Population Parameters – Chapter 9

Population – a group of organisms of the same species occupying a particular space at a particular time.

Demes – groups of interbreeding organisms• local population• smallest collective unit of a plant or animal population

Populations are units of study.

Population Attributes

• Density – number of organisms per unit area or per unit volume

• Affected by:– Natality – the reproductive output (birth rate) of

a population– Mortality – the death rate of organisms in a

population– Immigration – number of organisms moving

into the area occupied by the population– Emigration – number of organisms moving out

of the area occupied by the population

Unitary and Modular Organisms

• Unitary organisms – individual units such as humans or mice

• Modular organisms – organisms that do not come in simple units of individuals– Several grasses that are attached by runners– Corals, bryozoans

Population Density

Four primary population parameters:

Density Examples

Two fundamental attributes that affect our choice of techniques for population estimation are the size and mobility of the organism with respect to humans.

Abundance and Animal Size

Birds are less abundant than mammals of equivalent size.

Density Relationships

All slopes are negative

Two Types of Density Estimates

• Absolute Density – a known density such as #/m2

• Relative Density – we know when one area has more individuals than another

Measuring Absolute Density

• Total Count – count the number of organisms living in an area– Human census, number of oak trees in a

wooded lot, number of singing birds in an area– Total counts generally are not used very often

• Sampling Methods – use a sample to estimate population size– Either use the quadrat or capture-recapture

method

Quadrat Method• A Quadrat is a sampling area of any shape

randomly deployed. Each individual within the quadrat is counted and those numbers are used to extrapolate population size.– Example: a 100 square centimeter metal

rectangle is randomly thrown four times and all of the beetles of a particular species within the square are counted each time: 19, 21, 17, and 19. This translates to 19 beetles per 100 cm2 or 1900 per m2.

Transects as Quadrats

• Each transect was 110 meters long and 2m wide (220 m2 or 0.022/ha). All trees taller than 25 cm counted.

Capture-recapture Method

• Important tool for estimating density, birth rate, and death rate for mobile animals.

• Method:– Collect a sample of individuals, mark them,

and then release them– After a period, collect more individuals from

the wild and count the number that have marks– We assume that a sample, if random, will

contain the same proportion of marked individuals as the population does

– Estimate population density

Marked animals in second sample

Total caught in second sample

Marked animals in first sample

Total population size

=

520 N

16= N = 64

Peterson Method

= Proportions

Assumptions For All Capture-Recapture Studies

• Marked and unmarked animals are captured randomly.

• Marked animals are subject to the same mortality rates as unmarked animals. The Peterson method assumes no mortality during the sampling period.

• Marked animals are neither lost or overlooked.

Multiple Census Capture –Recapture Will Be Done in Lab.

Indices of Relative Density

• Assume that samples represent some relatively constant but unknown relationship to total population size.– # cars in the Piggly Wiggly parking lot

• Provides an index of abundance– Is population increasing, decreasing, or

staying the same– Are there more animals in one location than

another?– Can not quantify differences between sites

Twice the number of tracks does not = twice as many animals

Some Indices Used

• Traps• Number of Fecal

Pellets• Vocalization

Frequency• Pelt Records• Catch per Unit Fishing

Effort

• Number of Artifacts• Questionnaires• Cover• Feeding Capacity• Roadside Counts

Natality• Fecundity – physiological notion that

refers to an organism’s potential reproductive potential – Usually inversely proportional to the amount of

parental care given to young

• Fertility – Ecological concept that is based on the number of viable offspring produced during a specific period– Realized fertility – actual fertility rate

One birth per 15 years per human female in the child-bearing ages

– Potential fertility – potential fertility rate One birth per 10 to 11 months per human

female in the child-bearing ages

Mortality

• Opposite of mortality is survival• Longevity focuses on the age of death of

individuals in a population– Potential longevity – maximum lifespan by an

individual of a particular species Set by the organisms physiology (dies of old age) Sometimes described as the average longevity of

individuals living in optimal conditions– Realized longevity – actual life span of an organism

Measured as an average for all animals living under real environmental conditions

Determining Mortality• Mark several individuals and measure how many survive

from time t to t+1.• If abundance of successive age groups is known, then you

can estimate mortality between successive age groups.• Can use catch curves for fish:

147292

Survival between age 2 and 3=

147/292=0.50

Or develop regression equation

Immigration and Emigration

• Seldom measured

• Assumed to be equal or insignificant (island pop’s)

• However, dispersal may be a critical parameter in population changes

Limitations of the Population Approach

• How to determine what exactly is a population– How clear are the boundaries?

• Population does not exist as an isolate– Individuals interact with other members of the

community