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Study of populations relative to environment
- Age Structure
- Population Size
Group of individuals of single species living in same area.
- Number of individuals per unit area or volume.
- Result of processes that add or remove inividuals from population.
- Pattern of spacing of individuals in boundaries of population.
- Influenced by environmental and social factors.
- Impractical or impossible to count all individuals in a population.
- Sampling techniques estimate density and total population size.
- Population size estimated from small samples, index of population size, or mark-recapture method.
Three Methods for Marking:
- Bands on Feet (Birds)
- Radio Collars (Wolves)
- Recognizing Individual Uniqueness (Dorsal fin on Whales)
Assumption for Marking Techniques:
Method will not affect organisms way of life.
Processes that Change a Population Size:
- Immigration - Influx of new individuals from other areas
- Emigration - Movement of individuals out of a population
- Individuals aggregate in patches.
- Influenced by resource availability and behavior.
- Individuals are evenly distributed.
- Influenced by social interactions like territoriality.
- Position of each individual is independent of others.
- Occurs in absence of strong attractions or repulsions.
- Study of vital statistics of a population and change over time.
- Death rates and birth rates are important to demographers.
- Life tables to study death rates.
- Reproductive tables to study birth rates.
- Age-specific summary of the survival pattern of population.
- Best by following fate of cohorts.
A group of individuals of the same age.
Calculation of Death Rate:
[(Deaths During Year)/(Alive At Start)] = Death Rate
Graphic way of representing data in a life table.
Survivorship Curve Classification:
- Type I - low death rates early-mid life, increase in older age.
- Type II - constant death rate.
- Type III - high death rates early, lower death rate for survivors.
Species with sexual reproduction, concentrate on females.
- Also called a Fertility Schedule.
- Age-specific summary of reproductive rates in population.
- Describes reproductive pattern of population.
Calculation of Average Number of Female Offspring:
[(Females Weaning a Litter)X(Females in Litter)] = Female Offspring
Traits Affecting Reproduction and Survival Schedule:
- Age which reproduction begins.
- How often organism reproduces.
- How many offspring produced during reproductive cycle.
Life History Traits:
Evolutionary development in physiology and behavior of organism.
- Big-bang reproduction.
- Reproduce once and die.
- Variable or unpredictable environments favor big-bang reproduction.
- Repeated reproduction.
- Produce offspring repeatedly.
- Dependable environments favor repeated reproduction.
Survival and Reproduction:
Finite resources lead to trade-offs.
For Finite Resources:
- Reduced brood size has higher survivability because there of lower population of species (less consumption and more dispersion of resources)
- Enlarged brood size has lower survivability because there of higher population of species (more consumption and less dispersion of resources)
- Parental care of smaller broods facilitate survival of offspring.
- Some plants have many small seeds ensuring growth and reproduction.
- Other plants have few large seeds giving large energy store to help seedlings.
Modeling Population Growth:
- Change in population size = Births + Immigrants - Deaths - Emigrants
- Ignoring immigration and emigration, population's growth rate (per capita increase) equals birth rate minus death rate.
Per capita rate of increase(r):
- Difference of per capita birth rate(b) and per capita death rate(d)
- r = b-d
- Increasing if birth rate is higher than death rate (r>0).
- Decreasing when birth rate is lower than death rate (r<0).
- Zero population growth if birth rate equals the death rate (r=0).
Calculating population size (N):
Nt+1 = rNt + Nt
Population growth over many time intervals:
- ΔN/ Δt = rN
- Discrete time interval, no immigration or emigration.
Population Growth Instantaneously:
- dN / dt = rinstN
- rinst is the instantaneous per capita rate of increase.
Exponential Growth Model:
- To study population growth in idealized situation.
- Ideal situations help us understand capacity and factors of species to grow.
Exponential Population Growth:
- Population increase under ideal conditions.
- Rate of reproduction is at maximum (intrinsic rate of increase).
- dN = rmaxN
- If r > 0, population increase is rapid.
- Result of exponential population growth.
- Population growth depends on the value of N and r,.
- Thus population increase is greater as time passes.
- Characterizes some rebounding populations.
Logistic Growth Model:
- Exponential growth cannot be sustained for long in any population.
- Realistic population model limits growth by using carrying capacity.
- Fit few real populations but useful for potential growth.
- Carrying capacity(K) is max population size environment can support.
- Per capita rate of increase declines as carrying capacity reached.
- dN = rmax N(K-N)
- .dt _ _____ _ K
- (K-N)/K close to 1, per capita rate of increase approaches maximum rate.
- (K-N)/K is close to 0, per capita rate of increase is small.
- Result of logistic model of population growth.
- Some overshoot K before settling down to stable density.
- Some fluctuate greatly and difficult to define K.
- Some show an Allee effect.
Difficult time surviving/reproducing if population size is small.
Logistic Model and Life Histories:
- Life history traits favored by natural selection vary with population density and environmental conditions.
- K-selection and r-selection are oversimplifications but stimulated alternative hypotheses of life history evolution.
- Density-Dependent selection.
- Selects life history traits sensitive to population density.
- Density-Independent selection.
- Selects life history traits maximizing reproduction.
Factors Affecting Population Growth:
- Environmental factors that stop a population from growing.
- Radical Fluctuations in size over time.
Birth rate and death rate do not change with population density.
- Birth rates fall and death rates rise with population density.
- Negative feedback that regulates population growth.
- Affected by competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors.
Competition for Resources:
Crowded populations intensifies competition for resources resulting in lower birth rates.
- In vertebrates and invertebrates competition for territory limit density.
- Cheetahs use chemical communication to warn other cheetahs of their boundaries.
- Oceanic birds exhibit territoriality in nesting behavior.
- Influences the health and survival of organisms.
- Pathogens can spread more rapidly in dense populations.
Population growth attracts predator preferential.
Accumulation contribute to density-dependent regulation of population size.
Physiological factors regulate population size.
- Challenged hypothesis that large mammal populations are relatively stable.
- Biotic and abiotic factors influence population size.
- Some undergo regular booms-and-bursts cycles.
Global Human Population:
- No population can grow indefinitely.
- Increased slowly until 1650 and began to grow exponentially.
- Still growing but began to slow during the 1960s.
Regional Human Population Configurations:
- Zero population growth = High birth rate & High death rate
- Zero population growth = Low birth rate & Low death rate
- Demographic transition goes from first state toward second state.
- Associated increase in quality of health education.
- Most global population growth concentrated in developing countries.
- Relative number of individuals at each age.
- Age structure diagrams predict population's growth trends.
- Illuminates social conditions and help plan for the future.
- Infant mortality and life expectancy at birth vary greatly.
Global Carrying Capacity:
- Carrying capacity of humans on Earth uncertain.
- Average estimate is 10-15 billion.
- Ecological footprint concept summarizes land and water needed to sustain a nation.
- Measures proximity to carrying capacity of Earth.
- Great footprint size and ecological capacity variation.