1. Dynamics of Ecosystems: Population Ecology
Chapter 43
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2. Population Ecology
A population consists of individuals of a given species living together at the same place at the same time.
Population Distributions
Most species have a limited geographic range.
Population ranges change through time.
Environment Changes
Organisms circumvent previously inhospitable habitat.
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3. Population Dispersion
Randomly Spaced
Individuals are randomly spaced within a population.
Uniformly Spaced
Uniform spacing within a population often results from resource competition.
Clumped Spacing
Individuals clump into groups or clusters in response to uneven distribution of resources in their immediate environment.
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4. Population Dispersion
Human Effect
By altering the environment, humans have allowed some species to expand their ranges, although the overall effect has most often been detrimental.
Plant Dispersal Mechanisms
Light Seeds (Wind)
Hooks and Hairs (Animal Fur)
Fruits (Animal Digestive Systems)
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5. Metapopulations
Metapopulations are a network of distinct populations interacting with each other by exchanging individuals (genes).
Degree of interaction depends on dispersal.
Rate of extinction in habitats is balanced by the rate of colonization of empty habitats.
Prevent long-term extinction.
Source-Sink Metapopulations occur in areas where some habitats are suitable for long-term maintenance, while others are not.
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6. Demography
Demography - Statistical study of populations.
Factors Affecting Growth Rates
Sex Ratio
Effect also determined by mating habits.
Generation Time
Average interval between birth of an individual and the birth of its offspring.

7. Demography
Age Structure
Cohort - Individuals of the same age.
Fecundity - Number of offspring produced in a given period of time.
Mortality - Number of deaths in a given period of time.
Age Structure determine by the relative number of individuals in each cohort.

8. Demography Life Tables
Constructed by following the fate of a cohort from birth until death.
Survivorship Curves
Survivorship is the percentage of an original population that survives to a given age.
Type I - Full Life Span
Type II - Mortality un-related to age
Type III - Early Susceptibility

9. Cost of Reproduction
A life history is the complete life cycle of an organism.
All involve significant trade-offs.
Due to limited resources, increased reproduction may decrease survival and chances of future production.
Cost of Reproduction
Natural selection will favor the life history that maximizes lifetime reproductive success.

10. Cost of Reproduction Fecundity and Mortality Correlation in Birds
Investment Per Offspring
Key reproductive tradeoff concerns amount of resources to invest in producing any single offspring.
Number of offspring versus size of each offspring.
In many species, offspring size critically affects chances of survival.
Fecundity and Mortality Correlation in Birds
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11. Cost of Reproduction Reproductive Events Per Lifetime
Semelparity - Organisms focus all reproductive efforts on a single, large event.
Iteroparity - Organisms produce offspring several times over many seasons.
Age at First Reproduction
Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals.

12. Population Growth and Life History Models
K-selected species tend to produce relatively few, large offspring.
Large invest in parental care
R-selected species tend to produce many, small offspring.
Small investment in parental care.

13. Population Growth and Life History Models
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14. Biotic Potential
Exponential Growth Model
Rate at which a population of a given species will increase when no limits are placed on the rate of growth.
Innate capacity for growth of any population is exponential.
Even when rate remains constant, actual increase in number accelerates as the population size grows.

15. Biotic Potential Carrying Capacity
Carrying capacity (K) is the maximum number of individuals the resources in a given area can support.
Logistic Growth
As a population approaches carrying capacity, its growth rate slows as resources become depleted.
Sigmoid growth curve.

16. Influence of Population Density
Density Dependent Population Controls
As populations approach their carrying capacity, competition for resources can be severe, leading to decreased birth rate and increased risk of mortality.
In some cases, growth rates increase with population size.
Allee Effect

17. Influence of Population Density
Density Independent Population Controls
Growth of a population at any instant is limited by some factor other than population size.
External environmental conditions.
Population Cycles
Snowshoe Hares
Food Plants (willows) and Predators (Lynx) both control population.

18. Predator-Prey Cycle
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19. Exponential Growth
Throughout most of human history, human populations have been regulated by food availability, disease, and predators.
Resemble K-selected species.
Due to the easing of environmental restraints, the human population has grown explosively over the last 300 years.

20. Advent of Exponential Growth
Population Pyramids
Bar graph displaying number of people in each age category.
Stable - Rectangular
Rapid Growth - Triangular
Decreasing - Inverted Pyramid

21. Population Pyramids Uncertain Future
Earth’s rapidly growing human population is possibly the greatest challenge facing facing the biosphere.
Uneven Resource Distribution
Population Distribution
Wealth Gap

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