Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 41Community Ecology

(Sections 41.6 - 41.9)

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 climate2. Chance events such as the order in which species arrive3. 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 poles1. Tropical latitudes intercept more sunlight, receive more

rainfall, and their growing season is longer2. Tropical communities have been evolving for a long time3. 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