Understanding Population Ecology: Dynamics and Growth

1. Chapter 51 Population Ecology

2. Ecology Basics • Terms to know… • Ecology • Branch of biology • Relatively new science • Biotic factors • Abiotic factors • Environmental science • ecology + human interactions • Population • same species, same area, same time • Population ecology • numbers + changes

3. Features of populations • Properties that individuals lack: • Population density • Population dispersion • Birth/death rates • Growth rates • Survivorship rates • Age structure • Properties that communities lack: • Common gene pool • Reproductive success • Evolution • Economic importance (crops, forests, game animals…)

4. Population dynamics • Density– the number of individuals per unit of area or volume at a given period of time • Affected by: • Habitat • Season/Weather • Other populations • Limiting factors: the number of individuals in a population is controlled by the ability of the environment to support it • Density-dependent factors – the effect increases as population density increases • Examples: • Density-independent factors – affects the size of a population but is not influenced by changes in population density; typically abiotic • Examples:

5. Population dynamics… • Dispersion– spacing in relation to other members of the population • Three basic varieties: • Clumped (aggregated distribution, patchiness) • Individuals are concentrated in specific areas • Reasons: distribution of resources, social behavior of animals (herds, family groups), reproduction • Advantages: reduced chance of predation • Uniform • Individuals are fairly evenly spaced • Reasons: social behavior of animals (territories), high levels of competition between individuals • Advantages: reduced competition • Random • Individual spacing is unrelated to others in the population • Does not occur often in nature

7. Changes in population size • Per capita – per individual • Natality– average per capita birth rate (b) • Mortality– average per capita death rate (d) • Immigration– individuals entering a local population (i) • Emigration – individuals leaving a local population (e) • Population growth rate (r): • r = (b + i) – (d + e) • If r = positive number  population is increasing • If r = negative number  population is decreasing • If r = zero  population is staying the same • Examples:

8. Intrinsic rate of increase • Maximum rate of increase when: • Conditions are ideal • Resources are abundant • Population density is low • rmax • Factors which influence this: • Age at which reproduction begins • The fraction of the life span devoted to reproduction • The number of reproductive cycles • The number of offspring produced each cycle • Different species have different intrinsic rates… • Small organisms have high rates (bacteria); large species have low rates (elephants)

9. Exponential population growth • Optimal conditions allow a constant per capita population growth (rmax) • The larger the population gets, the faster it grows • J shape curve: • Organisms cannot reproduce this way indefinitely because of increased: • competition, predation, disease, wastes

10. Logistic population growth • Population growth rate nears zero • Occurs near the environment’s limits to support the population • Carrying capacity (K) – the largest population that an area can maintain indefinitely, assuming no changes in the environment • S shape curve:

11. Density-dependent regulatory factors • Cause an increase in death rate as the population increases • Affect a larger proportion of the population, not just a larger number • Can also cause a decrease in death rate as the population decreases • Tend to regulate a population at a relatively constant size near the carrying capacity • An example of a negative feedback system

12. Cyclical population fluctuations

13. Intraspecific competition • Interference competition • Also called contest competition • Dominant individuals obtain resources at the expense of others (they interfere) • Can cause small drops in population as the individuals unable to compete die • Exploitation competition • Also called scramble competition • All individuals share the limited resource • Can cause wide variation in population size

14. Life history traits • Reproductive strategies differ: • Semelparous– a single, large reproductive effort • Examples: • Iteroparous– repeated reproductive cycles over the life span • Examples: • Two extremes: • r-selected – rapid population growth rate • Small size, early maturity, short life span, large broods, little or no parental care • Found in variable, temporary, or unstable environments • K-selected – keeps population at or near carrying capacity • Large size, late reproduction, long life span, small broods, parental care of young • Found in constant or fairly stable environments

15. Survivorship curves • Measures the probability of survival to a particular age • Three basic types: • Type I – young have a high chance of survival, probability of survival decreases with age • humans • Type II – the probability of survival does not change with age • squirrels • Type III – the probability of death is highest early in life • oysters

17. metapopulations • Different populations within an ecosystem • Due to variations of habitats: • Source habitats – high quality habitats, plenty of resources, lots of reproductive success, greater population density • Sink habitats – lower-quality habitats, fewer resources, less reproductive success, lower population density • Humans are increasing the number of metapopulations as they fragment ecosystems

18. Human Populations Our population continues to increase, but this is due to a decrease in the death rate rather than an increase in the birth rate. This decrease is due to: Greater food production Better medical care Improved sanitation practices Zero population growth (r = 0)

19. Human Demographics Highly developed countries: Low population growth rates Highly industrialized Low infant mortality rates Example: Developing countries: Moderately developed Birth rates and infant mortality rates high but declining Medium industrialization Examples: Less developed Highest birth and infant mortality rates Lowest amounts of industrialization Examples:

20. Age Structure Diagram The number and proportion of people at each age of a population

21. Age Structure Diagrams… Rapid growth: Pyramid shaped – as children mature, they become parents of the next generation and there are more of them than the previous group Slower growth or decline in population: More tapered bases – the number of prereproductive and reproductive ages are close to the same  slower, but still positive growth Small bases – a higher percentages of postreproductive ages than reproductive and prereproductive ages  negative growth (decline)

22. Environmental degradation People overpopulation…. Developing countries Examples: Consumption overpopulation… Developed countries Examples: