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POPULATION ECOLOGY
I. Population Ecology = the study of how populations
interact with their environment
A. Population = group of individuals of the same species occupying a
common geographical area
B. Habitat - where a species normally lives
C. Characteristics of populations - Each population has
certain characteristics:
1. Population size = number of individuals making up
its gene pool
2. Population density = number of individuals per unit of
area or volume, e.g. persons/square mile
3. Population distribution = the general pattern in which
the population members are dispersed through its habitat, may be:
a. Clumped - most common type because:
Suitable physical, chemical, and biological conditions are patchy, not
uniform
Many animals form social groups
Many offspring are not highly mobile and are forced to live "where
they landed"
b. Uniformly dispersed - rare in nature. If it does occur, it is usually
the result of fierce competition for limited resources
c. Randomly dispersed - occurs if environmental conditions are rather
uniform in the habitat and members are neither attracting nor repelling each
other
4. Age structure defines the relative proportions of
individuals of each age:
a. Pre-reproductive
b. Reproductive
c. Post-reproductive
II. Population size and growth
A. Population size is dependent on births, immigration, deaths, and
emigration
1. Zero population growth designates a near balance of
births and deaths
B. Exponential growth:
1. If birth and death rates of a population remain constant they can be
combined into one variable r = net reproduction
per individual per unit time (rate of increase)
2. Population growth may be represented mathematically as: G =
rN
Where G = population growth per unit time, r = rate of increase and N=
the number of individuals
3. Plotted against time a graph in the shape of a J will be obtained
a. A J shaped growth curve denotes exponential growth,
i.e. one variable increases much faster than the other
b. As long as per capita birth rates remain even slightly above per
capita death rates, a population will grow exponentially - with
ever-increasing rates and shorted "doubling times"
E.g. It took 2 million years for the world's human population to
reach 1 billion, yet it took only 12 years to reach the fifth billion
4. If a population lives under ideal conditions it may display its biotic
potential - the maximum rate of increase under ideal conditions
a. However few populations live under ideal conditions because a number
of factors limit their growth
b. Limiting factor - any resource that is in short supply, e.g.
food, minerals, light, living space, refuge from predators, etc.
5. Carrying capacity = maximum number of individuals of a species
or population a given environment can sustain. Each habitat or area can only
support so many individuals
6. Because of limiting factors populations rarely exhibit J shaped growth
curves
C. Logistic growth
1. Early on populations will exhibit very rapid growth but as they near
the carrying capacity they will level off
2. This is called logistic growth and it produces an S
shaped curve
3. Logistic growth is density dependent, i.e. the growth is
affected by the density of individuals
a. For example - 26 reindeer were introduced onto an island off the
coast of Alaska in 1910. Within 30 years the herd increased to 2,000.
However, overgrazing reduced the food supply and the population crashed to
8 animals by 1950
b. High density and overcrowding put individuals at greater risk of
being killed, e.g. predators, parasites and pathogens have greater numbers
of prey and hosts in a smaller area, to interact with
Bubonic plague swept through Europe in the 14th
century, killing at least 25 million. The disease spread rapidly in
overcrowded cities where sanitary conditions were poor and rats were
abundant
4. Population size and growth may also be controlled by density-independent
factors, e.g. adverse weather, floods, droughts, cold
temperatures
III. Life history patterns
A. Not all individuals in a population are the same age.
B. Different populations may have very different age structures and
these will determine their growth patterns
1. Age structure refers to the proportions of pre-reproductive,
reproductive and post-reproductive age individuals in a population. The age
structure of a population will determine its future
C. Each species has a characteristic life span and the probability of dying
increases with age
D. Population ecologists, as well as insurance companies track cohorts
and construct life tables for populations
1. Cohort = a group of individuals born at the same time, e.g.
baby boomers are a large group of individuals born just after World War II
E. A life table is an age-specific death schedule. Such
a schedule is often converted to a more palatable survivorship
schedule. For each age interval there is an predicted life expectancy or
survivorship
F. Ecologists divide populations into age classes and assign birth rates
and mortality risks to each class. Absolute population numbers mean very
little unless their age structure is known
1. For example, population A might have many more members than population
B. However, all the members of A might be post-reproductive, whereas
population B might consist of mostly pre-reproductive and reproductive age
individuals. Population A might be in danger of extinction
IV. Life history strategies:
A. r-selected organisms - put most of their energy into rapid growth
and reproduction
1. This is common of organisms that occupy unpredictable environments,
e.g. weeds are usually annuals with rapid growth and early reproduction.
They produce large number of seeds containing few stored nutrients
B. K-selected organisms - put most of their energy into growth
1. They are common in stable environments near carrying capacity, e.g.
long lived trees such as redwoods take many years of growth to reach
reproductive age
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