55
Introduction
• The species that live together in a particular area
constitute an ecological community.
• Each species interacts in unique ways with other
species in its community and with its physical
environment.
• The species that form an ecological community,
together with the physical environment, constitute
an ecosystem.
55
Quiz
• Explain what species richness is and how it is
linked to primary productivity.
55
Communities: Loose Assemblages of Species
• Ecologists ask the following questions:
 What patterns exist in ecological communities
and ecosystems?
 How does the physical environment influence
those patterns?
 What are the relative roles of historical
accident and current interactions?
 How does evolution influence the assemblage
of species that live together?
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Process and Pattern
in Communities and Ecosystems
• Organisms need energy inputs, water, and
minerals for metabolism and growth.
• The sun is the source of energy, either directly or
indirectly, for almost all organisms.
• About 5% of solar energy is captured by
photosynthesis.
What happens to the rest?
55
Process and Pattern
in Communities and Ecosystems
• The rate at which plants assimilate energy is
called gross primary productivity.
• Some energy is used for their own metabolism;
the rest is stored or used for growth or
reproduction.
 primary production.
• The energy available to organisms that eat plants
is called net primary production.
• Gross – Amount used = Net
Figure 55.3 Energy Flow through an Ecosystem
55
Process and Pattern
in Communities and Ecosystems
• Where is primary production at it’s maiximum?
 Where the conditions allow for growth for
extended times.
 Tropics -Warm and wet.
• Where is it less?
 Cooler climates, high altitudes.
Figure 55.4 Primary Production in Different Ecosystems (Part 2)
Figure 55.5 Net Primary Production of Terrestrial Ecosystems
55
Process and Pattern
in Communities and Ecosystems
• Organisms in a community can be categorized into
trophic levels depending on how they get their
food.
 Primary producersAutotrophs/photosynthesizers.
 Primary Consumers-Heterotrophs/Eat plants
(herbivores)
 Secondary Consumers- Eat primary consumers.
Examples??????
55
Process and Pattern
in Communities and Ecosystems
• Detritivores or decomposers consume dead
organisms.
• Organisms that eat foods from primary producers
and another trophic level are omnivores.
• A sequence of linkages in which a plant is eaten
by an herbivore, and so on, is called a food
chain.
• Food chains are usually interconnected to make a
food web, because most species eat or are eaten
by more than one species.
Figure 55.7 Food Web of Isle Royale National Park
55
Process and Pattern
in Communities and Ecosystems
• Energy pyramids show how energy decreases
as it flows from lower to higher trophic levels.
 10% rule
 What happens to the rest?
 Lost as heat, waste, etc.
Figure 55.8 Pyramids of Biomass and Energy
55
Process and Pattern
in Communities and Ecosystems
• Much of the energy in biomass is consumed by
detritivores.
• Detritivores, such as bacteria, fungi, worms,
mites, and insects, transform the remains and
waste products of organisms into CO2, water, and
minerals.
• Continued ecosystem productivity depends on
rapid decomposition of detritus.
 Recycling of nutrients and energy.
55
Process and Pattern
in Communities and Ecosystems
• Species whose influences on ecosystems are
greater than would be expected on the basis of
their abundance are called keystone species.
• They may influence both the species richness of
communities and the flow of energy and materials
through ecosystems.
• Beavers, for example, create habitats for other
species by cutting down trees and building dams.
55
Process and Pattern
in Communities and Ecosystems
• Large grazing mammals, such as bison, change
the structure and composition of vegetation.
• Bison prefer grasses to forbs (small broad-leaved
plants). When bison are excluded from an area of
prairie, grasses dominate the ecosystem. When
bison are present, they eat the grasses and make
space for forbs.
• Bison urine is broken down quickly, providing
nitrogen for plant uptake. Plants in areas grazed
by bison have higher leaf nitrogen levels and grow
faster.
55
Process and Pattern
in Communities and Ecosystems
• Sea star Pisaster ochraceous of the North
American Pacific coast.
 Eats mussels to make room for other species.
 Sea stars removed
 28 species of animals and algae
disappeared.
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Disturbance and Community Structure
• Ecological succession is the sequence of
changes in the species composition of a
community over time.
• Primary succession begins with the
establishment of organisms on newly available
sites that previously had no organisms.
• Secondary succession begins when organisms
reestablish themselves on disturbed sites where
some organisms survived the disturbance.
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Disturbance and Community Structure
• The changes that take place when all or part of
the dead body of an animal or plant is
decomposed are examples of secondary
succession.
• The needle litter under pine trees is decomposed
by a succession of fungal species.
• Each group of fungi gets energy by decomposing
certain compounds and converting them to other
compounds that are used by the next group of
fungal species.