Species interactions and community ecology
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Species Interactions and Community Ecology. Chapter 6. Species interactions. Species interact in several fundamental ways. Species interactions. When multiple species seek the same limited resource Interspecific competition is between two or more species.

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Species Interactions and Community Ecology

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Species Interactions and Community Ecology

Chapter 6

Species interactions

Species interact in several fundamental ways.

Species interactions

When multiple species seek the same limited resource

  • Interspecific competition is between two or more species.

  • Intraspecific competition is within a species.

    Usually does not involve active fighting, but subtle contests to procure resources.

Interspecific competition

Different outcomes:

  • Competitive exclusion = one species excludes the other from a resource

  • Species coexistence = both species coexist at a ratio of population sizes, or stable equilibrium


Coexisting competitors may adjust their resource use, habitat use, or way of life to minimize conflict.

Fundamental niche

Realized niche

Interspecific competition

Adjusting resource use, habitat use, or way of life over evolutionary time leads to:

  • Resource partitioning = species specialize in different ways of exploiting a resource

  • Character displacement = physical characters evolve to become different to better differentiate resource use

Resource partitioning

Tree-climbing bird species exploit insect resources in different ways.


One species, the predator, hunts, kills, and consumes the other, its prey.

Predator–prey cycles

Population dynamics of predator–prey systems sometimes show paired cycles: ups and downs in one, drive ups and downs in the other.

Predation drives adaptations in prey


Fool predators

(here, caterpillar mimics snake)

Cryptic coloration:

Camouflage to hide from predators

Warning coloration:

Bright colors warn that prey is toxic


One species, the parasite, exploits the other species, the host, gaining benefits and doing harm.


One of the most common types of exploitation is herbivory, which occurs when animals feed on the tissues of plants.


Both species benefit one another.

Hummingbird pollinates flower while gaining nectar for itself.

Amensalism and commensalism

  • Amensalism = one species is harmed; the other is unaffected

  • Commensalism = one species benefits; the other is unaffected

Roles in communities

  • By eating different foods, organisms are at different trophic levels, and play different roles in the community.

  • Plants and other photosynthetic organisms are producers.


  • Animals that eat plants are primary consumers, or herbivores.

  • Animals that eat herbivores are secondary consumers.

  • Detritivores and decomposers eat nonliving organic matter; they recycle nutrients.

Trophic levels

Together, these comprise trophic levels:

Food chains and webs

We can represent feeding interactions (and thus energy transfer) in a community:

  • Food chain =simplified linear diagram of who eats whom

  • Food web = complex network of who eats whom

Food web for an eastern deciduous forest

Keystone species

  • Species that have especially great impacts on other community members and on the community’s identity

  • If keystone species are removed, communities change greatly.

A “keystone” holds an arch together.

Keystone species

When the keystone sea otter is removed, sea urchins overgraze kelp and destroy the kelp forest community.

Resistance and resilience

  • A community that remains stable despite disturbance is showing resistance to the disturbance.

  • A community shows resilience when it changes in response to disturbance but later returns to its original state.


A series of regular, predictable, quantifiable changes through which communities go

• Primary succession: Pioneer species colonize a newly exposed area (lava flows, glacial retreat, dried lake bed).

• Secondary succession:The community changes following a disturbance (fire, hurricane, logging).

Secondary terrestrial succession

Primary aquatic succession

1.Open pond

2.Plants begin to cover surface; sediment deposited

3.Pond filled by sediment; vegetation grows over site

Climax Community

  • The transitions between stages of succession eventually lead to a climax community.

  • The climax community remains in place, with little modification, until some disturbance restarts succession.

Clements vs. Gleason

  • Frederick Clements believed that communities are cohesive entities whose members remain associated over time and space.

  • Henry Gleason maintained that communities are not cohesive units, but temporary associations of individual species that can reassemble into different combinations.

Temperate deciduous forest

  • Temperature moderate, seasonally variable

  • Precipitation stable through year

  • Trees deciduous: lose leaves in fall, dormant in winter

  • Moderate diversity of broad-leafed trees

  • North America, Europe, China

Temperate grassland

  • Temperature moderate, seasonally variable

  • Precipitation sparse but stable

  • Grasses dominate; few trees

  • Large grazing mammals

  • North America, Asia, South America

Temperate rainforest

  • Temperature moderate

  • Precipitation very high

  • Trees grow tall

  • Dark moist forest interior

  • Pacific northwest region of North America, Japan

Tropical rainforest

  • Temperature warm, seasonally stable

  • Precipitation high

  • Trees tall; forest interior moist and dark

  • Extremely high biodiversity

  • Soil poor in organic matter; is aboveground

  • Equatorial regions

Tropical dry forest

  • Temperature warm, seasonally stable

  • Precipitation highly seasonally, variable

  • Trees deciduous: dormant in dry season

  • High biodiversity

  • Subtropical latitudes


  • Temperature warm

  • Precipitation highly seasonal, variable

  • Grassland interspersed with trees

  • Large grazing mammals

  • Africa and other dry tropical regions


  • Temperature warm in most, but always highly variable b/w day and night

  • Precipitation extremely low

  • Vegetation sparse; growth depends on periods of rain

  • Organisms adapted to harsh conditions

  • Southwestern region of North America, Australia, Africa


  • Temperature cold, seasonally variable

  • Precipitation very low

  • Vegetation very low and sparse; no trees

  • Low biodiversity; high summer productivity

  • Arctic regions

Boreal forest

  • Temperature cool, seasonally variable

  • Precipitation low to moderate

  • Coniferous (evergreen) trees dominate: monotypic forests

  • Low biodiversity; high summer productivity

  • Subarctic regions


  • Temperature seasonally variable

  • Precipitation seasonally variable

  • Evergreen shrubs dominate

  • Plants resistant to fire; burns frequently

  • California, Chile, West Australia

Altitude creates patterns

As altitude increases, vegetation changes in ways analogous to changes in latitude.


  • Dividing the world’s communities into major types, or biomes, is broadly informative.

  • Understanding how communities function at local scales requires understanding species interactions.

  • Types of species interactions include predation, parasitism, competition, and mutualism.


  • Feeding relationships are represented by trophic levels and food webs, and influential species are called keystone species.

  • Humans alter communities by introducing non-native species that may turn invasive.

  • Ecological restoration can undo the changes we have caused.


Which of the following lists of trophic levels is in the correct order?

a. Producer, secondary consumer, herbivore

b. Producer, herbivore, secondary consumer

c. Secondary consumer, producer, detritivore

d. Herbivore, carnivore, producer


Primary succession would take place on all of the following EXCEPT…?

a. The slopes of a Hawaiian volcano’s new lava flow

b. A South Carolina coastal forest after a hurricane

c. Alaskan land just uncovered as a glacier melts

d. A new island formed by falling levels of a reservoir in Ohio


Which biome has warm stable temperatures, highly seasonal rainfall, deciduous trees, and high biodiversity?

a. Tropical rainforest

b. Tropical dry forest

c. Temperate rainforest

d. Taiga

Question: Weighing the Issues

What would you recommend as the primary approach to dealing with invasive species in a bay area with heavy maritime trade, and why?

a. Establish tough restrictions with trading partners

b. Use chemical treatments, barriers, and removal

c. Use biological control

d. Some combination of the above (describe)

QUESTION: Interpreting Graphs and Data

What does the graph illustrate?

a. Paired predator–prey cycle

b. Competitor exclusion

c. Resource partitioning

d. Succession

QUESTION: Interpreting Graphs and Data

In this climatograph for Los Angeles, California, in the chaparral biome, summers are… ?

a.Warm and dry

b.Warm and wet

c.Mild and dry

d.Mild and wet

QUESTION: Viewpoints

A celebrity made headlines when she illegally brought an apple into the U.S. from abroad. How far should governments go to prevent invasive species?

a.As far as they can; preventing invasive species is worth the expense and inconvenience of strict regulations.

b. Governments need to be reasonable in allowing for important activities (e.g., ship movement).

c. The threat of invasive species is vastly overrated, and governments should focus on more important priorities.

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