Chapter: 6 Population Dynamics

Chapter: 6 Population Chapter: 6 Population DynamicsDynamics

To understand the factors regulating To understand the factors regulating populations in the habitat, community, populations in the habitat, community,

and ecosystemand ecosystem

I. Dynamics of Population GrowthI. Dynamics of Population Growth

• A. Exponential Growth and Doubling Times– Ideal environmental conditions can cause a

population to grow exponentially • Exponential growth is growth at a constant rate

(per unit of time)• Can be expressed as a constant fraction, or as an

exponent, by which the original population is multiplied

– Usually yearly in macro-organisms– Sometimes hourly or daily in micro-organisms


• A. (cont)• Ex. 22, where the 2 is the exponential growth rate

– 2, 4, 16, 64, etc.

– Also called Geometric growth• the sequence of growth follows a geometric pattern

of increase• Graphically looks like a J• Called a J-shaped curve

– Sometimes called “unfettered” growth


• A. (cont)– Population doubling times are necessary to

predict the effectiveness of changes to the habitat

• Useful rule of thumb to find doubling rates of the population is the 70 rule

• Divide 70 by the annual percentage growth rate


• A. (cont)– Ex. A population has an annual growth rate of 35%,

therefore, the doubling time will be 70 / 35 = 2. Thus, the population will double every 2 years

– Ex. The US population has a growth rate of 1.2%, What is the doubling time? (initial pop is 300 million)

• 58 years, 300 million to 600 million


• B. Arithmetic Growth– Less than ideal environmental conditions will

produce a population growth rate that is a constant fraction that is added to the original population

• Called Arithmetic Growth– Produces a straight line on a population graph


• C. Biotic Potential– Based on the ability of an organism to

reproduce– The maximum reproduction rate for an

organism is its Biotic Potential


• D. Population Oscillations and Irruptive Growth– In Vitro, populations can have no limits– In Situ, populations have limits– Negative growth rates occur when the

population exceeds the carrying capacity for the habitat

• The carrying capacity is the maximum number of organisms a habitat can have at any given time period


• D. (cont)– Negative growth rates are called Dieback

• The death rate is greater than the birth rate

– A small population growth above the carrying capacity is called overshoot

– A large population growth above the carrying capacity is called a population explosion

• A large negative population growth rate is called population crash


• D. (cont)– Malthusian Growth or Irruptive Growth is

when there is a population explosion followed by a population crash

• Populations grow until they exhaust resources (typically food)

• May occur repeatedly• Can occur irregularly

– Isle de Royal, Newfoundland Canada


• E. Growth to a Stable Population– Internal and external factors which regulate

population growth• “Harmony” with the environment

– May initially experience exponential growth, but slow as resources dry up

• Closer to carrying capacity

– Called Logistic Growth Model• Add to growth rates, environmental resistance • Looks like an S, graphically, then the tail moves

above and below the carrying capacity line– Also called a sigmoid curve


• F. Chaotic and Catastrophic Population Dynamics– Since many population growth curves don’t

follow linear growth curves, growth curves are called Chaotic

– Exhibit variability– Non-random events– Minute differences in conditions, change the

populations dramatically• Small events strung together form a large affect


• F. (cont)– Catastrophe theory is hypothetical

• Used by biologists to explain population dynamics showing abrupt discontinuities

• Catastrophic systems may jump from one state to another

– Chaotic systems can be predicted over a longer period of time, catastrophic can not


• G. Population Growth Strategies– Malthusian growth strategies are followed by

most animals in the lower trophic levels• Some are pioneers• Most are generalists• Use large numbers to offset predation• Little investment to the individual• Called Extrinsically (externally) controlled growth

or, r-selected (strategies) controlled reproduction• Most insects, rodents, marine invertebrates,

parasites, crustaceans use this method


• G. (cont)– Logistic Strategies are followed by animals

higher up the trophic levels• Larger organisms• Live longer• Mature slowly• Provides more care for offspring• Called intrinsically (internally) controlled growth, or

k-selected (strategies) controlled reproduction

II. Factors that increase or II. Factors that increase or decrease populationsdecrease populations

• A. Natality, Fecundity, and Fertility– Natality is the production of new individuals

• Tied to nutrition, climate, soil, water, and species interactions for success

– Fecundity is the physical ability to reproduce• Does not mean they will mate• Can have high fecundity without high Natality

– Fertility is the number of offspring produced


• B. Immigrations– Introduced organisms into a new habitat or

community• Ex: seeds, spores, boats, wind (floating)

– Increases population growth rates

• C. Mortality and Survivorship – Mortality is the ability to die

• Called death rate• Number of living divided by the number of deaths

in a given amount of time


• C. (cont)– Survivorship is more important to scientists

• The percent of the population/ that survives to the next year

• A cohort is all of the individuals that are born in a specific generation

– Life expectancy is the probable number of years an individual will survive

– Life span is the maximum number of years a person can survive

• Very different amongst organisms


• C. (cont)– 4 Survivorship patterns

• Type A– Tend to live full life expectancy– Low death rate in pre-reproductive and reproductive

years– Higher death rate in post-productive years – [k-selected reproduction]

• Ex bears, whales, humans, elephants



• Type B– Death rate is unrelated to age – [k-selected reproduction]

• i.e. constant over the life span• Ex. Seagulls



• Type C– Tend to have high mortality rate in the pre-reproduction

period (juvenile), once they reach the reproduction stage, very high survival rate until post-reproduction stage

– [r-selected reproduction]• Ex. Song birds, rabbits, deer, etc.



• Type D– Very high mortality rate in early life (most prey species),

when they reach reproduction stage very low mortality rate, even through post-reproduction stage

– [r-selected reproduction]• Ex. Crustaceans, fish, plants, insects


• D. Age Structure Diagrams– Combine mortality and natality– Proportions of individuals in various age

classes• Pre-reproduction• Reproduction• Post-reproduction

– Population momentum is dependant on the number of individuals in the pre-reproductive stage


• D. (cont)• Very large number, compared to reproductive

group, is increasing population growth• Same size number, compared to reproductive

group, is a stable population• Very small number, compared to reproductive

group, is a decreasing population


• E. Emigration– The movement of organisms out of a

population permanently– Different from migration

• Migration is temporary and the organisms will return during the next cycle


• F. Population Growth Equation– PG(R) = (BR + I) – (DR + E)

• PG = Population Growth (rate)• BR = Birth Rate• I = Immigration• DR = Death Rate• E = Emigration

– Can be positive or negative• Growing population is positive• Decreasing population is negative

III. Factors that Regulate III. Factors that Regulate Population GrowthPopulation Growth

• A. General Information– Can be intrinsic– Can be extrinsic – Can be biotic and/or abiotic– Can be density dependant– Can be density independent

• Biotic regulators tend to be density dependant• Abiotic regulators tend to be density independent

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Chapter: 6 Population Dynamics