Communities and
Ecosystems
Biological Community

The collection of all populations of all
species living together and potentially
interacting in a restricted area (biotic
components of the ecosystem).
• Examples are all of the organisms within a pond
or a forest.

Populations of different species in a
community interact and form a system with
its own special attributes that cannot be
predicted based on knowledge of the
individual populations alone.
Species Diversity


The variety of different organisms
included in a community.
Diversity has two components:
• species richness

the total number of different species.
• relative abundance

the number of individuals of each species.
Trophic Structure

The feeding relationships between and
among all of the species that make up the
community
• Food chains
• Food webs

The trophic structure determines the way
that energy and nutrients pass through
the community
• From photosynthesizing plants, through
herbivores, through carnivores, and ultimately
to the decomposers
Trophic Levels in a Food Chain
Bluegills
2o
consumers
Daphnia
1o
consumers
Humans
4o
consumers
Bass
3o
consumers
Algae - producers
Trophic Level Producer/Consumer Plant/Herbivore/Carnivore
Example
Fifth
Decomposers
Decomposers
Fungi &
Bacteria
Fourth
Tertiary
Consumer
Larger Carnivore
Mountain
Lion
Third
Secondary
Consumer
Carnivore
Weasel
Second
Primary
Consumer
Herbivore
Gopher
First
Producer
Photosynthetic
Green Plant
Wild
Mustard
The organization of a typical terrestial food chain.
Food Webs

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Food webs essentially show "who eats whom"
within a community or ecosystem.
The direction of energy flow is shown by the
arrows. Since food chains rarely follow a precise
linear sequence, the food web is a better way to
show energy flow between different trophic
levels.
The food web interconnects several different food
chains within the community.
Ecological Succession

The orderly, predictable sequence of
stages in the development of a mature
(climax) community over time.
• Start with a clear area with no communities
• Succession begins with a PIONEER community,
which includes opportunistic settlers, such as
low weeds and grasses
• Succession ends in a CLIMAX community, such
as a hardwood forest, which is quite stable in
the absence of natural disasters.
Primary Succession

Occurs in a new
habitat where no
community has
previously existed


Indiana Dunes
Volcanic island
Dune Succession (Indiana Dunes)
Beach and Foredune
Vegetation
Marram grass
Ammophila breviligulata
Pioneer species, roots help to
hold sand together. Holds
moisture and nutrients in the
soil.
Cutaway view of dune stabilized by
Marram grass
Stabilized Dune
Vegetation
Shrub Zone: note Junipers
and Bearberries
Pine Zone: mainly
Jackpines
Oak-Hickory Forest - the
Climax Community
Secondary Succession

Replacement of
one community by
another
• Old field succession
• Fire succession
Species Interactions
Keystone Species
Keystone Species
Based on idea of the
keystone in an arch.
Keystone takes the
weight of all the
other stones in the
arch.
Without the
keystone the entire
structure collapses.
Keystone Predator (Paine 1969)
Pisaster
Starfish
Mussels
Keystone Predator: a
species that preferentially
consumes and holds in
check another species
that would otherwise
dominate the system.
Pisaster keeps mussel
population under control
and thereby increases
species diversity of
rocky intertidal
community.
Rocky shoreline on
Northwest coast
Site of Paine’s experiments
Note mussel bed above
highest spray line.
Diverse algal and
invertebrate community
below spray line
Mussels cleared by Pisaster - Barnacles beginning to colonize
Keystone Species

A keystone species is any species
whose impact on its community or
ecosystem is large and
disproportionately large relative to
its abundance.
• Does not have to be a predator!
Maintenance of keystone species is
critical to conserving ecosystems.
PROBLEM??
We rarely know which species are
the keystones.
Disturbance
Disturbance and Diversity
Ecosystems


First Law of Thermodynamics: Energy
cannot be created or destroyed but can only
be transformed from one one form into
another.
Second Law of Thermodynamics: Whenever
energy is changed from one form to
another, there is always a "loss" of usable
energy.
• Electrical energy to light energy: the light bulb
get hot due to the lost energy
Energy in Ecosystems




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Energy enters ecosystems in the form of
sunlight
It is captured by plants and stored in the
chemical bonds of sugar (organic matter)
Sugar is broken down by all cells to
release the stored energy.
This process is not completely efficient,
and some of the energy is lost as heat.
Thus, energy passes through an
ecosystem, entering as light energy and
leaving as heat energy.
Photosynthetic Producers
Photosynthesis
6 CO2 + 6H2O
6 C6H12O6
Respiration
The sun is the ultimate source of energy for most
ecosystems on Earth
Pyramid of Energy



At each link in the food chain, energy that
was originally stored by the autotrophic
plants is dissipated along the food chain.
The more links in the food chain, the more
dissipated or unusable energy.
There is generally a 90% loss at each link
of the food chain, creating a pyramidshaped diagram that is wider at the
bottom and narrow at the top.
Pyramid of Mass & Numbers


The mass (weight) and numbers of
organisms decreases along a food chain
(e.g. grass-grasshoppers-frogs-snakeshawk).
It takes many pounds of grass (or
numerous grass plants) to support one
hawk at the top of a food chain.
• Why does the pyramid of numbers look
different for the oak woodland?
• Why do the two pyramids of biomass look the
same?
Pyramids of Numbers
Hawk
Hawk
Frogs
Frogs
insects
Grass
Grassland
insects
Oak Trees
Woodland
Pyramids of Biomass
Hawk
Hawk
Birds
Birds
Insects
Insects
Grass
Oak Trees
An Ocean Food Pyramid


It takes about 2500 pounds (1136 kg) of
phytoplankton to support 0.5 pound
(0.227 kg) of tuna.
This is roughly the amount of tuna packed
into a single can sold at the supermarket.
• Note: The decrease from 2500 to 500 is 80
percent. The other trophic levels in this
pyramid are decreased by 90 percent.
Human Food Chains


With a 90 percent decrease in energy for
each link of a food chain, the efficiency of
each link is approximately 10 percent. In
other words, about 10 percent of the food
energy available to each link is utilized
and the remainder is lost "dissipated.“
Food chains with fewer links have more
energy available to the top level
consumer.
The following table compares human food chains in the
U.S. and Asia. Asian food chains typically support more
people because they have fewer links and less energy lost
from the ecosystem.
Which diet can support more people: a diet rich in meat
proteins, or a diet based on rice and beans (also supply
all the necessary proteins).
U.S.
Asia
Plants
1000 kcal
Plants
1000 kcal
Beef
100 kcal
People
10 kcal
People
100 kcal
Biomagnification
The red dots represent
units of DDT.
Starting with a low
concentration in the
water, the DDT
concentration (density
of red dots) increases
within the bodies of
each successive
trophic level,
ultimately reaching a
lethal concentration in
the Grebes.