Organisms live as members of populations
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Organisms live as members of populations Population size Population density Population dispersal How do populations grow - PowerPoint PPT Presentation

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Organisms live as members of populations Population size Population density Population dispersal How do populations grow?. Exponential growth. Exponential growth growth without limits at a maximal rate (biotic potential) Actual growth rate: difference between

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Organisms live as members of populations

Population size

Population density

Population dispersal

How do populations grow?

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Exponential growth

growth without limits at a maximal rate

(biotic potential)

Actual growth rate: difference between

birth and death rates, corrected for migration

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Some species show a very high rate of

population growth (exponential)

reproduce at early age

produce a lot of offspring

life span is short

“r-selected adaptations” for organisms

with that type of life history (and

which don’t consume a lot of resources

and where resources are not limiting)

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Populations have innate capacity for

exponential growth

Usually this is temporary, until carrying

capacity is reached.

What sorts of things limit population growth?

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Most populations are affected by limitations

in their environment

“logistic growth model”: growth is rapid

(exponential) for awhile then tapers off

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Many species are adapted for life at or

near carrying capacity (K-selected)

Compete for resources

Reproduce late in life

Have few offspring-and care for the

offspring they have

Tend to live long lives and have low

mortality rate (low birth rate balanced

by low death rate)

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At carrying capacity, birth rate is equal

to the death rate

What sorts of factors affect population


Density-dependent factors:

increasing density limits availability

of resources for all

Limited food supply

Waste accumulation (toxicity)

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Density-independent factors

independent of population size but

do affect its growth

Weather (can contribute to population


Natural disasters

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Seen in populations of insects, birds

and mammals (e.g., lemmings and

other rodents)


Food supply?

Overcrowdingstress hormone imbalance

and infertility?

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How predators control prey population

As prey population increases, more

opportunities for predators. As predators

switch to alternatives, original prey can


Predators remove weaker prey.

Parasites weaken prey

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Competition within a community



Competition for what?

Food, water, space, breeding sites,

shelter, light…

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Introduced species can upset the whole

balance of activity




Zebra mussels

Prickly pear cacti in Australia- a predatory

cactus moth was introduced to control the


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Structure of populations


Dispersion- how organisms are grouped

in their geographical range

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Clumped pattern

Most common in nature

Unequal distribution of resources

Social behavior


safety in numbers

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Uniform distribution

Often results because of interactions

between individuals in the population

(to deal with scarce resources)

Creosote bushes in the desert- roots

compete for water and nutrients

Nesting penguin pairs (on islands)

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Random- no pattern

no factors contribute to attraction

or repulsion between individuals

Rare, but could occur when there are no

pressures due to environment

Exhibited mainly by plants (why?)

What kind of dispersion is shown by


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Demographics- statistical methods for

analyzing populations

Cohorts- age groups- birth and death rates

populations with many young members

tend to grow faster

Sex ratio- proportion of males to females

effect depends on mating practices

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Community interactions

Community- all the interacting populations

within an ecosystem

Interactions tend to maintain balance

between resources and consumers

Populations act as agents of natural

selection (e.g. predator-prey)

Over time- coevolution

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Major types of community interactions

Competition (harms both)

Predation (benefits predator, harms prey)


parasitism (helps one, harms the other)

mutualism (helps both)

commensalism (helps one, no effect on

the other)

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Competition: how much do ecological

niches overlap?

In plain English: ecological niche is

everything an organism needs to survive

No two species occupy exactly the same

niche, although some aspects of a

niche may be shared

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Competitive exclusion

One survives, the other dies

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Resource partitioning

Minimizes niche overlap

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Remember Darwin’s finches?

Finches had evolved different beak shapes

and feeding behaviors that minimized

competition among them

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Members of species, or closely related

species, compete for limited resources

Two species of barnacles that live in

intertidal zone

One species (A) dominates the upper zone

The other (B) predominates in the lower

(wetter) zone)

When the lower species is removed, the

upper species spreads into the area

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Conclusion: B outcompetes A (A can’t

grow in the lower region unless

B is removed)

However, A has an advantage on the upper

shore because it can tolerate low tide

(studies of Connell et al.)

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Predator-prey interactions

Prey usually outnumber predators

Herbivores “prey” on plants

Predators and prey exert a lot of pressure

on each other:

Prey evolve to become more difficult to catch

Predators evolve to become better hunters

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Coral snake


King snake


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Monarch butterfly has adapted to milkweed

(a toxic plant)

Lays eggs on milkweed

Larvae feed on milkweed

Caterpillars store toxin in their own tissues

as defense against predators!

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Even grasses and their consumers have


Grasses have tougher consistency

Herbivores evolved teeth for grinding

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Keystone species- has an especially

important role in its community

“Top predator”

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Predatory starfish Pisaster

When it is removed, its prey (mussels)

overgrow and outcompete algae and other


Elephant is another example

Grazes on shrubs and small trees; helps

maintain savanna

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Primary succession can begin on bare rock

Secondary succession happens when an

ecosystem has been disturbed.

Time span is much shorter

Primary succession has been studied on

Isle Royale in Lake Superior

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As rock is weathered, nutrients are released.

Lichens are the pioneering organisms

(washed up by the water?)

Mosses grow in the cracks, and trap soil

Dead mosses provide nutrients and trap


Seeds of larger plants germinate; as they die

they contribute organic material to soil

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Woody shrubs take advantage of the soil

(and their shade kills off the lichens and


Eventually trees take root and overshadow

the shrubs

Larger trees overtake the smaller trees and

form the climax forest

This takes several hundred years.

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Secondary succession is seen in abandoned


fieldweeds perennial plants shrubs 

pines and fast-growing deciduous trees 


Common theme:

plants get progressively bigger, and

their shade kills off the previous plants

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Climax community


Varies depending on the biome- from rain

forest to desert to tundra

Many ecosystems are maintained at


Prairie does not progress to forest

Cultivated land (farm or lawn)