Population Concepts and Growth Patterns

1. Chapter 7 Population Concepts • Population - a group of individuals of the same species occupying an area and capable of interacting with each other. •  Natality - # of individuals added to population through reproduction. “birthrate”—born per 1000 individuals per year • Mortality - # of individuals removed from population through death. “deathrate”—die per 1000 individuals per year

2. Chapter 7 Ability of a population to grow is a function of: •  sex ratio: number of males to the number of females. •  age distribution: number of individuals in each “age bracket”  • pre-reproductive age <15 • reproductive age 15-44 • post-reproductive age >44 • - survivorship curves - population pyramids

3. Chapter 7 Population size influenced by dispersal  • Movement from dense population to new areas • emigration - outward migration • Pressure (competition) is typical reason individuals leave a population. • Typically juveniles leave, relieve overcrowding •  immigration - inward migration • Enter a population where pressure is less severe. • Bring genetic/cultural characteristics

4. Chapter 7 Population Growth Curve • A population’s inherent reproductive capacity is called its biotic potential. • Plant versus animal  • Typically, biotic potential is higher than that need to simply replace the parents. • è most (many) offspring die, with few surviving to be reproductive adults. • Usually more than 2 survive. è population size grows.

5. Chapter 7 Growth is typically “exponential”, until a stable size is reached. • Lag phase • It takes time for the first mating pairs to reach sexual maturity, mate, and produce offspring. • Exponential growth phase • As more and more individuals mature, more offspring are produced. • Continues as long as birth rate exceeds death rate • Stable equilibrium phase • Birth rates and death rates become equal. • Typically, death rate increases first. Often followed by a decrease in birth rate.

6. Chapter 7 Carrying Capacity • The number of individuals that can be supported in an area over time. Determined by environmental resistance. • Limiting Factors • Key components to limiting size of population • availability of raw material • availability of energy • accumulation of waste products (can result in “death phase”) • interactions between individuals • If fix one, carrying capacity can increase, but new primary limiting factor emerges

7. Chapter 7 K-strategist Start 2/25/02 • Have long lives, few offspring and provide much care for each offspring. • Usually large organisms • Strategy: exert much energy into a few offspring that have a good chance of surviving. • Examples—lion, deer, human • Limited by density-dependent factors. • Becomes more severe as size of population grows

8. Chapter 7 r-strategist • Have short lives, many offspring and provide little or no care for the offspring. • Usually smaller organisms • Strategy: If only a very small percentage of offspring survive, produce MANY offspring. • Examples: frogs, fish, dandelions  • Oyster may produce millions only couple survive • Typically limited by density-independent factors and do not reach carrying capacity. • Changing weather • Pond dries up • Destruction of food source

9. Chapter 7 Human Population Growth • Current rate of increase is 1.4%. • Current population size is around 6 billion. • Very long lag time. • Explosive growth in last 200 years. • Currently in exponential growth phase • Major reason for continuing increase is reduction in death rate • Method of reduction shared throughout the world • No change in birth rate

10. Chapter 7 Environmental Resistance for Humans • raw materials • available energy • waste disposal • interactions with others  • We will, with absolute certainty, reach our carrying capacity. What that level is, nobody knows. Earth’s population will double again by 2050.