Population Ecology (5.3 & G.5)
R2
Population Dynamics
• Four main factors:
– Natality – the number of new members of the species due to
reproduction;
– Mortality – the number of deaths;
– Immigration – members arriving from other places;
– Emigration – members leaving the population;
• Example – Mount Saint Helens Eruption:
– Mortality from the eruption and emigration afterwards greatly
decreased population.
– But after only a few months life began to return to the scene through
the immigration of species to the devastated area, which then caused
birth rates to slowly increase.
Population Growth Curve
• There can be a dramatic increase in population numbers even after
complete eradication.
• Through reproduction, the size of a population will continue to increase at
steadily growing rates until a steady balance is reached between mortality
and natality rates.
• Described as sigmoid, the S-shaped
curve has 3 phases:
– A) Exponential Growth Phase - # of individuals
increases at a faster and faster rate.
– B) Transitional Phase – Growth rate slows
considerably. Population increasing but much
slower.
– C) Plateau Phase – The # of individuals is
stabilized. Carrying Capacity has been
reached.
Causes of Exponential Phase
• Ideally, a population can double its size regularly.
• Predators are often introduced into populations that exhibit
uncontrolled natality rates so that the population size can be
regulated.
• Causes for this exponential growth are:
– Plentiful resources such as food, space or light;
– Little or no competition from other inhabitants;
– Favorable abiotic factors such as temperature or dissolved oxygen
levels;
– Little or no predation or disease.
Causes of the Transitional
Phase
• Factors that allow for exponential growth of a population
eventually become no longer true.
• This leads to the transitional phase.
• Causes of this phase are:
– Increase in competition due to increase in population.
– Predators begin to immigrate into the area due to the increase in food
supply.
– Opportunities for disease increase due to the large number of
individuals living together in limited space.
Causes of the Plateau Phase
• The plateau phase, or stationary phase, occurs when the
habitat’s resources begin to be insufficient to support the
ever-growing population.
• Less space is available for seed germination as vegetation
continues to take up the fertile land, which leads to less plant
growth.
• Growing numbers of herbivores compete over limited food
supply, leading to less offspring.
• Predators and disease continue to effect the population as
they have been, so the mortality and emigration rates
become balanced with the natality and immigration rates.
Carrying Capacity (K)
• Carrying capacity (K) – the maximum number of individuals
that a particular habitat can support.
• It is reached when there are not enough resources left in a
specific habitat to allow for the continued growth of a
population.
• Carrying capacity is defined by limiting factors of a habitat,
such as:
– Availability of water, food, sunlight, shelter, space, or oygen.
– Excess waste, such as excrement, or excess carbon dioxide.
– Predation.
– Disease.
Harder Stuff (G.5)
R- and K-Strategies
Distinguishing R&K
• R-Strategy – The strategy of disposable offspring, such as with the zebra
mussel, which produces thousands of offspring at a time.
– The majority of them do not make it past the larval stage because they are eaten.
– But the surviving minority is still a sizeable amount of offspring, often a few hundred.
• K-Strategy – The strategy of nurturing, as with the zebra.
– Young zebras need a very specific quality of care from their mothers. Without it they will
not survive as easily, if at all.
– Zebras and other mammals only produce a few offspring at a time and they have long
periods of gestation, so it is important that each newborn receives extensive maternal
care to help them survive.
• So if a natural disaster were to occur, the zebra mussel would be much
more likely to survive. Why?
Characteristic
R-strategy
K-strategy
Life span
Short
Long
Number of offspring
Many
Few
Onset of maturity
Early
Late – after long period of
parental care
Body size
Small
Large
Reproduction
Once during lifetime
More than once during
lifetime
Parental care
None
Very likely
Environment
Unstable
Stable
Environmental Conditions of
R&K
• The r-strategy means that in an unstable environment it is better to
produce many offspring as quickly as possible, such as with weeds in
frequently disturbed ground.
• The K-strategy means that if a stable environment is available then the
nurturing of offspring is ideal.
• Using the dune as an example, inhabitants of the foredune are usually rstrategists because of the constant sand erosion and salt spray that make
the environment unstable.
• As the dune progresses inland and becomes more mature, K-strategists
begin to appear. This is because the mature dune is out of reach of the
wind and salt spray of the coast. The soil also contains nutrients to help
sustain K-strategist plants.
• Habitats are diverse when they are filled with many different K-strategist
species.
Ecological Disruption
• Ecological disruption favors r-strategists.
• The natural ecology of farms, for example, is severely disrupted and
contains little diversity because humans control the interactions between
the species.
• But in a balanced ecosystem there are natural interactions between all
species and they regulate each other’s populations.
• A forest that has natural ecology will have an assortment of trees,
meaning that not all of them will necessarily be wiped out if there is a pest
attack. A man-made plantation of one type of tree, however, risks being
completely wiped out if such an attack were to happen.
Capture-Mark-ReleaseRecapture
• It is a sampling technique that enables you to estimate the
number of animals in an ecosystem.
• It involves catching some of a population, marking them, and
releasing them back into the population.
• The marked animals mix with the unmarked and a second
sample of individuals is taken.
• The proportion of marked individuals to unmarked in the
second sample is the same as the proportion of the
individuals in the first sample to the entire population.
C-M-R-R Continued
• There are limitations to this technique:
– Marking animals could injure them.
– Marks may be too conspicuous and attract
predators. Death of marked animals makes the
second sample unreliable.
– It assumes a closed population (no immigration or
emigration).
Commercial Fish Stocks
• Size of fish populations can be determined in several ways:
– Studying catches
– Gathering information from fishermen.
– Using research vessels
• Research vessels collect information by:
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Casting nets in hundreds of selected locations
Using sound to monitor fish populations
Calculating the age of fish in a population
Using coded wire tag detectors
Analyzing data using mathematical models
More Fish Stuff
• The Maximum Sustainable Yield (MSY) is the highest
proportion of fish that can be removed from the total
population without jeopardizing this maximum yield in the
future.
• To maintain the MSY enough fish must be left to produce a
new population of fish.
– Too small fish stock = not enough adult fish to produce offspring
– Too large fish stock = too much competition among fish so
reproduction rates decrease.
Save the Fish
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A 29% decline in seafood population over 3 years started fish conservation
movements.
What can we do?
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Regulate bottom trawling of the ocean, it is basically stripping the ocean floor.
Rebuild depleted fish populations ASAP. Pay attention to MSY.
Eliminate wasteful fishing practices like by-catching, where unwanted fish are thrown back dead or
dying. Careless reduction of biodiversity.
Enact strong national fish quote programs based on MSY.
Establish programs to make safety-conscious fishing gear. Sea turtle doors.
Improve scientific research to better keep track of catches.
Fishermen and scientists should become friends.
Marine reserves and no-catch zones. It would help stop the reduction in biodiversity.
Problems – Countries don’t like each other. Increase of fish prices worldwide.
Limiting catches hurts the fisher economy, which nobody wants in an already
unstable economy.
Game?!
Indeed.
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You will receive a notecard that tells you if you are marked or unmarked. DO NOT SHOW
ANYONE WHAT YOU ARE. Not even me. 
You are all working competitively and in the interest of survival. Your goal is to identify the
marked individuals, which will make predators aware of them and they will be eaten.
You are doing this because you are trying to confuse the humans and make their data
unreliable. When the humans recapture a sample of species, the complete absence of any
marked individuals will throw off their data and make it harder for them to intervene in your
habitat.
In an astounding display of cunning for an animal, your job is to write down the name of one
person in the class whom you believe to be marked. You probably shouldn’t tell that person
you’re writing their name down. The person whose name you write down will be eaten by
predators, even if they are unmarked.
If you are marked you are to write down the name of someone else you think is also
marked. If you choose correctly, that person is immune to predation. If you choose
incorrectly, they die. But… you have to choose someone.
If you are killed by a predator, the person whose name you wrote down still dies.
At the end we will see if there are any marked individuals remaining and determine if the
conspirators were successful or not.