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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.