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Biological Communities
Reading: Smith and Smith, Chapters 21-23
• Biological community- a group of populations
of different species that exist in the same place
and are, at least in principle, capable of
interacting with each other.
– The true nature of biological communities is a
subject of intense debate.
Are communities collections of
interdependent components, like a
wristwatch?
Or, are they ad-hoc assemblages of
species that just happen to be able to live
together?
• Descriptive Properties of a community
• scale size of community-boundaries are sometimes arbitrary
• species diversity diversity of species in a community
• species richness
number of species in a community
• Functional properties-these imply
interdependence and some degree of functionality
• trophic structure
Pattern of energy flow based on who eats whom
• habitat structure
– spatial and temporal structure
• stability resistance of a community to change due to
extinction or disturbance
• disturbance and succession
Every community has some degree of disturbance,
and most are part of a series of communities that
replace one another due to the process of
ecological succession
A saltwater
food web
from San
Francisco
Bay, CA
• Biological Communities are frequently
nested within one another
Example-pitcher plants, sp. are members of bog
communities.
• In practice, communities tend to be defined
by their most conspicuous members.
– Ecotone-the boundary between biological
communities
This is an ecotone
between California
coastal forest and
wetland at the
Elkhorn Slough
Some species belong to more than one
biological community.
– Example-dragonflies inhabit freshwater ponds
as larvae, emerge and prey upon insects in
neighboring forest, wetland, and grassland
communities
Example-warblers migrate seasonally,
moving from temperate forests and
grasslands to subtropical forests
• Each of the properties of a community affects the
others, for example, a change in species richness
will affect habitat structure, which will affect
stability and trophic structure
• Example-eliminating disturbance due to fire
causes prairie communities to be gradually
replaced by woodland
Keystone species-single species that exert a
major influence on composition, and function
of a biological community
• Example-California sea
otters
– are predators of marine
fish and invertebrates.
– are one of the few
important predators of sea
urchins.
– Their range formerly
extended along the Pacific
rim, from Japan to Baja
California
• in Coastal California, sea urchins were hunted
nearly to extinction in the eighteenth and
nineteenth century.
– sea urchins graze rocks of encrusting algae,
preventing the establishment of new kelp plants, and a
gradual destruction of the Kelp forests-and the
enormous diversity of life forms that live in kelp
– the disappearance of the sea otter is the likely cause of
the decline in Kelp forests that occurred during the
20th century-more sea urchins=less kelp.
– With the recovery of the California Sea Otterdensities of sea urchins have dropped (also
commercial abalone, however), and kelp communities
have stabilized and started to return.
Strongylocentrotus purpuratus
Pacific Kelp,
macrocystis sp. forms
dense forests
underwater
• Introduction of an exotic species can completely
change a biological community as well.
– Example-Buckthorn; Introduced buckthorn Rhamnus
cathartica is a big problem in the midwest. Originally
an ornamental, this species is an incredibly efficient
competitor for light, spreading through the forest
understory and shading out the other forest-floor
species.
– If the trees succumb to disease or old age, a thicket
forms, which resists invasion by younger trees.
– The result is a much simpler ecosystem, with fewer
producers, and the disappearance of specialized
herbivores, pollinators, and mutualists of the trees and
plants replaced by buckthorn
In communities dominated
by buckthorn, the most
conspicuous major food
source for birds is an
annoying, laxative berry. This
spreads buckthorn to other
sites
Are Biological Communities Real?
• Some ecologists view communities as ad-hoc
assemblages of species that just happen to occur
together and be compatible with one another.
– This view was promoted by the 20th century
botanist, H.A. Gleason
• Other ecologists view communities as interdependent
assemblages of species that actively facilitate and
promote each other’s existence.
– This view was promoted by the ecologist, F.E.
Clements.
• In fact, there may be a continuum between open and
closed communities in nature.
• In nature, “closed communities” tend to occur in special
environments with tightly defined boundaries.
– This leaves it an open question as to whether the
species in these environments are together because
they are truly interdependent with specific members
of that community, or because their physical
tolerances are so specific they just happen to be the
only species that can live there.
• They also occur under circumstances where one
organism is essential for the survival of every other
species-so the physical tolerances of this species
determine the distribution of the entire community
Some “closed” communities
• Geothermal hot springs– organisms that can tolerate high temperature are
called thermophiles or hypothermophiles
– may include photosynthetic green and purple sulfur
bacteria, bacteria and archaeans that act as
chemoautotrophs and others that act as
decomposers.
• Most of species are probably unknown to science and
cannot be cultured in a lab.
• Majority of species cannot live anywhere else.
– this is a truly ancient assemblage of organisms, the
first communities may have resembled these
These communities are often zoned by temperature, with green and
red algae growing at the cool edges, cyanobacteria in the middle,
and hyperthermophile prokaryotes near the center.
The archaean, Sulfolobus sp.
gains energy by oxidizing sulfur
granules around hot springs, and
thus lower the Ph.
• Cave communities– includes specialized prokaryotes that grow
under low energy conditions with constant, low
temperature and special Ph. Also may include
blind cave fish (catfish, tetras, gobies), cave
crickets and other specialized arthropods.
– not completely insulated from outsidedependent upon energy and nutrients from
outside-for instance, bats have the potential to
bring enormous amounts of energy and
nurtrients into a cave from outside, and harbor
many parasites.
• In other cases, replacement of one species for
another occurs along a gradient or continuum. As
conditions change, the assemblage of species
changes.
– Even in the most “open” community, however,
species assume roles -decomposers, producers,
herbivores, pollinators, parasites, etc. and
depend upon other species for nutrients, food,
habitats, and other things needed for survival.
• Gradient Analysis suggests that most major
terrestrial plant communities are “open”
– open communities might be the rule rather than
the exception
Predators Influence the Diversity of Lower Trophic Levels
• Predators may exert a major influence food web
organization.
– Paine compared the food webs of rocky intertidal
communities in Washington and the Gulf of California.
– In Washington, the sea star Pisaster dominates.
– In the Gulf of California, the sea star Heliaster
dominates.
– In the Gulf of California, other predators were
abundant, including snails, crabs, and fish.
– Herbivore and producer trophic levels were also more
diverse, despite the lower primary productivity there.
• These findings motivated his removal experiments.
Removal experiments
• Paine is known for two removal experiments that
attempted to elucidate the influence of particular
species on the organization of communities.
– Both were conducted on rocky intertidal communities in
coastal Washington
• Removal of the predatory sea star, Pisaster, vs. unmanipulated
control.
– Result; Pisaster removal plots became dominated by
mussels, with a decline in the number of other species.
• Removal of “herbivorous” sea urchin, Strongylocentrotus vs.
unmanipulated controls
– Result, sea urchin removal plots became dominated by a
small number of competitively superior algae, with a
decline in the diversity of producers.
Productivity Affects the Diversity and Organization of
Communities
• There is a rough correlation between the productivity of
communities, and their diversity• I.e., tropical rainforests are more diverse for most groups, and
more productive than temperate forests, which are in turn more
diverse for most groups, and productive than taiga.
– In general, more productive communities are more
diverse, but this is not always the case.
• I.e., for many groups, the relatively unproductive desert scrub
community is more diverse than the enormously productive salt
marsh community
– Does productivity affect diversity?
• -experiments where the productivity of an existing ecosystem is
boosted by adding nutrients usually lead to a decrease in
diversity
“Top Down vs. Bottom Up”
• “Bottom up” control of communities implies that the
diversity and abundance of organisms at each trophic
level is determined by the amount of energy coming
up from below.
• “Top down” control implies that the diversity and
abundance of organisms at each trophic level is
determined by the presence or absence of predation
from higher trophic levels.
– Consumers depress biomass at the level immediately
below themselves, indirectly increasing the biomass at the
level below that.
• Real communities show evidence of both.
– In a well known study of ponds by Matthew
Leibold, it was demonstrated that the biomass
of herbivores (zooplankton) was positively
correlated to the biomass of producers (algae),
indicating a top down effect.
– He intentionally introduced fish to some ponds,
The result was a decrease in zooplankton and
increase in producers, indicating a top down
effect.
Assembly Rules
• A big, unanswered question in community
ecology concerns how species become assembled
into biological communities.
– Obviously, only some sequences may occur-for
instance, consumers will not persist if they arrive
before producers, predators will not persist if they
arrive before prey.
– Removal experiments, and laboratory experiments
with microcosms suggest that such rules exist, though
ecologists have not worked out the details.