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Chapter 7. Community Ecology. Core Case Study: Why Should We Care about the American Alligator?. Hunters wiped out population to the point of near extinction. Alligators have important ecological role. Figure 7-1. Core Case Study: Why Should We Care about the American Alligator?.

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chapter 7

Chapter 7

Community Ecology

core case study why should we care about the american alligator
Core Case Study:Why Should We Care about the American Alligator?
  • Hunters wiped out population to the point of near extinction.
  • Alligators have important ecological role.

Figure 7-1

core case study why should we care about the american alligator1
Core Case Study:Why Should We Care about the American Alligator?
  • Dig deep depressions (gator holes).
    • Hold water during dry spells, serve as refuges for aquatic life.
  • Build nesting mounds.
    • provide nesting and feeding sites for birds.
    • Keeps areas of open water free of vegetation.
  • Alligators are a keystone species:
    • Help maintain the structure and function of the communities where it is found.
community structure and species diversity
  • Biological communities differ in their structure and physical appearance.

Figure 7-2









rain forest









Fig. 7-2, p. 144

species diversity and niche structure different species playing different roles
Species Diversity and Niche Structure: Different Species Playing Different Roles
  • Biological communities differ in the types and numbers of species they contain and the ecological roles those species play.
species diversity and niche
Species Diversity and Niche
  • Species diversity: combination of:
    • species richness - the number of different species it contains
    • species evenness - abundance of individuals within each of those species
species diversity and niche structure
Species Diversity and Niche Structure
  • Niche structure: how many potential ecological niches occur, how they resemble or differ, and how the species occupying different niches interact.
  • Geographic location: species diversity is highest in the tropics and declines as we move from the equator toward the poles.
species diversity on islands
Species Diversity on Islands
  • species equilibrium model
  • at some point the rates of immigration and extinction should reach an equilibrium based on:
    • Island size
    • Distance to nearest mainland
types of species
  • Native
  • Nonnative
  • Indicator
  • Keystone
  • Foundation species

Play different ecological roles in communities.

types of species1
  • Native: those that normally live and thrive in a particular community.
  • Nonnative species: those that migrate, deliberately or accidentally introduced into a community.
invasive species nonnative introduced
Invasive species Nonnative(Introduced)
  • They displace native species
  • They lower biodiversity
  • The can adapt very quickly to local habitats
  • They contribute to habitat fragmentation
  • They can reproduce very quickly
importation of species
Importation of Species
  • Ex. The Chinese chestnut had a fungus that spread & virtually eliminated the American chestnut.
  • Kudzu
keystone species major players
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

foundation species other major players
Foundation Species: Other Major Players
  • Expansion of keystone species category.
  • Foundation species can create and enhance habitats that can benefit other species in a community.
    • Elephants push over, break, or uproot trees, creating forest openings promoting grass growth for other species to utilize.
indicator species biological smoke alarms
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 why are amphibians vanishing
Indicator species: Why are Amphibians Vanishing?
  • Frogs serve as indicator species because different parts of their life cycles can be easily disturbed.

Figure 7-3


Adult frog(3 years)

Young frog


Tadpole develops

into frog





Fertilized egg


Egg hatches

Organ formation

Fig. 7-3, p. 147

case study why are amphibians vanishing
Case Study: Why are Amphibians Vanishing?
  • Habitat loss and fragmentation.
  • Prolonged drought.
  • Pollution.
  • Increases in ultraviolet radiation.
  • Parasites.
  • Viral and Fungal diseases.
  • Overhunting.
  • Natural immigration or deliberate introduction of nonnative predators and competitors.
species interactions competition and predation
  • Species can interact through competition, predation, parasitism, mutualism, and commensalism.
  • Some species evolve adaptations that allow them to reduce or avoid competition for resources with other species (resource partitioning).
resource partitioning
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

the fundamental niche
The Fundamental Niche
  • The fundamental niche of an organism is described by the full range of environmental conditions (biological and physical) under which the organism can exist.
  • The realized niche of the organism is the niche that is actually occupied. It is narrower than the fundamental niche.
    • This contraction of the realized niche is a result of pressure from, and interactions with, other organisms.
niche specialization
Niche Specialization
  • Niches become separated to avoid competition for resources.

Figure 7-6


Number of individuals

Species 2

Species 1



niche overlap

Resource use

Number of individuals

Species 1

Species 2

Resource use

Fig. 7-6, p. 150

species interactions competition and predation1
  • Species called predators feed on other species called prey.
  • Organisms use their senses their senses to locate objects and prey and to attract pollinators and mates.
  • Some predators are fast enough to catch their prey, some hide and lie in wait, and some inject chemicals to paralyze their prey.
  • Some prey escape their predators or have outer protection, some are camouflaged, and some use chemicals to repel predators.

Figure 7-8



(a) Span worm

Fig. 7-8a, p. 153



(b) Wandering leaf insect

Fig. 7-8b, p. 153


Chemical warfare

(c) Bombardier beetle

Fig. 7-8c, p. 153


Chemical warfare

& warning coloration

(d) Foul-tasting monarch butterfly

Fig. 7-8d, p. 153


Chemical warfare

& warning coloration

(e) Poison dart frog

Fig. 7-8e, p. 153



(f) Viceroy butterfly mimics

monarch butterfly

Fig. 7-8f, p. 153


Deceptive looks

(g) Hind wings of Io moth

resemble eyes of a much

larger animal.

Fig. 7-8g, p. 153


Deceptive behavior

(h) When touched, snake caterpillar changes shape to look like head of snake.

Fig. 7-8h, p. 153

species interactions parasitism mutualism and commensalim
  • Parasitism occurs when one species feeds on part of another organism.
  • In mutualism, two species interact in a way that benefits both.
  • Commensalism is an interaction that benefits one species but has little, if any, effect on the other species.
parasites sponging off of others
Parasites: Sponging Off of Others
  • Although parasites can harm their hosts, they can promote community biodiversity.
    • Some parasites live in host (micororganisms, tapeworms).
    • Some parasites live outside host (fleas, ticks, mistletoe plants, sea lampreys).
    • Some have little contact with host (dump-nesting birds like cowbirds, some duck species)
mutualism win win relationship
Mutualism: Win-Win Relationship
  • Two species can interact in ways that benefit both of them.

Figure 7-9


(d) Lack of mycorrhizal fungi on juniper seedlings

in sterilized soil

Fig. 7-9d, p. 154

commensalism using without harming
Commensalism: Using without Harming
  • Some species interact in a way that helps one species but has little or no effect on the other.

Figure 7-10

ecological succession communities in transition
  • New environmental conditions allow one group of species in a community to replace other groups.
  • 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.


  • The process where plants & animals of a particular area are replaced by other more complex species over time.
primary succession starting from scratch
Primary Succession: Starting from Scratch

Primary succession begins with an essentially lifeless are where there is no soil (bare rock). Soil formation begins with lichens or moss.

primary succession
Primary Succession
  • Primary begins with a lifeless area where there is no soil (ex. bare rock). Soil formation begins with lichens or moss.
primary succession1
Primary Succession
  • Primary succession refers to colonization of a region where there is no pre-existing community. Examples include:
    • Newly emerged coral atolls, volcanic islands
    • Newly formed glacial moraines
    • Islands where the previous community has been extinguished by a volcanic eruption

Hawaii: Local plants are able to rapidly recolonize barren areas

primary succession2
Primary Succession
  • A classical sequence of colonization begins with lichens, mosses, and liverworts, progresses to ferns, grasses, shrubs, and culminates in a climax community of mature forest.
    • In reality, this scenario is rare. Unless there is the formation of a new island or volcanic eruption.

Mature, slow growing trees

Shrubs and fast growing trees

Grasses and herbaceous plants

Mosses and liverworts

Bare rock

and lichens

secondary succession starting over with some help
Secondary Succession: Starting Over with Some Help
  • Secondary succession begins in an area where the natural community has been disturbed.

Figure 7-12


Mature oak-hickory forest

Young pine forest

with developing

understory of oak

and hickory trees


and pine



weeds and





Fig. 7-12, p. 157

secondary succession
Secondary Succession
  • Secondary succession occurs where an existing community has been cleared by a disturbance that does not involve complete soil loss.
  • Such disturbance events include cyclone damage, forest fires, hillside slips and clear-cutting.
    • Clear-cutting reduces ecosystem services provided to humans like oxygen, food, and removal of carbon


Forest fire

secondary succession1
Secondary Succession
  • Because there is still soil present, the ecosystem recovery tends to be more rapid than primary succession, although the time scale depends on the species involved and on climatic and edaphic (soil) factors.

Mature forest

Young fast growing trees

Shrubs and small trees

Grasses and herbaceous plants

Pioneer community (annual grasses)

Bare land

pioneer communities
Pioneer Communities

Pioneer community, Hawaii

  • A succession proceeds in stages, until the formation of a climax community, which is stable until further disturbance.
  • Early successional (or pioneer) communities are characterized by:
    • Simple structure, with a small number of species interactions
    • Broad niches
    • Low species diversity

Broad niches

climax communities
Climax Communities
  • In contrast to early successional communities, climax communities typically show:
    • Complex structure, with a large number of species interactions
    • Narrow niches with typically large old growth plants.
    • High species diversity

Climax community, Hawaii

Large number of species interactions

  • Land – rock  lichen  small shrubs  large shrubs  small trees  large trees
can we predict the path of succession and is nature in balance
Can We Predict the Path of Succession, and is Nature in Balance?
  • The course of succession cannot be precisely predicted.
  • Succession involves species competing for enough light, nutrients and space which will influence it’s trajectory.
ecological stability and sustainability
  • Inertia (persistence): the ability of a living system to resist being disturbed or altered.
  • Constancy: the ability of a living system to keep its numbers within the limits imposed by available resources.
  • Resilience: the ability of a living system to bounce back and repair damage after (a not too drastic) disturbance.
ecological stability and sustainability1
  • Having many different species appears to increase the sustainability of many communities.
  • Human activities are disrupting ecosystem services that support and sustain all life and all economies.