COMMUNITY ECOLOGY

• Overview: What Is a Community?– Is an assemblage of populations of various species living

close enough for potential interaction• The various animals and plants surrounding this watering hole

– Are all members of a savanna community in southern Africa

Figure 53.1

Species interactionsSpecies interact in several fundamental ways.

Species interactionsWhen 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.

The Competitive Exclusion Principle• The competitive exclusion principle

– States that two species competing for the same limiting resources cannot coexist in the same place

• However, ecologically similar species can coexist in a community– If there are one or more significant difference in their

niches

When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.

The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche.

RESULTS

CONCLUSION

Ocean

Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.

EXPERIMENT

In nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata.

Low tide

High tide

Chthamalusfundamental niche

Chthamalusrealized niche

Low tide

High tideChthamalus

Balanusrealized niche

Balanus

Ocean

Figure 53.2

Ecological Niches• The ecological niche

– Is the total of an organism’s use of the biotic and abiotic resources in its environment

• The niche concept allows restatement of the competitive exclusion principle– Two species cannot coexist in a community if their

niches are identical

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

Fundamental niche Realized niche

• As a result of competition– A species’ fundamental niche may be different from

its realized niche

Interspecific competitionAdjusting 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

A. insolitususually percheson shady branches.

A. distichus perches on fence posts and

other sunny surfaces. A. distichus

A. ricordii

A. insolitus

A. christophei

A. cybotes

A. etheridgei

A. alinigar

Figure 53.3

Resource Partitioning•Resource partitioning is the differentiation of niches

– That enables similar species to coexist in a community

Resource partitioning

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

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

Predator–prey cyclesPopulation 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

Cryptic coloration:Camouflage to hide from predators

Warning coloration:Bright colors warn that prey is toxic

Mimicry:Fool predators(here, caterpillar mimics snake)

• In Batesian mimicry– A palatable or harmless species mimics an

unpalatable or harmful model

(a) Hawkmoth larva

(b) Green parrot snake

Figure 53.7a, b

• In Müllerian mimicry– Two or more unpalatable species resemble each

other

(a) Cuckoo bee

(b) Yellow jacketFigure 53.8a, b

Parasitism

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

Interspecific Competition

Brown Headed Cowbird lays her eggs in nest of different species of bird (referred to as brood parasitism)

Herbivory

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

MutualismBoth species benefit one another.

Hummingbird pollinates flower while gaining nectar for itself.

Mutualism• Mutualistic symbiosis, or mutualism an

interspecific interaction that benefits both species

Figure 53.9

Commensalism• In commensalism

– One species benefits and the other is not affected

Figure 53.10

Amensalism• Amensalism = one species is harmed; the

other is unaffected

• Ex: Sheep and Cattle trample grass to make a trail, kill the grass

• Ex: bread mold penicillium secretes penicillin, a chemical that kills bacteria.

Interspecific Interactions and Adaptation

• Evidence for coevolution– Which involves reciprocal genetic change by interacting

populations, is scarce.Example: snapdragons that many people plant in their

gardens are designed for a bumblebee of just the right weight to trip the opening mechanism.

Example: Predator – Prey

Energy Passes among trophic levels 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.

Consumers• 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 levelsTogether, these comprise trophic levels:

Food chains and webs

• Food chain = simplified linear diagram of who eats whom

• Food web = complex network of who eats whom

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

Food web for an eastern deciduous forest

QUESTION: Review

a. Producer, secondary consumer, herbivore

b. Producer, herbivore, secondary consumer

c. Secondary consumer, producer, detritivore

d. Herbivore, carnivore, producer

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

Answer: B

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

• Gopher Tortoise – Florida Keystone Species

What Is Disturbance?

• A disturbance– Is an event that changes a community– Removes organisms from a community– Alters resource availability

• Fire– Is a significant disturbance in most terrestrial

ecosystems– Is often a necessity in some communities

(a) Before a controlled burn.A prairie that has not burned forseveral years has a high propor-tion of detritus (dead grass).

(b) During the burn. The detritus serves as fuel for fires.

(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

Figure 53.21a–c

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 (without human interference)

Ecological Succession• Ecological succession

– Is the sequence of community and ecosystem changes after a disturbance

i.e. It is a series of regular, predictable, quantifiable changes through which communities go

Succession

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

Life has never existed there, no soil present.

• Secondary succession: The community changes following a disturbance (fire, hurricane, logging). Life once existed there….soil is already present.

Secondary terrestrial succession

Primary terrestrial succession

Examples of Primary Succession

Newly exposed rock from receding glacier

Newly formed volcanic Island

1. Open pond

2. Plants begin to cover surface; sediment deposited

3. Pond filled by sediment; vegetation grows over site

Primary aquatic succession

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.

Human Disturbance• Humans

– Are the most widespread agents of disturbance

• Human disturbance to communities– Usually reduces species diversity

• Humans also prevent some naturally occurring disturbances– Which can be important to community structure

QUESTION: Review

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

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

Answer: B

Invasive Species What is an invasive species?

(1)nonnative to the ecosystem under consideration, and(2)whose presence causes or is likely to cause economic or environmental harm, or harm to human health

Terminology

Invasive : Alien, nonnative, exotic, naturalized, weed

Native: Indigenous Endemic – species lives only in one specific area

Costs of Invasive Species • Economic >$120 billion annually in the U.S.

• Health Introduced pathogens and diseases Example: Dutch Elm Disease,, West Nile Disease

• Biodiversity - 2ndmost important cause of loss of biodiversityIn the U.S., >1/2 of the species listed as threatened or

endangered are at risk due to competition with or predation by nonnative species

• Ecological systems in disturbance

Invasive Species - SuccessCharacteristics of Invasive species that enable them to thrive in new environments: •Generalist species•Excellent dispersal mechanisms allowing it to spread•R-selected/r-strategist example: many offspring produced, early onset of reproduction, frequent reproduction •Superior defenses against predators in new environment (Novel weapons hypothesis – ex: Plants….chemicals exuded in the native range are benign to surrounding (co-evolved) plants. In new areas these compounds are often toxic ex: Cane Toad)

•Superior skills as a predator in new environment

Characteristics of new environment that enable them to thrive…•Lack of natural predators, parasites, pathogens to keep population in check

Invasive Species – Success

Invasive Species – most common crimes

• Compete for resources with native species

• Prey on native species

Zebra Mussels – Case Study • Introduced into Great Lakes – first discovered in

lakes 1988

How did it get there? Ballast water dumping by large ocean-going vessels from Europe

Mechanisms for Spread • Within 2-3 years – spread to all 5 Great Lakes, Hudson

River, Illinois River to Mississippi River• By 1994 colonized waters in 19 US states and 2 Canadian

provinces• Main Reason – Recreational activities…Transport of boats

or boat trailers with mussels attached• Flooding of isolated lakes with floodwater containing

mussels• Transport by animal vectors….example: brought with fish

used to stock lake waters• Building canals between infested and noninfested lakes

Impacts Economical Impacts: •Clogging up water intake pipes•Large densities - Damage boat engines, docks, fishing gear, and sink buoys ships use for navigation

Ecological Impacts • Consume entire population of phytoplankton –

diminish other populations that compete for this resource

Ex: zooplankton, fish, other native mussels• Disrupt food chains • Suffocate native mussels – attaching to their shells

Ecological Effect • Benefit bottom dwellers of lake – create lake clarity

by consuming algae….more sunlight available to bottom dwelling plants for photosynthesis

• Wastes – more organic material at bottom of lake • Increased populations of “visual feeders” – improved

ability to “see” prey…ex: water fowl…certain fish species

• Alter water chemistry – through filtering and digestion of their food, also shells store minerals from the water

Methods that might be used to prevent spread?

• Cleaning of boats before transport or launch• Refrain from building connecting canals between

waterways• No catching or distributing mussels for use as bait or pets

in aquariums• No transporting of water from one lake to the next (bait

buckets…should be dumped on land when possible• Education to discourage practices causing spread • Study its behavior and biology – determine best strategies

for controlling spread

Biological Control

Method of decreasing population of invasive species by re-introducing some of the natural enemies that exert control on it (using live organisms)

Biological Control Gone Bad – Cane Toad

Cane Toad – biological control gone bad

• Introduced in Australia in 1935 as biological control against Greyback beetle that were destroying sugar cane crops

• Native to South and Central America• Had been successfully used as biological control against

beetles in Hawaii • Lack of research on Cane Toad was at fault: 1. Sugar reaches heights of 6-8m, beetles feed on tops of

plant….toads unable to reach that high 2. Timing – beetles are out in daytime, toad feeds at night 3. Wrong habitat – Sugar Cane fields much drier than

native habitat of toads…toads need wet conditions….quickly moved from sugar cane fields to moister areas.

Spread of Cane Toad…1. Generalist species – can live in a variety of habitats, eat a

variety of foods2. Predator to many native species (eats them)3. Competition – Out competes native species for food and

breeding sites4. R- selected species – can lay 8,000-30,000 eggs at a

time....Australia’s native frogs only lay 1,000-2,000 eggs per year.

5. Poisonous – few predators have resistance to this venom6. No successful predators or agents in reducing their

population

Biological Control – Success Stories Cottony Cushion Scale

Biological Control Success Stories Cottony Cushion Scale

• The insect sucks the phloem sap from the leaves, twigs, branches, and trunk from California citrus trees

• Named after the white cottony secretion it produces • First discovered in citrus groves in 1868• Natural predator Ladybird beetle • 1988 514 beetles introduced into citrus groves of

California…by 1890 all infestations of cottony cushion scale were eliminated

Some benefits of Biological Control

• Cost effective • No chemicals – environmentally safe• Self sustaining