
territorial so they must be controlled by density
dependence
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Density dependence in mammals Tigers fighting
for territory
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• SO FAR THEN . We see that
• Populations don't just grow or shrink
  indefinitely. Something keeps
• them from taking over the world, or crashing down
  to nothing.
• - Density dependent control is particularly
  likely in larger animals,
• which are mostly territorial at some point during
  their life.
• - Density dependence is probably in large part
  responsible for the
• 'balance of nature' the overall stability of
  most populations.

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What does the detailed pattern of population
growth look like on a graph?
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Imagine a situation where the population can grow
for a while almost free of competition. e.g. A
seed arrives in a landscape previously
devastated by a volcano
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Or the population of an aquatic species reaches a
new lake...
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The result of increase without competition is the
exponential curve
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Do populations out in nature show the exponential
growth curves we expect?
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An example of exponential growth ...trees coming
back after the last ice age
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Human population of the USA showed exponential
growth (dots) from about 1800 to 1960. (But
partly due to immigration!).
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But it cant go on forever, or thatspecies will
take over the planet
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So what happens next?Back to theory..
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Once competition starts to bite, you tend to get
the logistic curve...
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Carrying capacity (K) the level the population
tends to stabilize at.
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To predict what happens when competition begins
to bite, we usethe logistic equation
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When competition starts to act
Rate of increase of population per unit time
Number of offspring each produces (multipled
by....) current population size (multiplied
by.....) remaining space for the population to
grow.
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Here is the actual Logistic Equation.... (same
as in previous slide) dN/dt rN (K-N)/K
N population size t time r potential
maximum rate of population growth per capita K
upper asymptote or maximal value of N.
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Can we see real populations behaving like the
mathematics predicts?
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Lab populations showing what looks like
logistical growth...(kind of)
Two separate populations of flour beetles
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Yeast population in the laboratory
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The protozoan Paramecium
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But do WILD populations out in nature show
logistic growth?
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Examples of wild populations showing logistic
growth
Barnacle geese, England
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Trees in Japan after last ice age, from pollen
records
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Trees in America after the ice age
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Sometimes things dont quite work out like the
logistic model predicts
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Overshootdeath rates or decreased birth rates
dont occur soon enoughto bring population under
control..The population wobbles, or crashes
down!
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If competition effects on fertility bitefast
(relative to growth rate), then you get overshoot
and damped oscillations
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With still more rapid (stronger overshoot), you
get bigger oscillations
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So 3 ways a population might grow Exponential,
logistic, or fluctuating
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Still more rapid onset ...the population starts
to wobble around chaos
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Very strong overshoot.. population goes extinct!
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Reindeer on St Paul's Island, Canada show An
example of this a population crash
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Human population is growing
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What might happen with human populations if we
are not careful..
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• Summary
• Lack of food, territory, or disease and predation
  keep each
• species from taking over the Earth (keeping a
  balance in nature)
• - In a pristine environment, a population will
  tend to grow
• Exponentially (with an exponential curve)
• - As competition increases, the population should
  tend to plateau,
• at 'carrying capacity'...(a logistic curve)
• ... Or if it overshoots, maybe fluctuate around
  carrying capacity,
• or even crash!