Demography Lecture Notes

Population Ecology

• Population Ecology: Studies factors affecting the number of individuals of a species in a location over time.
• Population: A group of interacting individuals of a single species in a single location.

Factors Influencing Populations

• Biotic Factors: Living components (e.g., lions, tigers, human poaching).
• Abiotic Factors: Non-living components (e.g., temperature, rainfall).
• Dispersion: Pattern of individuals in space.
  • Random: Unpredictable spacing.
  • Uniform: Even spacing due to interactions like chemical inhibition or territoriality.
  • Clumped: Clustering due to resource distribution, social groups, or reproductive behavior.

Population Size and Density

• Population Density: Number of individuals in a defined area.
• Population Size: Total number of individuals in a population across all areas.
• $population : size = density \times area$

Measuring Populations

• Full Census: Counting all individuals (feasible in some cases).
• Sample Plots (Quadrats): Count individuals in randomly selected areas, average density, and extrapolate.
• Mark-Recapture: Capture, mark, release, and recapture individuals to estimate population size.
• Population Genetics: Using DNA to identify individuals.

Sample Plots

• Divide the study area into subunits (quadrats) and count individuals in selected plots.
• Calculate population density: $Population : Density = \frac{Total : individuals : in : all : sample : plots}{Total : area : of : plots}$
• Extrapolate total population size from density:
  • $Estimated : Population : Size = Population : Density \times Total : Habitat : Size$
• Limitations: Non-uniform distributions can lead to inaccurate estimates.

Mark-Recapture

• Estimate population size using the proportion of marked animals in the recapture sample.
• Formula: $\frac{M}{N} = \frac{r}{n}$, where:
  • $M$ = number marked and released in sample 1,
  • $n$ = total captured in sample 2,
  • $r$ = number of recaptures,
  • $N$ = estimated population size.
• Solve for N: $N = \frac{Mn}{r}$
• Assumptions:
  • Marked and unmarked individuals have an equal probability of sampling.
  • No birth, death, immigration, or emigration during resampling.
  • The population mixes thoroughly between samples.
• If many are recaptured in the second sample, the population size is relatively small. If few are recaptured, the population size is relatively large.
• Violations of Assumptions: Can lead to over or under-estimation of population size.
  • Animals avoid recapture: population size inflated.
  • Animals prefer recapture: population size underestimated.

DNA Sampling

• Genetic Profiling: Using DNA to identify individuals, especially useful for elusive species.
• Create genetic profiles from samples.
• Compare eggshell samples to databases of adult females.

Demography

• Demography: The study of populations and how they change over time.
• Populations are described by age and sex structure, survivorship, and life history.

Age & Sex Structure

• Age Structure: Distribution of individuals at different ages in a population.
• Sex Structure: Ratio of males to females.
• Relative proportions impact population stability and growth.
• Age structure predicts population growth or decline.
  • Many young, few old: Rapidly Expanding
  • Few young, many middle aged: Contracting
• Population increases if births > deaths, decreases if births < deaths.
  *Differential survival skews sex ratio.
  *Many species tend towards 1:1 sex ratio (M:F)
  *Skewed sex ratios impact reproduction & population growth. Strong skews in either sex decrease mating opportunities & increase inbreeding

Survivorship

• Life Tables: Summarize mortality, survival, and reproduction in a population by age group.
• Cohort Life Table: Tracks a group (cohort) from birth to death.
• Static Life Table: Records the number of individuals of each age at a single time point.
• Survivorship Curves: Summarize life table data, showing deaths over time.
• Can compare intervention strategies for diseases using survivorship curves.

Types of Survivorship Curves

• Type I: Low death rates early and middle life, steep increase in old age.
• Type II: Constant death rate over lifespan.
• Type III: High death rates for the young, low death rates for survivors.

Life History

• Natural selection shapes traits related to reproduction and survival.
• Key components:
  • Reproductive Onset: When reproduction begins.
  • Reproductive Frequency: How often to reproduce.
  • Reproductive Output: How many offspring each time.

Tradeoffs

• Tradeoffs determine fecundity (reproductive capacity).
• Caring for more young can lower parental survival.
• High current reproductive investment reduces future reproductive success, and vice-versa.
• Reproductive Onset: Early reproduction risks death without offspring, while later reproduction risks not surviving to reproduce.
• Reproductive Frequency: Semelparous (reproduce once) vs. Iteroparous (reproduce repeatedly) species.
• Reproductive Output: Number of offspring reflects parental investment.

Semelparity vs. Iteroparity

• Semelparous: Invest all resources in one reproductive event.
• Iteroparous: Balance investment in each reproductive event.

r/K Selection Theory

• Parental investment balances demands in habitats varying in stability.
• Reproductive strategies produce the most surviving offspring over an individual’s lifetime.