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

• 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)

• 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 timeAbundance (rare, common, etc.)Frequency of encounters in sampled plots or in

time intervalsCounts 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 pre- is 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 3-dimensional 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)