Communities and Ecosystems
55 Communities and Ecosystems
• Introduction
• Communities: Loose Assemblages of Species
• Process and Pattern in Communities and
Ecosystems
• Disturbance and Community Structure
• Dispersal, Extinction, and Community Structure
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 Communities: Loose Assemblages of Species
• Ecological communities contain many species that
interact with one another. The species and
interactions may change over time.
• In 1926, Henry Gleason argued that communities
were loose assemblages of species, with each one
distributed individualistically according to its unique
interactions with the physical environment.
• In 1936 Frederick Clements argued that plant
communities were tightly integrated
“superorganisms.”
55 Communities: Loose Assemblages of Species
• Detailed studies of plants generally supported
Gleason’s view.
• Studies of mountain vegetation in Oregon showed
that different combinations of plant species are
found at different locations.
• Species enter and drop out of communities
independently over environmental gradients.
• However, where environmental conditions change
abruptly (e.g., at the edges of lakes and streams),
the ranges of many species may terminate
abruptly.
Figure 55.1 Plant Distributions along an Environmental Gradient
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?
55 Communities: Loose Assemblages of Species
• A few interactions may determine the features of
a community.
• Although hundreds of species live in oak forests in
the eastern U.S., the ecological interactions are
dominated by the oak trees, white-footed mice,
gypsy moths, and white-tailed deer.
• Mice and deer survive well during years of heavy
acorn production.
• Gypsy moth larvae eat oak leaves and pupate on
the trunks, where they may be eaten by mice;
every few years the moths become extremely
abundant.
55 Communities: Loose Assemblages of Species
• To test the hypothesis that the mice generate
fluctuations in the gypsy moth populations,
ecologists did an experiment during a year when
mice populations were high and gypsy moth
populations were low.
• By removing mice from experimental plots, they
determined that the mice were preventing
development of the gypsy moths by eating all the
pupae from the tree trunks.
55 Communities: Loose Assemblages of Species
• In another experiment, acorns were added to
experimental plots during a year of low acorn
production.
• Mouse populations became much more dense on
plots with added acorns than on control plots.
• The research found that mice control gypsy moth
populations, allowing oak trees to recover from
defoliation by the moth and produce large crops of
acorns. When mouse populations are reduced by
a low acorn crop, moth populations rise, resulting
in the defoliation of the oak trees.
55 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.
• Fossil fuels, such as coal, oil, and natural gas, are
stored solar energy.
• About 5% of solar energy is captured by
photosynthesis.
• The remaining energy is either radiated back into
the atmosphere as heat or consumed by the
evaporation of water.
55 Figure 55.2 Interactions within Communities Control Populations
Oak trees produce
large crops of acorns
When larvae do not every few years.
defoliate oaks, the
trees produce more
acorns.
Dense mouse populations
keep gypsy moth populations
low by eating gypsy moth pupae.
55 Process and Pattern
in Communities and Ecosystems
• The rate at which plants assimilate energy is
called gross primary productivity.
• Plants use some of this energy for their own
metabolism; the rest is stored or used for growth
or reproduction. The accumulated energy is called
primary production.
• The energy available to organisms that eat plants
is called net primary production; gross primary
production minus the energy used by the plants.
• The energy content of an organism’s net
production—its growth plus reproduction—is
available to other organisms that consume it.
Figure 55.3 Energy Flow through an Ecosystem
55 Process and Pattern
in Communities and Ecosystems
• The distribution of primary production worldwide
reflects the distribution of land masses,
temperature, and moisture.
• Tropical areas with high temperatures and
adequate water all year are most productive.
• In lower-latitude and mid-latitude deserts, primary
production is low because plants are limited by
lack of moisture.
• At higher latitudes with adequate moisture, low
temperatures during much of the year limit
production.
Figure 55.4 Primary Production in Different Ecosystems (Part 1)
Figure 55.5 Net Primary Production of Terrestrial Ecosystems
55 Process and Pattern
in Communities and Ecosystems
• Production in aquatic systems is limited by light,
nutrients, and temperature.
• Primary productivity influences two other
community characteristics: species richness and
food web structure.
55 Process and Pattern
in Communities and Ecosystems
• The species richness of an ecological
community is correlated with gross primary
productivity—to a point.
• Species richness often increases with productivity
at first, but then decreases.
• One hypothesis to explain the decrease
postulates that interspecific competition becomes
more intense with higher productivity, resulting in
competitive exclusion.
• This hypothesis is supported by a long-term
experiment in England, in which species richness
of plants in unfertilized plots has remained
unchanged, and species richness in fertilized
plots has declined.
Figure 55.6 Local Species Richness Peaks at Intermediate Productivity
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.
• Photosynthesizers (autotrophs) are the primary
producers; they get energy from sunlight and
produce the organic molecules that other
organisms (heterotrophs) consume.
• Organisms that eat plants are called herbivores or
primary consumers.
• Secondary consumers eat herbivores. Those that
eat secondary consumers are tertiary consumers,
and so on.
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 ENERGY AND BIO-MASS PYRAMIDS
• Most communities have only three to five trophic
levels.
• Only a portion of energy captured at one trophic
level is available to organisms at the next higher
level. Energy pyramids show how energy
decreases as it flows from lower to higher trophic
levels.
• At each tropic level in a food chain, energy is
used by the organisms at that level to maintain
their own life process. Because of the 2nd law of
energy, some energy is lost to the surroundings
as heat. It is estimated that in going from one
tropic level to the next, about 90 % of the energy
is lost.
55 ENERGY AND BIO-MASS PYRAMIDS
• A biomass pyramid illustrates the amount of
biomass available at a given time for organisms at
the next trophic level.
• The shapes of the pyramids depend on the
dominant organisms and how they allocate their
energy.
Figure 55.8 Pyramids of Biomass and Energy
55 Process and Pattern
in Communities and Ecosystems
• In most terrestrial systems, the primary producer
level contains a large biomass.
• However, trees store much of their energy in
difficult-to-digest wood, whereas much of the
primary net production of grasslands is
consumed.
• Thus the herbivore level has a relatively larger
biomass in grasslands than in forests.
55 Process and Pattern
in Communities and Ecosystems
• Most aquatic communities have dominant primary
producers that are bacteria and protists.
• These have such high rates of cell division that a
small biomass can feed a much larger biomass of
herbivores.
• This pattern can produce an inverted biomass
pyramid, even though the energy pyramid for the
same ecosystem has the typical shape.
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.
55 Process and Pattern
in Communities and Ecosystems
• Does species richness influence ecosystem
productivity?
• Ecologists hypothesized the following:
Species richness might enhance productivity
because a richer mixture of species should result
in a more complete use of resources.
If the environment changes, a species-rich
system is more likely to contain species already
adapted to the new conditions.
A species-rich ecosystem should be more
stable—it should change less over time in terms
of both productivity and species composition.
55 Process and Pattern
in Communities and Ecosystems
• To test this, ecologists planted grasses in plots with
various mixtures of species.
• In 11 years of measurements, the plots with more
species had greater biomass (greater net primary
productivity) and varied less from one year to
another.
• However, population densities of individual species
varied independently of the plot’s species richness,
because different species performed better during
drought and during wet years.
• Although species richness and productivity were
positively correlated, this could have resulted if only
one or a few species had exerted very strong
influences on the ecosystem.
Figure 55.9 Species Richness Enhances Community Productivity and Stability (Part 1)
Figure 55.9 Species Richness Enhances Community Productivity and Stability (Part 2)
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.
Figure 55.10 Grazing Increases Plant Species Richness and Productivity (Part 2)
55 Process and Pattern
in Communities and Ecosystems
• Another keystone species is the sea star Pisaster
ochraceous of the North American Pacific coast.
• In the absence of the sea star, mussels take over
the intertidal zone and crowd out other animals.
• By consuming mussels, the sea star creates bare
spaces for a variety of other species.
• When sea stars were removed experimentally
from parts of the intertidal zone, 28 species of
animals and algae disappeared.
Figure 55.11 Sea Stars are Keystone Predators
55 Disturbance and Community Structure
• A disturbance is an event that changes the
survival rate of one or more species in an
ecological community.
• A disturbance can be limited to a small area or it
can be large in its effects (e.g., a hurricane or
volcanic eruption).
• Disturbances have different effects according to
how often they occur and the pattern of the
damage that they cause.
• Forest fires in Yellowstone National Park created
a mosaic of patches that burned with varying
intensity.
Figure 55.12 Fires Create Mosaics of Burned and Unburned Patches
55 Disturbance and Community Structure
• In general, communities with very high levels of
disturbance and those with very low levels have
fewer species than communities with intermediate
levels.
• This observation generated the intermediate
disturbance hypothesis:
There is low species richness in areas with
high disturbance because only species with
great dispersal abilities and rapid reproductive
rates can persist.
Species richness declines with low levels of
disturbance because competitively dominant
species displace other species.
55 Disturbance and Community Structure
• In an experiment with different-sized boulders on
intertidal beaches, it was determined that
medium-sized boulders had more species on
them than small or large boulders.
• The idea that the small boulders were disturbed
by wave action too frequently to have many
species was tested by gluing down small
boulders. The secured small boulders had more
species than unsecured small boulders.
• This experiment also showed that the number of
species in a community changes over time
following a disturbance.
Figure 55.13 Species Richness Is Greatest at Intermediate Levels of Disturbance
55 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.
55 Disturbance and Community Structure
• A good example of primary succession can be
found in glacial deposits (moraines) in Alaska that
were left by the retreat of a glacier over the last
200 years.
• By comparing moraines of different ages,
ecologists have been able to infer the order of
primary plant succession on them.
Figure 55.14 Primary Succession on a Glacial Moraine
55 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.
Figure 55.15 Secondary Succession on Pine Needles
55 Dispersal, Extinction, and Community Structure
• Immigration and emigration influence the structure
of communities, and species introduced by
humans often come to dominate the communities
they invade.
• Throughout the history of Earth, species have
colonized new areas and others have gone
extinct.
• The rate of introduction of new species and the
extinction of existing ones has been increased
greatly by human activities over the past few
centuries.
55 Dispersal, Extinction, and Community Structure
• Before the Central American land bridge formed
about 4 million years ago, South America had
evolved a distinctive mammalian fauna..
• Thereafter, many mammals dispersed across the
newly established land bridge, mostly North
American mammals going south.
• The North American invasion caused the
extinction of several kinds of marsupial carnivores
and the large herbivores they preyed on.
• Subsequently, the invaders formed new species
that today exist only in South America.
Figure 55.16 North and South America Exchanged Mammals (Part 1)
Figure 55.16 North and South America Exchanged Mammals (Part 2)