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Species Interaction and
Biological Diversity
Chapter Overview Questions
• What determines the number of species in
a community?
• How can we classify species according to
their roles in a community?
• How do species interact with one another
and control each other’s populations?
• How do communities respond to changes
in environmental conditions?
• Does high species biodiversity increase
the stability and sustainability of a
What is community structure?
• Structure or spatial distribution. Is described by
four characteristics
– Physical appearance: relative sizes, stratification and
distribution of populations and species
– Species diversity or richness: number of different
– Species abundance: number of individuals of each
– Niche structure: number of ecological niches, how
they resemble each other and how they interact
• Biological communities differ in their structure
and physical appearance.
Figure 7-2
Structure in a Mature Deciduous
How do communities differ in
physical appearance?
• They usually consist of a mosaic of vegetation
patches of differing size. Results in:
– Sharp edges and
– Ecotones (wider and more diffuse transition zones
between one community and another)
• Edge Effects: caused by differences in physical
structures at boundaries and in ecotones
– Example: edge area between a forest and open field.
It will have a different combination of species than the
forest and field interiors.
Sharp Edge
Edge between woodlands
and meadow
Community Structure
• Structure: patterns of spatial distribution of
individuals and populations within community
and relation of a community to its surroundings
Patchiness: graininess in spatial distribution
Vertical and horizontal (eg, rain forest is vertical)
Community Structure
• Most populations live in clumps although other
patterns occur based on resource distribution.
Figure 8-2
Desert plant spacing of
Creosote Bush and Ocotillo,
Sonoran Desert.
Clustered community;
school of fish
Abundance and Diversity
• Abundance: total number of organisms in
a biological community
• Diversity: measure of number of different
species, ecological niches, or genetic
variation present in an ecosystem
• In general, species diversity decreases,
but abundance within species increases
as one travels from equator toward poles.
Species diversity and abundance
• Most species-rich environments are
tropical rain forests, coral reefs, deep sea,
and large tropical lakes.
• Diversity affected by:
– Latitude (highest in tropical, lowest in polar)
– Depth in aquatic systems (increases from
surface to depth of 2000 m, and at sea bottom
– Pollution (diversity decreases as pollution
General types of species
• Native: normally live and thrive in a particular
• Nonnative: species that migrate or are
deliberately or accidentally introduced into an
ecosystem by humans. Also known as exotic or
alien species
• Indicator: serve as early warnings of damage to
a community or an ecosystem
• Keystone: species whose role is much more
important than their abundance suggests
Case Study:
Species Diversity on Islands
• MacArthur and Wilson proposed the
species equilibrium model or theory of
island biogeography in the 1960’s.
• Model projects that at some point the rates
of immigration and extinction should reach
an equilibrium based on:
– Island size
– Distance to nearest mainland
Introduced or Invasive Species
• If introduced species preys upon or
competes more successfully with one or
more native populations, the whole
community can be altered.
• Considered to be artificially introduced
(I.e., by man)
– kudzu, privet, zebra mussels, cane toads,
Asian ambrosia beetle
Chinese Privet
Cane toad
in Australia
Asian ambrosia
beetle and tree
Zebra mussel and
Indicator Species:
Biological Smoke Alarms
• Species that serve as early warnings of
damage to a community or an ecosystem.
– Presence or absence of trout species
because they are sensitive to temperature
and oxygen levels.
Indicator Species
• Birds are excellent indicators because
they are found everywhere and respond
quickly to environmental change.
• Fish are good indicator species because
they need high quality water.
• Amphibians live partly on land and partly
in water, so can be good indicator species
Examples of Indicator Species
Keystone Species: Major Players
• Keystone species help determine the
types and numbers of other species in a
community thereby helping to sustain it.
Figures 7-4 and 7-5
Critical roles of keystone species
• Pollination of flowering plant species
(bees, bats, humming birds)
• Dispersion of seeds by fruit-eating animals
• Habitat modification
• Predation by top carnivores that helps
control the populations of various species
• Improving the ability of plant species to
obtain soil minerals and water
• Efficient recycling of animal wastes
Sea urchins eating kelp
(a type of algae)
Sea Otter
Gopher Tortoise Burrow
Long Leaf Pine
Gopher Tortoise
Habitat modifications of keystone
• Elephants push over, break, or uproot trees, creating
forest openings in Africa.
• Gopher tortoises dig burrows that provide refuge to more
than 360 animal species, including indigo snake,
burrowing owl, and Florida mouse.
• Bats and birds regenerate deforested areas by
depositing plant seeds in their droppings.
• Beavers build dams, which produce lakes. This provides
habitat for many other animals.
• Top predator keystone species exert a stabilizing effect
on their ecosystems (wolf, leopard, lion, alligator)
• Dung beetles remove, bury, and recycle animal wastes.
• Is a kind of antagonistic relationship within a
– Compete for growing space, energy, and specific
sites for all life activities
• Intraspecific: competition among members of the
same species
– Territoriality: individuals of same species define an
area surrounding their homesite and defend it
• Interspecific: competition among members of
different species
Competitive Exclusion Principle
• No two species will occupy the same niche and
compete for same resources in same habitat for
– Resource partitioning: response to competition for
• Different species use different parts of same resource to
• Different species use the same resources at different times of
• Different species have different morphologies that allow them
to use slightly different resources (beak length vs depth of
Resource Partitioning
• Each species minimizes
competition with the
others for food by
spending at least half its
feeding time in a distinct
portion of the spruce tree
and by consuming
somewhat different
insect species.
Figure 7-7
Species Interactions
• Predation and competition for scarce resources
are major factors in evolution and adaptation
• Predator: feeds directly upon other living
– Includes omnivores, herbivores and carnivores, but
not detritivores, scavengers and decomposers
• Parasite: feeds on host organism or stealing its
resources without killing it
• Pathogen: organism that causes disease
Mutualism, parasitism and commensalism are types of
symbiosis, which is defined as the intimate living together of
members of two or more species.
• Complex influence on population balance of
• Is an important factor in evolution
– Reduces number of individuals that are slowest, least
fit and weakest (populations with predators are in best
balance with environment
• Prey species, in response to predation, have
evolved many specialized protective or
defensive adaptations
– Coevolution: species exert selective pressure on each
• Some prey escape
their predators or
have outer
protection, some
are camouflaged,
and some use
chemicals to repel
Figure 7-8
Types of Symbiosis
Commensalism: Using without
• Some species
interact in a
way that helps
one species
but has little or
no effect on the
Figure 7-10
Mutualism: Win-Win
• Two species
can interact in
ways that
benefit both of
Figure 7-9
Lichens are a combination of
a fungus and an alga or
Parasites: Sponging Off of
• Although parasites can harm their hosts,
they can promote community biodiversity.
– Some parasites live in host (micororganisms,
– Some parasites live outside host (fleas, ticks,
mistletoe plants, sea lampreys).
– Some have little contact with host (dumpnesting birds like cowbirds, some duck
A tomato hornworm is covered
with cocoons of pupating braconid
Adult heartworms
in a dog’s heart
Defensive Mechanisms
• Defensive adaptations to protect species
from predation
– Examples
• Noxious odors
• Poisonous secretions
• Mimicry
– Batesian: species that are harmless will evolve colors,
patterns, or body shapes that mimic noxious species
– Mullerian: two species, both of which are noxious, have
evolved to look alike
• Camouflage
• New environmental conditions allow one group
of species in a community to replace other
• Ecological succession: the gradual change in
species composition of a given area
– Primary succession: the gradual
establishment of biotic communities in lifeless
areas where there is no soil or sediment.
– Secondary succession: series of
communities develop in places containing soil
or sediment.
Primary Succession
• Pioneer species
– first ones on site
• Ecological development
– accumulation of organic debris allowing
germination of seeds
– orderly sequence of stages occurs as
organisms modify the environment in ways
that allows one species to replace another
Primary Succession:
Starting from Scratch
• Primary
begins with an
lifeless area
where there is
no soil in a
Figure 7-11
Secondary Succession
• Occurs due to disruption of community
– Bare soil colonized by rapidly growing annual
• Annual plants replace by perennial plants
• Biomass accumulates and allows richer soil,
better shelter, and allows a more complex
Climax woodland community
Climax Forest on
Stone Mountain
Beech Forest (Fernbank
Science Center)
Damage done by
Yellowstone Fire in 1988.
Pictures taken in May, 2010
Slow succession due to
destruction of beneficial soil
bacteria and fungi
This is primary succession on a recent lava flow in the
Galapagos Islands, Ecuador. The vegetation near the
center of the photo is mangrove, on the slopes there are
cactus and other desert plants.
This is the northern lower flank of Mt. Saint Helens taken about 24 years
after the eruption. This area was buried under hundreds of feet of ash, so
all the plant life you can see has come in from wind-blown seed. The
plants are things like willows, cottonwoods, grasses and forbes that have
fine, easily wind-transported seeds. In the thin ash area, most of the new
vegetation arises from perennial plants whose roots survived the blast
This is reforestation of the blast zone around Mt. Saint
Helens. All of the trees were planted (by Weyerhaeuser)
about 20 years ago.
This photo shows plant succession following the retreat of the Emmons Glacier on Mt. Rainier. The glacier began
retreating around the time of the Civil War and has moved up the valley several miles. The glacier itself is the dark
mass (not the white area) that terminates in the lower left third of the picture. You can see plant succession
marching up-valley toward the snout of the glacier. If you look closely you can see the White River emerging from
the snout. This river flows all the way to Puget Sound at Tacoma. To the right is a beautiful lateral moraine. Plant
succession is visible on the right (north) slope of the moraine but is being impeded by hot dry conditions on the
south (left) slope of the moraine. Most of the pioneering vegetation is alder and willow, with a few conifers
gradually becoming established. The seeds are coming down from the forests above. The dark old growth forest
to the right is mainly mountain hemlock and subalpine fir.
Climax Community
• Culmination of successional process; is a
stable, complex mature form.
• Equilibrium communities
– Never reach a stable climax because they are
characterized by and adapted to periodic
• Example: fire-climax communities (Long-leaf pine
Fire Ecosystem
• The longleaf/wiregrass community is part
of a fire ecosystem.
• Fire is required to clear brush to allow pine
seedlings to grow
• Wire grass is most likely to bloom and set
seed after a late spring or early summer
Jackson Pine Forest, Lake Tahoe, CA
Resilience and Stability
• Resilience or stability: Resistance to
change. Three types:
– Constancy: ability to keep populations within
the limits of available resources
– Inertia: ability to resist change due to
– Resilience: ability to repair damage after
• Having many different species appears to
increase the sustainability of many
• Human activities are disrupting ecosystem
services that support and sustain all life and
all economies.