Populations Dynamics

Chapter 36

I. Environmental Factors

Living organisms are influenced by a wide range of environmental factors. These can be two types:

Abiotic factors – nonliving environmental factors (water condition, air quality, salinity, temperature, soil content, etc.)

Biotic factors – living organisms that interact with the organism we are studying (prey, predators, infectious agents etc.)

II. Population ecology Population ecology is the study of how and

why populations change. Also studies population numbers, where you find the studied population and how many organisms live in it, how/why it increases/decreases in numbers.

Population – a group of individuals of the same species that live in the same general area (same resources, same environmental influences, common breeding, social interactions)

III. Population Growth The number of individuals comprising a population

may fluctuate over time. These changes make populations dynamic.

A population in equilibrium have about the same individuals from one generation to the next.

Factors that can change the number of individuals in a population:

Birth (B) Death (D) Immigration (I) Emigration (E)

IV. Population Structure (density, dispersion and age structure)A. Population Density – the number of

individuals of a species per unit area or volume.

Measuring population density: By counting Sampling techniques (

http://www.biologycorner.com/flash/mark_recap.swf )

Patterns of distribution Clumped Uniform Random (rare, temporary)

B. Age structure is determined by Life tables – Determine the average

lifespan to study the dynamics of population growth.

http://www.ssa.gov/OACT/STATS/table4c6.html

Suvivorship Curves – they plot the # of individuals alive in each age Type I – produce few offspring, take care of their young,

many survive into maturity Type III – high death rates for the very young, mature

individuals survive longer, usually involves very large # of offspring with little or no parent care.

Type II – intermediate, more constant mortality over the entire lifespan

V. The Patterns of Growth The Exponential Growth Model

The rate of population increases exponentially under ideal conditions (high birth rate, low death rate).

The population multiplies by a constant factor during each time interval.

Endless resources J-shaped curve Colonizing populations http://www.pbs.org/wgbh/nova/earth/global-population-growth.html

During exponential growth, all members of the population will have the maximum capacity of reproduction.

B) Carrying Capacity – Logistic Growth Environmental factors limit population growth. With limiting factors included growth curve resembles a

logistic growth curve (S-shaped curve)

K : Carrying capacity – the maximum population size that a particular environment can support. It’s value depends on the species and environmental resources.

Ultimately, a population would stabilize on the carrying capacity (K) – birth rate equals death rate.

C) Factors Limiting Population Growth Density-dependent factors – factors that have

greater effect when the population density is higher. Competition for food, territory Health of organisms Predation Parasites Physiological factors (reproduction, growth, hormonal

changes)

Density-independent factors – Regardless of population density, these factors affect individuals to the same extent. Weather conditions Acidity Salinity Fires Catastrophies

Boom-and-bust cycles – the number of individuals within the population seems to show a cyclic change. Predator/prey relationships Changing food supply

Figures 36.5C and 36.6 on page 733.

VI. Age Structure Diagrams

Human population can also be described by age structure diagrams. These diagrams are frequently dependent on the economy and social state of the country that they are measured in.