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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 41
Community Ecology
(Sections 41.6 - 41.9)
Albia Dugger • Miami Dade College
41.6 Parasites, Brood Parasites,
and Parasitoids
• Some plants and animals benefit by withdrawing nutrients
from other species
• Some species trick others into providing parental care
Parasitism
• With parasitism, one species (the parasite) feeds on another
(the host), without immediately killing it
• Endoparasites live and feed inside their host
• Ectoparasites feed while attached to a host’s external
surface
• parasitism
• Relationship in which one species withdraws nutrients
from another species, without immediately killing it
Endoparasites: Intestinal Roundworms
Ectoparasites: Ticks
Parasites (cont.)
• Parasites include a diverse variety of groups:
• Bacterial, fungal, protistan, and invertebrate parasites feed
on vertebrates
• Lampreys attach to and feed on other fish
• Some plants parasitize other plants
• Many parasites are pathogens that cause disease in hosts
Dodder (Devils Hair): A Parasitic Plant
Coevolution in Parasites and Hosts
• Parasites are adapted with traits that allow the parasite to
locate hosts and to feed undetected
• Ticks move toward heat and carbon dioxide
• Hosts adapt with traits that minimize the negative effects of
parasites
• Sickle-cell allele protects against malaria
Strangers in the Nest
• Presence of brood parasites, such as North American
cowbirds, decreases the reproductive rate of the host species
and favors host individuals that detect and eject foreign young
• Brood parasitism also evolved in some bee species
• brood parasitism
• One egg-laying species benefits by having another raise
its offspring
Cowbird and Foster Parent
Parasitoids
• Parasitoids reduce a host population in two ways:
• Parasitoid larvae withdraw nutrients and prevent the host
from reproducing
• Presence of larvae leads to death of the host
• As many as 15% of all insects may be parasitoids
• parasitoid
• An insect that lays eggs in another insect, and whose
young devour their host from the inside
Biological Controls
• Parasites and parasitoids are commercially raised and
released in target areas as biological control agents – an
environmentally friendly alternative to pesticides
• A biological control agent must be adapted to a specific host
species, and survive in that species’ habitat
• Introducing a biological control species into a community is
risky – they sometimes go after nontargeted species
Parasitoid Wasp Deposits
an Egg in an Aphid
Key Concepts
• Forms of Species Interactions
• Commensalism, mutualism, competition, predation, and
parasitism are interspecific interactions
• They influence the population size of participating species,
which in turn influences the community’s structure
ANIMATION: Succession
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41.7 Ecological Succession
• Species often alter the habitat in ways that allow other
species to replace them (ecological succession)
• The first, opportunistic colonizers of new or newly vacated
habitats are pioneer species, which have high dispersal
rates, grow and mature fast, and produce many offspring
• pioneer species
• Species that can colonize a new habitat
• Mosses, lichens, some flowering annuals
Primary Succession
• Primary succession begins when pioneer species colonize a
barren habitat with no soil, such as a new volcanic island or
land exposed by the retreat of a glacier
• Pioneers help build and improve the soil
• primary succession
• A new community becomes established in an area where
there was previously no soil
Primary Succession
Ecological Succession
• Seeds of later species grow in mats of pioneers
• Organic wastes and remains accumulate and help other
species take hold
• Later successional species often shade and eventually
displace earlier ones
Ecological Succession
Secondary Succession
• In secondary succession, a disturbed area within a
community recovers.
• Occurs in abandoned agricultural fields and burned forests
• secondary succession
• A new community develops in a site where the soil that
supported an old community remains
Factors That Influence Succession
• Species composition of a community changes frequently, and
unpredictably – random events can determine the order in
which species arrive , and affect the course of succession
• Example: 1980 eruption of Mount Saint Helens
• Presence of some pioneers helped later-arriving plants
become established
• Other pioneers kept the same late arrivals out
Mount St. Helens: 1980
Mount St. Helens: 1990
Mount St. Helens: 2002
Intermediate Disturbance Hypothesis
• intermediate disturbance hypothesis
• Species richness is greatest in communities where
disturbances are moderate in intensity or frequency
Factors in Community Composition
• Three factors affect species composition of communities:
1. Physical factors such as soil and climate
2. Chance events such as the order in which species arrive
3. The extent of disturbances in the habitat
Key Concepts
• Long-Term Change in Communities
• The array of species in a community changes over time,
although the exact outcome of these changes is difficult to
predict
• When a new community forms, the early-arriving species
often alter the habitat in a way that facilitates their own
replacement
41.8 Species Interactions
and Community Instability
• Loss or addition of even one species (keystone species)
may destabilize the number and abundances of species in a
community
• keystone species
• A species that has a disproportionately large effect on
community structure
A Keystone Species: Sea Stars
• Experiment: Sea stars in a rocky intertidal zone in California
• Sea stars prey mainly on mussels
• Sea stars were removed from experimental plots
• With sea stars gone, mussels took over, crowding out
seven other species of invertebrates
• Conclusion: Sea stars a re a keystone species
• They normally keep number of prey species high by
preventing competitive exclusion by mussels
A Keystone Species: Periwinkles
• The impact of a keystone species can vary between habitats
that differ in their species arrays
• Example: Periwinkle snails
• In tidepools, periwinkles eat the most competitive algal
species, allowing less competitive species to survive
• In the lower intertidal zone, periwinkles eat the less
competitive algae, giving dominant algae an advantage
Effects of Algal Predation by
Periwinkle Snails
• Algal diversity in
tidepools
• Algal diversity in
intertidal zone
Adapting to Disturbance
• Some species adapted to being disturbed are at a competitive
disadvantage if the disturbance does not occur
• Example: Areas subject to periodic fires
• Some plants produce seeds that germinate only after a
fire, or resprout quickly after a fire
• Because different species respond differently to fire, the
frequency of fire affects competitive interactions
Adapting to Fire
• Toyon resprouts
from roots after a
fire
• In the absence of
occasional fire,
toyons are
outcompeted by
other species
Indicator Species
• Indicator species are the first to do poorly when conditions
change, so they can provide an early warning of
environmental degradation
• Example: Trout are highly sensitive to pollutants and cannot
tolerate low oxygen levels
• indicator species
• Species that is especially sensitive to disturbance and can
be monitored to assess the health of a habitat
Species Introductions
• Exotic species can dramatically alter a natural community
• More than 4,500 exotic species have become established
in the United States
• Visit the National Invasive Species Information Center at
www.invasivespeciesinfo.gov
• exotic species
• A species that evolved in one community and later
became established in a different one
Exotic Species
• Kudzu native to
Asia is
overgrowing
trees across
the
southeastern
United States
Exotic Species
• Gypsy moths
native to Europe
and Asia feed on
oaks through
much of the
United States
Exotic Species
• Nutrias native to
South America are
abundant in
freshwater
marshes of the
Gulf States
Key Concepts
• Species Effects on Community Stability
• Removing a species from a community or adding one to it
can have a dramatic effect on other species
• Some species are adapted to disturbances and a change
in the frequency of disturbances can affect their number
BBC Video: The Devastation of Exotic Species
in Hawaii
ABC Video: Circle of Life (Ecotourism)
41.9 Biogeographic Patterns
in Community Structure
• Biogeography is the study of how species are distributed in
the natural world
• Species richness correlates with differences in sunlight,
temperature, rainfall, and other factors that vary with latitude,
elevation, or water depth
• species richness
• Of an area, the number of species
Latitudinal Patterns
• Species richness is usually greatest in the tropics and
declines from the equator to the poles
1. Tropical latitudes intercept more sunlight, receive more
rainfall, and their growing season is longer
2. Tropical communities have been evolving for a long time
3. Species richness may be self-reinforcing; more plant
species leads to more herbivores, which leads to more
predators, and parasites
Species Richness by Latitude (Ants)
Island Patterns
• According to the equilibrium model of island
biogeography, the number of species living on any island
reflects a balance between immigration rates for new species
and extinction rates for established ones
• Colonization rates depend on the distance between an island
and a mainland source of colonists (distance effect)
• An island’s size affects it species richness (area effect)
Key Terms
• equilibrium model of island biogeography
• Predicts the number of species on an island based on the
island’s area and distance from the mainland
• area effect
• Affects immigration and extinction rates; larger islands
have more species than small ones
• distance effect
• Affects immigration rates; islands close to a mainland
have more species than those farther away
Island Colonization: Surtsey
• 1960s: Island formation
• 1983
Island Colonization: Surtsey
• Number of vascular plant species found in yearly surveys
Island Biodiversity Patterns
ANIMATION: Area and distance effects
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Key Concepts
• Global Patterns in Community Structure
• Biogeographers identify regional patterns in species
distribution
• They have shown that tropical regions hold the greatest
number of species and that characteristics of islands can
be used to predict how many species an area will hold
Fighting Foreign Fire Ants (revisited)
• Red imported fire ants (RIFAs) did not evolve in North
America, so there are few predators, parasites, or pathogens
to hold them in check
• Global climate change is expected to help RIFAs extend their
range in the US