Chapter: 6 Population Dynamics To understand the factors regulating populations in the habitat, community, and ecosystem

I. 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

I. Dynamics of Population Growth A. (cont) Ex. 2 2, 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

I. Dynamics of Population 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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth C. Biotic Potential –Based on the ability of an organism to reproduce –The maximum reproduction rate for an organism is its Biotic Potential

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth F. Chaotic and Catastrophic Population Dynamics –S–Since many population growth curves don’t follow linear growth curves, growth curves are called Chaotic –E–Exhibit variability –N–Non-random events –M–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

I. Dynamics of Population Growth 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

I. Dynamics of Population Growth 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 decrease 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations C. (cont) –4 Survivorship patterns Type B –Death rate is unrelated to age –[k-selected reproduction] i.e. constant over the life span Ex. Seagulls

II. Factors that increase or decrease populations C. (cont) –4 Survivorship patterns 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.

II. Factors that increase or decrease populations C. (cont) –4 Survivorship patterns 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations 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

II. Factors that increase or decrease populations 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 Population 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