Download BIOLOGY 154: ECOLOGY and ENVIRONMENTAL ISSUES

BIOLOGY 403:
PRINCIPLES OF ECOLOGY
(Populations)
POPULATIONS
What is a population?
all the individuals of a certain species in a
particular area
adjacent populations of the same species usually
have some degree of interaction (immigration,
emigration, gene flow, resource exchange, etc.)
thus forming a METAPOPULATION
EMERGENT PROPERTIES
• Each level of organization has certain properties.
• As we go from one level to the next (e.g. from
atoms to molecules or individuals to populations)
we see that the higher level has many of the
properties of the lower level(s) that make it up.
• HOWEVER, we also see properties or attributes
‘emerging’ in the whole which were not evident
in the parts that make it up.
• In other words, the whole is more than the sum of
its parts.
Some Important Population
Factors / Attributes
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Natality (crude, age or sex-specific) (0 or +)
Mortality (crude, age or sex-specific) (0 or +)
Growth Rate (+ or 0 or -)
Carrying Capacity (K)
Density
– Density Dependent Factors
– Density Independent Factors
Age Distribution
Dispersion
Survivability
Competition (intra species)
Evolution
POPULATION GROWTH I
The rate at which a population grows depends on:
• Natality
• Mortality
• Immigration
• Emigration
• Of course, each of the above factors is itself
affected by other factors.
POPULATION GROWTH II
• All rates (growth, birth , death) can be expressed
by the general formula: N / T
• r = biotic potential (the ability of a population to
increase)
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r depends on:
Survival to reproductive age
Age of 1st breeding
Duration of reproductive portion of the life span
Number of offspring per reproductive episode
FACTORS AFFECTING POPULATION SIZE
SURVIVORSHIP
• Mortality slows population growth
• Concentrate on those that live rather than
those that die
• The ‘reciprocal’ of mortality is survivability
• Rate at which organisms die often is not
uniform during the lifespan of a species
GENERALIZED
SURVIVORSHIP CURVES
SURVIVORSHIP REVISITED
• Not all organisms fit neatly into the
generalized categories
• Herring gulls have Type III early in life and
Type II later
• Oysters & Salmon have Type III
• Hydra, many annual plants and some reptiles
show Type II
• Humans?????
TYPES OF GROWTH (I)
• Linear Growth
– a quantity increases by a constant amount
per unit of time; additive
– produces a straight line when graphed
• Exponential Growth
– a quantity increases by a fixed percentage of
the whole per unit of time; same
phenomenon as compound interest
– produces a curve when graphed
TYPES OF GROWTH (II)
POPULATION GROWTH PATTERNS
• Exponential (J-shaped curve)
largely density independent
• Logistic (s-shaped or sigmoid)
more density dependent
factors such as territoriality, aggression,
inter- and intraspecific competition,
predation and disease are of major
importance here
POPULATION GROWTH
CURVES: J vs. S (I)
POPULATION GROWTH
CURVES: J vs. S (II)
POPULATION GROWTH
CURVES: J vs. S (III)
• Populations which follow a J-shaped curve usually
lack control by density dependent factors.
• Populations which follow an S-shaped curve have
one or more DENSITY DEPENDENT factors
controlling their growth (e.g., territoriality,
aggression, inter- or intraspecific competition,
predation, disease).
• All populations can be affected by DENSITY
INDEPENDENT factors such as catastrophic
weather, earthquakes, volcanic activity, etc.
FORMULAE FOR J & S CURVES
• J Curve:
Growth = r N
• S Curve:
Growth = r N {1 – (N / K)}
• When N is small, (N / K) is small, and thus
{1 – (N / K)} is relatively large and growth is rapid
• When N is larger, (N / K) is larger and thus
{1 – (N / K)} is smaller and growth slows
• When N = K then (N /K) = 1 and
{1 – (N / K)} = 0 and growth stops
HUMAN POPULATION GROWTH
• What type of Growth pattern? J or S?
• Why?
HUMAN POPULATION
GROWTH
• EXPONENTIAL ?????
• Until rather recently in our evolutionary
history human numbers were held in check
by famine, disease, war, lack of technology,
etc. (= Environmental Resistance)
• Recently these factors have been greatly
minimized (= less environmental resistance).
• Population is growing rapidly due to good
death control but poor birth control.
SOME ADDITIONAL
POPULATION GROWTH ITEMS
• Replacement Level Fertility
• Zero Population Growth
• Doubling Time For A Population
The “Rule” of 70
70 / % growth rate = doubling time
e.g.: 70 / 1.6% = 43.75 years
70 / 5% = 14 years
70 / 2% = 35 years
DENSITY I
• Population size (numbers or biomass) per unit
of area or volume
• Absolute Density (actual count of numbers or
biomass)
• Relative Density (some type of sampling)
Number or biomass per unit of time
Abundance (rare, common, etc.)
Frequency of encounters in sampled plots or
in time intervals
Counts in random selected plots
DENSITY II
• Population size can be controlled by Density
Independent Factors and Density Dependent
Factors
• Density Independent (usually abiotic)
weather, earthquakes, landslides, volcanic
activity
• Density Dependent (usually biotic)
predation, parasitism, stress, territoriality
and other behaviors
DENSITY III
• Density Independent factors can affect both J
(exponential) populations and S (logistic)
populations
• Organisms with logistic growth (S) have one
or more internal or external Density
Dependent factors regulating their population
size
POPULATION AGE STRUCTURE I
• The proportion of various age groups in a
population can have a profound effect on
growth of a population.
• It depends on a number of factors
• Age group studies in a population often bring
to light interesting / important aspects of the
species
• Three “Ecological” Ages
Pre-reproductive
Reproductive
Post Reproductive
POPULATION AGE STRUCTURE II
• % of life span spent in various stages varies
considerably
• Mayflies: mostly pre- (a year), 2-3 days
repro, no real post-
• Cicadas: mostly pre- (years), a month or two
repro-, maybe a little post• Trees: years/decades pre-, decades/centuries
repro-, maybe no post• Humans: at one time no post- ???? Now preis the shortest, repro- long and post- longer??
POPULATION AGE STRUCTURE III
• Humans (and some other organisms as well)
have two types of pre-reproductive ages.
Absolute (= biological)
12 to 15 years ?????
Dropping a bit over the last 50 years ?????
Socially Acceptable
varies with the culture
AGE CLASSES
• Chronological classes
• Do not have to be equal
• 0-1 year, 1-5, 5-10, 10-20…….70-75
• Age class data can be presented as a table, bar
graph or Age Polygon
POPULATION AGE STRUCTURE (I)
POPULATION AGE STRUCTURE (II)
POPULATION AGE STRUCTURE (III)
POPULATION AGE STRUCTURE (IV)
DISPERSION I
• Dispersion refers to the pattern of the
organisms on the landscape (or in a 3dimensional system)
• Dispersion types:
– Random
– Non-Random
• Uniform
• Clumped (aggregated)
–Random c, Uniform c, Aggregated c
DISPERSION II
• Causes of the dispersions:
– Response to local habitat conditions
• Indigenous
• Produced by the organism (allelopathy,
etc.)
– Response to daily / seasonal weather
changes
– Reproductive processes
– + or - social interactions
– Extrinsic biological factors (predation, etc.)
RANDOM DISPERSION
• There is no pattern in the distribution of
individuals in the population.
• No factors are working on this population to
influence the association of individuals.
• THEREFORE, where an organism is found is
due to chance.
• RARE!
UNIFORM DISPERSION
• Spacing is fairly regular. Generally each
individual has its own area.
• This can be due to competition, Allelopathy or
other antagonistic behaviors.
CLUMPED DISPERSION I
• Organisms are more likely to be found
associated with others
• Can result from:
– Asexual reproduction (especially in plants)
– Heavy fruits or seeds in plants
– Social interactions in animals (these often
allow for protection, learning, division of
labor)
– Habitat irregularities
CLUMPED DISPERSION II
• The aggregates of organisms can then be
distributed:
• Randomly --- rare
• Uniformly --- bunchgrass in arid regions
• Clumped --- due to habitat irregularities so
that certain critical resources are located only
in a few areas; social interactions
DETERMINING DISPERSION
PATTERNS I
• Poisson Distribution Test – a test for
randomness
• Dependent on count data
• Divide area in subplots (? how small ?)
• Use Poisson Formula to determine the
number of plots expected to contain 0, 1, 2, 3,
etc. organisms
• Count the number of plots ACTUALLY
having 0, 1, 2, 3, etc. organisms
DETERMINING DISPERSION
PATTERNS II
• Use Chi2 to determine if the deviations
between the observed and predicted numbers
are likely due to chance
• IF Chi2 is significant then most likely it is a
non-random distribution
• If more plots than expected with one
organism, then probably uniform
• If more plots with 2 and higher and fewer
with 0 or 1, then probably clumped
COMPETITION
• COMPETITION --- a striving for something
that (usually) is in short supply
• INTRAspecies competition
• A negative interaction
• Usually gets more intense as density increases
• At the population level this usually means
that one or more of the following will be
inhibited to some degree:
– Density, Organism size, Population
energy flow
EVOLUTION
• Competition and Evolution go ‘hand in hand’
• Who survives competition?
• REMEMBER: FITNESS is really an average
arrived at from the interaction of many
factors
DISPERSAL
• How a species moves
• When it moves
• Types of disseminules
• Migration patterns (diurnal, seasonal, once
during the life cycle or many times)