Chapter 8: Properties of Populations 11/5/15 9:31 AM

A population is a group of individuals of the same species that inhabit a

given area and are able to interbreed

Populations have structure:

o Density

o Spacing

o Age distribution

Populations are dynamic, changing over time

o When populations crashes, what has happened? The

environment is changing, lost, disappearing; disease?

Huge shift in population = invasive species

Organisms may be unitary or modular

Unitary organisms exists as individuals

Modular organism, the zygote develops into a module, a unit of

construction

o Plants are modular

- Cut basil plants right above “starter leaves” and above the nodes

Stolon and Rhizome

Reproduction in Plants

Suckers – many stems that sprout from surface roots and may

appear to be individuals

Genet – plant produced by sexual reproduction, a genetic individual

Ramet – plant produced asexually; an ecological unit

Clones and Parent trees communicate through each other in their root

systems underground; can also spread disease through roots as well

Individual = genet

Module = ramet

Both of the individual and module must be recognized; therefore

ramets are often counted as individual members of the population

The distribution of a population defines its spatial location

Determine areas where individuals are present and where they are

absent

Map the position of each individual

The geographic range of a species encompasses all of the

individuals of a species

Individuals are found in suitable habitats within that geographic

range

This range is limited by tolerance to different environmental

conditions, including soil moisture, temperature, etc

Ubiquitous species have a geographically widespread distribution

Endemic species have a geographically restricted distribution

Why we care? They have a lot of biodiversity but if we wipe out an

area, the plant will go extinct

We want to know where they are to not lose the biodiversity

There are many types of geographic barriers that reduce or prevent

individuals from moving and colonizing new areas

Bodies of water, including rivers

Mountains

Large areas of unsuitable habitat such as deserts

Interactions with other species can also serve as barriers

Competition

Predation

Range

Individuals not distributed equally across a geographic range

Suitable habitat within that range

o Distribution of how organism is in the landscape influences

the way you count the organism and figure out the population

Metapopulations

Collection of local subpopulations

Ecologists can only study a small population

Subpopulations are spatially separated but connected by the

movement of individuals

Metapopulation is to take all the subpopulations and do a statistical

analysis and look at overall picture

Accrue density

Number of individuals per unit area

Abundance = # of individuals in the population

Population density: = # of individual/area

Distribution

In a clumped distribution, individuals are found in groups

The most common spatial distribution is CLUSTERS

o Suitable habitat or resources found in patches

Nearly Regular distribution = good estimate of landscape from anywhere you

sample

Ecological density – the number of individuals per unit of

available living space

o Only look at areas where animals can live

Determining Density Requires Sampling

Population (abundance)

o Population density x the area occupied

A complete count may be possible if both the abundance and area

occupied are small, or if an area is very open so that all individuals

can be seen

Often times organisms are cryptic

Citizen Scientists

It is impossible to take complete census of a highly mobile animal

Geo-referencing = identify where sightings are made, can reveal

changes in the distribution

Secchi – a secchi disk tool to measure water turbidity

o Depth is then recorded on the app and uploaded to a

database

o Can give a number count of how many plankton is distributed

in that location

Random Sampling = have bias for the sample of your species so reduce

sampling is to have random sampling and as many sample sizes

Common method:

Mark-recapture = is the most commonly used technique to measure

animal population size

HOW DOES IT WORK?

o Capture an organism and mark it, release them back into te

habitat; give enough time to intermingle and come back and

capture the organism again

o If you find 4 marked ones then you can use a proportion and

figure out approximately how many are found in the

population

Proportion:

o N = total population

o M = initially capture and marked

o S = captured animal on second visit

o R = number of individuals marked on second capture

Assumptions:

No effect of marking on probability of recapture

Mixing of marked and unmarked – mix into the entire population

Capture individuals are representative of the whole population

Marks are not lost

Measure of population structure including age, developmental stage, and

size

Abundance does not provide information about age structure

A population with overlapping generations has an age structure

o There are individuals in different age classes

Populations can be dividing into three ecologically important age

classes

o Pre-reproductive

o Reproductive

o Post-reproductive

- age is hard to measure in animals so we measure via size to judge the age

the most accurate method is to mark young individuals in a

population and follow their survival through time; this is expensive

and takes a lot of resources

o many individuals must be marked and subsequently checked

at regular intervals, often over many years

trees with seasonal growth produce annual growth rings

dendrochronology – counting annual growth rings to determine

the age of a tree

size of the tree based on diameter at breast height (dbh)

recent studies suggest that there are annual growth rings in the

roots that can be used to provide information of its age

How do you trees die if you don’t cut it down?

Starts to show distress

As it gets older and closer to an age; starts to give off hormones

that show signs of distress, then fungus comes and starts to digest

the tree and it starts to rot inside out

Then the tree eventually falls out

Sex ratios in population may shift with age

In birds, over time the number of males tends to be higher than

females

o Nesting females are more susceptible to attack and predation

Individuals move within the population

Dispersal is the movement of individuals in space

o Emigration = when individuals leave a subpopulation

o Immigration = when individuals enter a population

Many organisms disperse passively

- animals can be important dispersers of plant seeds

dispersed when an animal eats the fruit

wind carries the young of spiders

water carries the larvae of invertebrates downstream

active dispersal in mobile animals may occur in any age class or sex

o in birds, most dispersal occurs in the young

- migration is movement of organisms that is round-trip

Invasive Species

Dispersal can affect the spatial distribution or can establish a new

subpopulation in a previously unoccupied habitat

Species introduced into an area where they did not previously live

can expand into new areas – if they expand in exponential way they

often become invasive

These introductions may be intentional or unintentional

Example: Asian Longhorned Beetle

How have humans aided in the dispersal of many species around

the world?

o World cargo shipping lanes

11/5/15 9:31 AM

11/5/15 9:31 AM