Chapter 54

Community Ecology

• What is a Community?• It is an assemblage of species living close

enough together for potential interaction.• Communities differ

in their species richness, thenumber of speciesthey contain, andthe relativeabundance ofdifferent species.

Introduction

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• There are different interspecific interactions, relationships between the species of a community.

Introduction

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• Competition (-/-)• Predation (+/-)• Mutualism (+/+)• Commensalism (+/o)

• Competition.

– Interspecific competition: can occur when resources are limited.

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Ecological niche = the "role" a species "plays" in the ecosystem.

An organisms use of biotic and abiotic resources in its environment

Contrast the ecological niche with the "habitat" which is the physical environment in which the organism lives.

The ecological niche of a species, therefore includes:

species’ habitat

abiotic & biotic interactions

No two similar species occupy the same niche at the same time.

Extinction of one species

G. F. Gause (1934) tested competitive exclusion principle

Constant food supply

extinction

• Classic experiments confirm this.

Fig. 53.2

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Resource partitioning: splitting the niche

Sympatric species consume slightly different resources or use resources in slightly different ways

Insect-eating warblers

Character displacement:

two similar species evolve in such a way as to become different from each other by accentuating their initial minor differences

Allopatric vs Sympatric populations

Allopatric populations:

Similar beak morphologies and eat similar sized seeds

Avoids competition

• Predation.

– A predator eats prey.

– Herbivory, in which animals eat plants.

– In parasitism, predators live on/in a host and depend on the host for nutrition.

– Predator adaptations: many important feeding adaptations of predators are both obvious and familiar.• Claws, teeth, fangs, poison, heat-sensing

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• Offset oscillations in the population sizes of the predator and prey

• Coevolution of predator and prey

– Plant defenses against herbivores include chemical compounds that are toxic.

– Animal defenses against predators.• Behavioral defenses include fleeing, hiding, self-

defense, noises, and mobbing.

• Camouflage includes cryptic coloration, deceptive markings.

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• Mechanical defenses include spines.• Chemical defenses include odors and toxins• Aposematic coloration is indicated by

warning colors, and is sometimes associated with other defenses (toxins).

Devil scorpionfish

backside

• Mimicry is when organisms resemble other species.– Batesian mimicry is where a harmless species

mimics a harmful one.

monarch viceroy

• Müllerian mimicry is where two or more unpalatable species resemble each other.

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Fig. 53.8

monarch queen

Cuckoo bee Yellow jacket

blenny (Aspidontus taeniatus) mimics Hawaiian cleaner wrasse

shortnose wrasse mimics Potter's angel which sports a defensive spine

Mimicry

– Parasites and pathogens as predators.• A parasite derives nourishment from a host,

which is harmed in the process.• Endoparasites live inside the host and

ectoparasites live on the surface of the host.

• Parasitoidism is a special type of parasitism where the parasite eventually kills the host.

• Pathogens are disease-causing organisms that can be considered predators.

• Mutualism is where two species benefit from their interaction.

• Commensalism iswhere one speciesbenefits from theinteraction, but other is not affected.

Coral polyp & zooxanthellae

• Coevolution and interspecific interactions.

– Coevolution refers to reciprocal evolutionary adaptations of two interacting species.• When one species evolves, it exerts

selective pressure on the other to evolve to continue the interaction.- flowers and pollinators- hermatypic coral and zooxanthellae- predator and prey- parasite and host

Lobelia Hawaiian honeycreeper

• The trophic structure of a community is determined by the feeding relationships between organisms.

• The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is called the food chain.

Trophic structure is a key factor in community dynamics

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Food ChainsFood Chains

• Artificial devices to illustrate energy flow from one trophic level to another

• Trophic Levels: groups of organisms that obtain their energy in a similar manner

• Total number of levels in a food chain depends upon locality and number of species

• Highest trophic levels occupied by adult animals with no predators of their own

• Secondary Production: total amount of biomass produced in all higher trophic levels

Food ChainsFood Chains

Antarctic Food Web

Keystone SpeciesA species whose presence in the community exerts a significant influence on the structure of that community.

Paine’s study on Pisaster and blue mussels

An Ecological Mystery

Kelp Forests

Keystone Species

Keystone Species

An Ecological MysteryAn Ecological Mystery• Long-term study of sea otter populations

along the Aleutians and Western Alaska• 1970s: sea otter populations healthy and

expanding• 1990s: some populations of sea otters

were declining• Possibly due to migration rather than

mortality• 1993: 800km area in Aleutians surveyed

- Sea otter population reduced by 50%

Vanishing Sea OttersVanishing Sea Otters

• 1997: surveys repeated• Sea otter populations had declines by 90%

- 1970: ~53,000 sea otters in survey area- 1997: ~6,000 sea otters

• Why?- Reproductive failure?- Starvation, pollution disease?

Cause of the DeclineCause of the Decline• 1991: one researcher observed an orca

eating a sea otter• Sea lions and seals are normal prey for

orcas• Clam Lagoon inaccessible to orcas- no

decline• Decline in usual prey led to a switch to sea

otters• As few as 4 orcas feeding on otters could

account on the impact- Single orca could consume 1,825 otters/year

Ecological Succession

The progressive change in the species composition of an

ecosystem.

The progressive change in the species composition of an

ecosystem.

Ecological Succession

Climax StageClimax Stage

New Bare SubstrateNew Bare Substrate

Colonizing StageColonizing Stage

Successionist StageSuccessionist Stage

Temperate Old-field Succession

Pioneer

Seral stage

Years afterabandonment

Herbs Grasses Shrubs PineForest

HardwoodForest

1-2 1-10 7-25 25-70 75+

Climax

Hanauma Bay Tuff Ring(shield volcano)

Succession after Volcanic Eruption

What organisms would appear first?

How do organisms arrive, i.e., methods for dispersal?

Volcanic eruption creates sterile environment

Mechanisms of Succession

Facilitation

Inhibition

Tolerance

Early species improve habitat.

Ex. Early marine colonists provide a substrate conducive for settling of later arriving species.

As resources become scarce due to depletion and competition, species capable of tolerating the lowest resource levels will survive.

Competition for space, nutrients and light; allopathic chemicals.

First arrivals take precedence.

r & K Selected Species

Pioneer species- 1st species to colonize a newly disturbed area

r selected

Late successional species

K selected

low competitive abilityshort life spanhigh growth rate

higher maternal investment per offspringlow reproductive output

high reproductive output

slow growth ratelong life spanhigh competitive ability

r & K refer to parameters in logistic growth

equation

Ecological Succession on a Coral Reef

Successional Models and their Impacts

• Case 1: No Disturbance (Competitive Exclusion Model)

• Case 2: Occasional Strong Disturbance (Intermediate Disturbance Model)

• Case 3: Constant Strong Disturbance (Colonial Model)

Case 1: No Disturbance(Competitive Exclusion Model)

• As the reef becomes complex, organisms compete for space.• Dominant organism outcompetes other species.• Occurs in stable environments. • Results in low species diversity.• Highly protected patch reefs within lagoons or protected bays• Deeper water

Case 2: Occasional Strong Disturbance(Intermediate Disturbance Model)

• Storms and hurricanes allow for other species to move in• Dominant species would not be allowed to reach competitive exclusion• After each disturbance have a recovery period• Area of high diversity

Case 3: Constant Strong Disturbance(Colonial Model)

• Constant exposure to disturbance• Shallow environment• High turnover of species• r-selected species

Reef

Case 3

Case 2

Case 1Deep reef slope

Reef slope beneath reef crest

Near reef crest

Ecological Succession on a Coral ReefThe Big Island

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Ecological Succession on a Coral Reef

Successional Models and their Impacts

Successional Models and their Impacts

Ohia lehuasuccession

• Because of their size and isolation, islands provide great opportunities for studying some of the biogeographic factors that affect the species diversity of communities.– Imagine a newly formed island some distance

from the mainland.• Robert MacArthur and E. O. Wilson developed a

hypothesis of island biogeography to identify the determinants of species diversity on an island.

Species richness on islands depends on island size and distance from the mainland

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• Two factors will determine the number of species that eventually inhabit the island.–The rate at which new species immigrate to the

island.

–The rate at which species become extinct.

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• Studies of plants on many island chains confirm their hypothesis.

Number of plant species on the Galapagos Islands in relation to the area of the island