Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 53.1 Biotic and Abiotic Factors Affect Population Density, Dispersion, and Demographics
Populations are groups of the same species living in a defined area and shaped by both biological interactions and physical conditions. Ecologists study their size, density, and spatial patterns, along with demographic trends like survival and reproduction.
1. What Is a Population?
- Population: individuals of a species in a defined area, sharing resources and potentially interbreeding
- Boundaries can be natural (lake, island) or arbitrary (e.g., county line)
- Key features: size, density, dispersion
2. Density and Dispersion
- Density: individuals per unit area/volume
- Estimated by direct counts, sampling (e.g., quadrats), or indirect indicators (e.g., feces)
- Mark-recapture method for mobile species:
N = (s × n) / x
where s = initially marked, n = total recaptured, x = recaptured marked individuals
- Dispersion: spatial pattern of individuals
- Clumped: due to resources or social behavior (e.g., fungi, sea stars)
- Uniform: even spacing, often from territoriality (e.g., penguins)
- Random: no pattern, rare; e.g., wind-dispersed plants
3. Population Dynamics
- Population changes:
- Births and immigration add individuals
- Deaths and emigration remove individuals
- Example: Hector’s dolphins show seasonal immigration/emigration patterns
4. Demography and Life Tables
- Demography: study of birth, death, and migration rates
- Life tables: track a cohort over its life
- Focus on females due to reproductive output
- Example: Belding’s ground squirrels increase reproduction until midlife
5. Survivorship Curves
- Type I: low death in early/midlife, high in old age (e.g., humans)
- Type II: constant death rate (e.g., squirrels)
- Type III: high early mortality, few survivors (e.g., oysters)
6. Reproductive Rates
- Measured as average female offspring per female per age group
- Example: female squirrels peak at 4–5 years
- PCR genetic profiling enables non-invasive reproductive studies (e.g., sea turtles)
In a Nutshell
Population structure and growth depend on spatial patterns, birth/death rates, and individual behaviors. By using tools like life tables and mark-recapture studies, ecologists quantify how populations change over time and space—insights crucial for conservation, management, and understanding species survival strategies.