ENVIRONMENTAL SCIENCE Chapter 8 Understanding Populations 8.1 How Populations Change in Size

8.1 How Populations Change in Size Objectives Describe the three main properties of a population. Describe exponential population growth. Describe how the reproductive behavior of individuals can affect the growth rate of their population. Explain how population sizes in nature are regulated.

Introduction While researching population growth, Charles Darwin calculated that one pair of elephants could theoretically produce about 19 million descendants in just 750 years! The actual number of individuals is controlled by their environment. In order to understand species better, they are often studied at the population level.

What is a Population? All the members of a species that live in a particular place at a particular time is referred to as a population. A population makes up an interactive gene pool. The term population can also be used to refer to the number of individuals that make up the population.

Properties of Populations Three major properties of populations may be measured to describe them. Density – the number of individuals per unit area or volume Dispersion – the relative distribution or arrangement of individuals such as even, clumped, or random Size – the total number of individuals in the gene pool at a given time

How Does a Population Grow? As offspring are produced, a population gains individuals. As individuals perish, the population declines. Growth rate is the change in size of a population over a given period of time. Births – Deaths = Change in Population Size

How Does a Population Grow? Birth rates and death rates can fluctuate, thus causing growth rates to change. Growth rates can be positive, negative, or zero. For a growth rate to be zero, the average number of births must equal the average number of deaths for the given time. If adults lost from a population are not replaced by new offspring, the growth rate will be negative, resulting in a shrinking population.

How Fast Can a Population Grow? Over her lifetime, a female sea turtle can lay 2,000 eggs. If all of those eggs produced turtles that survive to adulthood, the turtle population would explode exponentially. They do not all survive. The population of any species tends to remain stable from year to year due to environmental factors.

How Fast Can a Population Grow? Environmental factors have an impact on how a species evolves. The fastest rate at which a species can grow is called it’s biotic potential. The biotic potential is determined by the reproductive potential of the species – the maximum number of offspring that each member of a species can produce. The reproductive potential of species can vary greatly.

How Fast Can a Population Grow? In contrast to the biotic potential of a pair of elephants, a bacterium can produce 19 million descendants in a matter of a few days. Generation time is the average time it takes a member of the population to reach the age when it reproduces. Reproductive potential varies on three points. Number of offspring produced at a time Time between reproductive cycles Point in life when reproduction begins

How Fast Can a Population Grow? When graphing population growth, most populations will fit either an “S – curve” or a “J – curve” prior to reaching carrying capacity. Carrying capacity for the ecosystem for a particular species is the maximum population that can be supported without sustainability of the environment being compromised. Most large mammal populations fit the “S curve” pattern whereas insects typically fit the “J – curve” pattern. Humans are an exception – we fit the “J – curve” pattern.

What Limits Population Growth? Populations do not grow to their reproductive potential because conditions are not perfect or constant. Resources are limited, environments change, new predators arrive, diseases spread through populations, and other factors place pressure on populations. Only some members of a population, typically those best adapted – or fit – tend to survive. Because of this, gene pools and frequency of alleles change over time.

What Limits Population Growth? Though a population may increase beyond carrying capacity, it cannot remain at that level. Carrying capacities for species may be difficult to predict because of changing environmental factors. It could be said that a species reaches carrying capacity when it consumes a particular natural resource at the same rate at which the ecosystem replaces that resource.

What Limits Population Growth? Members of a particular species eventually have to compete for available resources. Indirect competition for social dominance or for territory also exists. Territory is an area defended by an individual or group against other individuals. The territory provides all of the resources necessary for survival, therefore, it is important to defend and maintain.

Two Types of Population Regulation Mortality in a population may be density dependent or density independent. Mortality that is density dependent occurs more rapidly in a crowded population than in a sparse population, such as disease. Density independent mortality affects a certain portion of the population no matter how dense or sparse the population, such as a winter storm or volcanic eruption.

References Orca Population - http://seattlepi.nwsource.com/local/248515_orcas16.html Sea Turtle Hatchlings - http://www.floridamarine.org/features/view_article.asp?id=21026 Fern Sporangium - http://io.uwinnipeg.ca/~simmons/16labman05/lb3pg6.htm African Elephants - http://www.wallpapers-zone.com/wallpapers-animaux-elephants-elephants_120.jpg-1-24.html

References Bacteria - http://www.carnegieinstitution.org/first_light_case/horn/lessons/images Growth Curves - http://www.chem.duke.edu/~bonk/Chem8304/enote1405.html Carrying Capacity - http://online.moraine.cc.il.us/WebSupported/BIO112/plants_and_ecology_notes.htm