Population Biology

Definition

A population is a group of organisms of the same species, interbreeding or closely related through interbreeding and evolving as a unit.

Review

Community: a group of interacting plants and animals forming an identifiable group

Review

Biosphere: the entire part of the earth where organisms are

found

Review

Habitat: the place where an organism naturally lives or grows

Review

Niche: position or function of an organism in its community – its occupation

Population Size Calculations

Simplest calculations involve changes in populations over a time period in a defined space.

gr= N/ t

Ex) What is the growth rate?

Initial Population 20020 gulls move in(immigration) 200 gulls move out(emigration) 032 chicks hatch(natality) 3210 chicks die -102 adults die -2Time 1 year

Total 240Annual Increase of +40 birds per year

Density

Formula: D = N/A or N/V

Density equals number of organisms divided by area (or volume or space)

Eg. 200 bison in a 100 acre pasture is a density of 2 bison per acre

Rate of Change

Often uses density

R = D / t

Rate of density change equals change in density over change in time.

Example

In 1996 there were 10 Grizzly Bears in a 10 000 ha forest. In 2005 there are only 8. What is the rate of density change?

R = D / t

R = 0.0008 – 0.001 / 2005 – 1996R= - 0.0000222 bears/ha/y

Per Capita Growth Rate

Or cgrThe amount that a population changes

per individual over a set period of time

Cgr = N / NPer capita growth rate = change in

number divided by initial population size

CGR

A lynx population was 19 per 10 000 sq. km in 1991. In 1993 it was 3 per 10 000 sq. km. What was the cgr of this population from 1991 to 1993?

Cgr = N/ N

= -16 / 19

= - 0.84 per lynx

Distributions of Populations

Can be clumped – more individuals together than apartOften involves cooperation among group

members (eg. Herd, pack)

Distribution of Populations

Can be random – not seen often in natureOrganisms have no effect on each other

Distribution of Populations

Can be uniform: evenly distributed Usually due to competition between

individuals Territories, etc.

Open Populations

Are those where organism can enter or leave

Often have S-shaped curves

Closed Populations

True closed populations are rareOn islands, isolated communities

Population Growth Graphs

typically have numbers on vertical axis and time on horizontal axis

Logistic Growth

S shaped curves are typical of stable populations.

Eg. Wild Horses on reserve land in AB

Exponential Growth

J shaped curve (initially) occurs with short-lived populations that rapidly deplete their environment

Eg. Flies on a carcass

Overshoots

Result when k is greatly exceeded and the environment deteriorates

Carrying Capacity

Is the number of organisms a habitat can sustain over the long term

“k” Influenced by Biotic

Potential and Environmental Resistance

Biotic Potential

maximum number of offspring produced

capacity of offspring to survive to reproducenumber of times per year an organism

reproducesage at which offspring are reproductively

mature

Generally speaking, smaller, simpler organisms have a higher biotic potential than larger organisms.

Environmental Resistance

limiting factors on a population Availability of resources

(food, water, space, etc.) Competition for resources

with other organismsIntra-specific –

within a speciesInter-specific –

between a species Predators Disease Climate change

Environmental Resistance

Puts brakes on biotic potential (B) – maximum reproductive rate

Environmental Resistance

Environmental Resistance

B

K

Environmental Resistance

Limiting Factors on Populations

Law of the Minimum: if any one of many needed nutrients/limiting factors is reduced below the required levels, the population growth rate declines

Limiting Factors

Can be density independent – those that will affect a population regardless of its sizeEg. Cold winter

Limiting Factors

Can be density dependant – those that increase when the population size increasesEg. Disease

PredationFood Supply

Gause’s Law

Competitive exclusion

No two species can remain in competition for a limited resource

Other GraphsSurvivorship curves

Other Graphs

Age distribution pyramids

Population Histograms

are graphs showing the composition by age and gender of a population at a specific time. Population histograms have the following characteristic shapes:

R and K Population Strategies

This is a continuumMost populations fall between these two

extremes

K Selection

k-selected species is one that typically has:Stable environmental conditionsSlow growing individualsLow reproduction rate (B)Parental care of offspring

R- Selection

An R-selected species is one that typically has:Unpredictable environmentSmall individuals with short life spansReproduce at a high rateLittle or no parental care

Life History Patterns

Some organisms undergo regular patterns of growth and decline known as population cycles

Small rodents, rabbits, lemmings often cycle every 1 – 4 years

Population Cycles

Can be due to fluctuations in food supply, predation, or both

Predator - Prey Cycles

Chaos Theory

Used by population biologists to study the general trends in populations

Small uncertainties in short-term prediction of individual events may be magnified to such an extent that complex systems become quite unpredictable

Chaos

The ‘butterfly effect”This is the sensitivity of a system to the

initial conditionsChange any starting parameter slightly

and the resulting changes magnify until the result is very different from the initial prediction

Technologies

Include:Radio collars

Technologies

Sampling methods Quadrats –

counting organisms in defined areas

Transects – counting organisms that touch line

Technologies

Mark/recapture studies

Symbiosis – a relationship between two individuals of different species.

Parasitism – one species lives in or on another where it obtains food and resources. The host is usually harmed by the relationship. Ex. Tapeworm

Symbiotic Relationships

Many parasites are r-selectedEg. Tapeworm

Commensalism – one species lives on or near another, but while one species benefits, the other is unaffected.

Mutualism – two species that live in close association with one another, where both benefit from the relationship.

Ex. honeybee and flower

Ex. shark and remora

Ecological Succession

Ecological succession – is the gradual and orderly change of a community as it is either developed from bare land or replaced by another community.

Succession

Primary Succession – the gradual colonization of an area that has not supported an ecosystem before. (from bare rock)

PrimarySuccession

Secondary Succession – the colonization of an area that once supported an ecosystem that was destroyed by fire, flood, etc.

Exploring Time Gallery Display

Pioneer Community – is the first species to appear during succession.

Climax Community – is the final stable community that results at the end of succession.

Generalizations about Succession:

Species composition changes more quickly at earlier stages.

Total number of species increases dramatically at early stages, levels off at intermediate phases and declines at the climax stage.

Food webs develop in complexity as succession progresses.

Total biomass increases during succession and levels off at the climax stage.