Chapter 56 Fall 2008faculty.weber.edu/rokazaki/Zoology1110/Chapter 56 Fall... · 2008. 12. 5. · Community Ecology Chapter 56 2 ... abundance of a different tree species • Community

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Community Ecology

Chapter 56

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Biological Communities

• Community: all organisms living together in a specific place– Evolve together– Forage together– Compete– Cooperate

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• Communities: characterized either by their constituent species or by their properties– Species richness: # of species present– Primary productivity: amount of energy

produced• Interactions among members govern many

ecological & evolutionary processes


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• Community interactions– Predation

large predator consuming sequentially smaller prey

– Symbiosis: an intimate relationship between two speciesCommensalismMutualismParasitism

• Assemblage: species included only a portion of those present in community


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• Two views of structure & functioning of communities– Individualistic concept: H.A. Gleason

community: nothing more than an aggregation of species that happen to occur together at one place

– Holistic concept: F.E. Clements community: an integrated unit; superorganism-more than sum of its parts


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In communities, species respond independently to changing environmental conditions

• Community composition changes gradually across landscapes

Most ecologists today favor individualistic concept

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Biological Communities• Abundance of tree

species along a moisture gradient in Santa Catalina Mts of Southeastern Arizona

• Each line represents abundance of a different tree species

• Community composition changes continually along gradient

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• Sometimes abundance of species in a community does change geographically in a synchronous pattern

• Ecotones: places where environment changes abruptly

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Ecological Niche

• Niche*: total of all ways an organism uses resources of its environment– Space utilization– Food consumption– Temperature range– Appropriate conditions for mating– Requirements for moisture & more

Billock

*address & occupation of an organism

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Ecological Niche

• Interspecific competition: occurs when two species attempt to use same resource & there is not enough resource to satisfy both

• Interference competition: physical interactions over access to resources– Fighting– Defending a territory– Competitive exclusion: displacing an

individual from its range

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Ecological Niche

• Fundamental niche: entire niche that a species is capable of using, based on physiological tolerance limits & resource needs

• Realized niche: actual set of environmental conditions, presence or absence of other species, in which species can establish a stable population

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Ecological Niche

J.H. Connell’s classical study of barnacles

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Ecological Niche

• Other causes of niche restriction– Predator absence or

presence• Plant species

– Absence of pollinators– Presence of herbivores

Billock

Billock

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Ecological Niche

• G.F. Gause’s classic experiment on competitive exclusion using three Paramecium species shows this principle in action

Principle of competitive exclusion: if two species are competing for a limited resource, the species that uses resource more efficiently will eventually eliminate the other locally

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Ecological Niche

• Niche overlap & coexistence• Competitive exclusion redefined: no

two species can occupy same niche indefinitely when resources are limiting

• Species may divide up resources àcalled resource partitioning

• Gause: found this occurring with two Paramecium species

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Resource partitioning

among sympatric

lizard species

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Ecological Niche

• Resource partitioning often seen in similar species that occupy same geographic area

• Thought to result from the process of natural selection

• Character displacement: differences in morphology evident between sympatric species– May play a role in adaptive radiation

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Ecological Niche

Character displacement in Darwin’s finches

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Ecological Niche

• Detection of interspecific competition can be difficult– If resources not limited there may be no

competition– Small vs. large population size– May be environmental conditions that cause

decline of a species, not competition

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Ecological Niche

• Experimental studies of competition• Seed-eating rodents & Kangaroo rats

– 50m x 50m enclosures– Enclosures had openings large

enough for seed-eating rodents but not the Kangaroo rats

– Monitor # of small rodents

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Ecological Niche

Detecting interspecific competition

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Ecological Niche

• Interpreting field data: Negative effects of one species on another do not automatically indicate the existence of competition– Adults may prey on juveniles of other

species– Presence of one species may attract

predators that prey on both species• Experimental studies are not always feasible

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Predator-Prey

• Predation: consuming of one organism by another

• Predation strongly influences prey populations• Prey populations can have explosions &

crashes– White-tail deer in Eastern US– Introduction of rats, dogs, cats on islands– New Zealand: Stephen Island wren extinct

because of a single cat

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Predator-PreyPredator-prey in microscopic worldWhen prey (Paramecium) used up in test tube,

predator (Didinium) also dies

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Predator-Prey

• Introduction of prickly-pear cactus in Australia – Cactus population exploded– Introduced predator a moth (caterpillar eats

the cactus) & population came under control

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Predator-Prey

• Predation & coevolution– Predation provides strong selective

pressure on prey population– Features that decrease probability of

capture strongly favored– Predator populations counteradapt to

continue eating prey– Coevolution race may ensue

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Predator-Prey

• Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves– Chemical defenses: secondary compounds

• Oils, chemicals to attract predators to eat herbivores, poison milky sap & others

– Herbivores coevolve to continue eating plants

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Predator-Prey

Insect herbivores well suited to their plant hosts: cabbage butterfly

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Predator-Prey

• Chemical defenses in animals– Monarch butterfly caterpillars feed on

milkweed & dogbane families– Monarchs incorporate cardiac glycosides

from plants for protection from predation– Butterflies eaten by birds, but Monarch

contains chemical from milkweed that make birds sick

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Predator-Prey

Blue Jay learns not to eat Monarch butterflies

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Predator-Prey

Poison-dart frogs of the family Dendrobatidae produce toxic alkaloids in mucus that covers their brightly colored skin

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Predator-Prey

• Defensive coloration– Poisonous insects & other animals use

warning coloration = aposematism– Organisms lacking specific chemical

defenses seldom brightly colored– Camouflage or cryptic coloration help

nonpoisonous animals blend with their surroundings

– Camouflaged animals do not usually live together in groups

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Predator-Prey

Inchworm caterpillar closely resembles a twig

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Predator-Prey

• Mimicry allows one species to capitalize on defensive strategies of another– Resemble distasteful species that

exhibit warning coloration– Mimic gains an advantage by looking

like distasteful model– Batesian mimicry– Müllerian mimicry

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Predator-Prey

• Batesian mimicry– Named for Henry Bates– Discovered palatable insects that

resembled brightly colored, distasteful species

– Mimics would be avoided by predators because they looked like distasteful species

– Feed on plants with toxic chemicals

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Predator-Prey

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Predator-Prey• Müllerian mimicry

– Fritz Müller– Discovered that several unrelated but

poisonous species come to resemble one another

– Predator learns quickly to avoid them– Some predators evolve an innate

avoidance • Both mimic types must look and act like

dangerous model

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Predator-Prey

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Species Interactions

• Symbiosis: two or more kinds of organisms interact in more-or-less intimate permanent relationships

• All symbiotic relationships carry potential for coevolution

• Three major symbiotic types:Symbiont Host

– Commensalism + o– Mutualism + +– Parasitism + -

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Species Interactions• Commensalism:

benefits one species & neutral to other– Spanish moss: an epiphyte hangs from trees

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• Commensalism may not be commensalism• Oxpickers & grazing animals

– Oxpickers eat parasites off of grazers– Sometimes pick scabs & drink blood– Grazers could be unharmed by insects

oxpickers eat

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Species Interactions• Oxpickers on an impala• Is it commensalism, parasitism or mutualism ?

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Species Interactions• Mutualism benefits both species• Coevolution: flowering plants & insects

Ants & acacias– Acacias provide hollow thorns & food– Ants provide protection from herbivores

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• Not all ant & acacia relationships mutualistic• In Kenya, several ant spp. live on acacias

– One sp. clips acacia branches to prevent other ants from living in tree

– Clipping branches sterilizes tree– A parasitic relationship

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• Parasitism benefits one species at expense of another

• External parasites: – Ectoparasites: feed on exterior surface of

an organism– Parasitoids: insects that lay eggs on living

hosts• Wasp, whose larvae feed on host body,

killing it

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External parasite: yellow vines flowering plant dodder, it is a parasite obtainings its food from host plant it grows on

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• Internal parasites– Endoparasites: live inside host– Extreme specialization by parasite as

to which host it invades– Structure of parasite may be

simplified because of where it lives in its host

– Many parasites have complex life cycles involving more than one host

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• Dicrocoelium dendriticum: flatworm living in ants as an intermediate host with cattle as its definitive host

• To go from ant to a cow it changes ant behavior • Causing ant to climb to top of a blade of grass

to be eaten with grass

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• Ecological processes à interactive effects– Predation reduces competition

• Predators choice depends partly on relative abundance of prey options

• Superior competitors may be reduced in # by predation

• Allows other species to survive when they could have been out competed

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Starfish eat barnacles, allowing other species to thrive instead of being crowded out by the explosive population of barnacles

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• Parasitism may counter competition– Parasites may affect sympatric species

differently, changing the outcome of interspecific interactions

– Flour beetles & a competition experiment• Without parasite T. castaneum dominant• With parasite: T. confusum dominant

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• Indirect effects: presence of one species may affect a second by way of interactions with a third species

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• Keystone species: absence or extreme abundance of a species alters dramatically composition of communities

• Sea star predation on barnacles greatly alters species richness of marine community

• Keystone species can manipulate environment in ways that create new habitats for other species


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Beavers construct dams & transform flowing streams into ponds, creating new habitats for many plants & animals

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Succession & DisturbancePrimary succession:

occurs on bare, lifeless substrate- Open water- Rocks- Organisms gradually move into an area &

change its nature

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Succession & Disturbance

Primary succession on glacial moraines

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• Secondary succession: occurs in areas where an existing community disturbed but organisms still remain– Example: field left uncultivated– Forest after a fire

• Succession happens because species alter habitat & resources available in ways that favor other species entering habitat

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• Three dynamic concepts in process– Tolerance: early successional species

characterized by r-selected species tolerant of harsh conditions

– Facilitation: early successional species introduce local changes in habitatK-selected species replace r-selected species

– Inhibition: changes in habitat caused by one species inhibits growth of original species

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• Animal species in a community can also change over time

• Krakatau island– Volcanic eruption– Fauna changed in synchrony with

vegetation– Changes in animals affect plant

occurrences; pollination, animal dispersion

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Succession after a volcanic eruption

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• Communities: constantly changing as a result of– Climatic changes– Species invasions– Disturbance events

• Nonequilibrium models emphasize change rather than stability used to study communities & ecosystems


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• Intermediate disturbance hypothesis:communities experiencing moderate amounts of disturbance have higher levels of species richness than communities experiencing either little or great amounts of disturbance

Disturbance

Co

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(D

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High

HighLow

Low

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–Patches of habitat exist at different successional stages

–May prevent communities from reaching final stages of succession


Intermediate disturbance hypothesis:

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• Disturbance common, rather than exceptional in many communities

• Understanding role disturbances play in structuring communities an important area of ecology

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Chapter 56 Fall 2008faculty.weber.edu/rokazaki/Zoology1110/Chapter 56 Fall... · 2008. 12. 5. · Community Ecology Chapter 56 2 ... abundance of a different tree species • Community