• A community is a group of interacting populations, all living in the same place at the same time– the focus is on the interactions between species

or populations including competition, predation , succession, invasion, mutualism, predation, etc.

Community Ecology

Community Structure and Change

• COMMUNITY STRUCTURE - a description of the community members (species list) and their relative abundances

• COMMUNITY DYNAMICS - the changes that occur over time and space in a community. (Even though communities have an underlying structure, the structure may change over time.

Emergent PropertiesEmergent Properties

• Properties not predictable from study of component populations

• Only apparent at level of community

• Properties not predictable from study of component populations

• Only apparent at level of community

Why is this important?Why is this important?

Appropriate unit of study:

- If the community is more than the sum of its parts, then we must study the entire community

(holistic approach)

- If not then entire picture can be put together from individual pieces

(reductionist approach)

Appropriate unit of study:

- If the community is more than the sum of its parts, then we must study the entire community

(holistic approach)

- If not then entire picture can be put together from individual pieces

(reductionist approach)

The Study of Ecological Communities

• Properties & patterns– Diversity (Number

of species)– Species’ relative

abundances– Morphology

– Succession

• Processes– Disturbances

– Trophic interactions

– Competition

– Mutualism

– Indirect effects

Two Views on Communities

• Community as a superorganism (equilibrium community, Clements)

• Species not replaceable • Species need one another to survive • Community as a group of individual species

(non-equilibrium community, Gleason) • Species are replaceable • Random association of species

Community Dynamics: Succession

• Succession - The change in numbers and kinds of organisms in an area leading to a stable (climax) community. Replacement of communities.

• Pioneer community - the first community to develop in a successional sequence

• Seral stage - any successional community between pioneer and climax community

1. Primary – situation where barren substrate is available for habitation (inorganic substrates= lava flows/ spreading centers

2. Secondary – occurs in areas where communities have previously existed (after fires or hurricane; much more rapid)

1. Primary – situation where barren substrate is available for habitation (inorganic substrates= lava flows/ spreading centers

2. Secondary – occurs in areas where communities have previously existed (after fires or hurricane; much more rapid)

Types of SuccessionTypes of Succession

Succession in community traits

increasing size of organisms

increasing longevity of organisms

shift from predominantly "r-selected" to predominantly "K-selected" species

shift from "individualistic" to predominantly "interactive" organization

increasing biomass

increasing independence of physical/chemical environment

Succession in community traits (2)

decreasing rate of change

increasing species diversity

increasing complexity of physical and trophic structures

increasing habitat modification and buffering of environmental extremes

increasing complexity of energy and nutrient flows

increasingly closed systems re-cycling of organic and inorganic materials

Opposing Views of CommunitiesOpposing Views of Communities Superorganism View (Clements, 1916)

- tightly evolved, interacting

- functions as a single organism

- developmental process (succession)

- homeostasis (self maintaining – stable)

- underlying “balance of nature”

Superorganism View (Clements, 1916)

- tightly evolved, interacting

- functions as a single organism

- developmental process (succession)

- homeostasis (self maintaining – stable)

- underlying “balance of nature”

Individualistic View (Gleason, 1925)

- randomly assembled

- Similar resource requirements

Individualistic View (Gleason, 1925)

- randomly assembled

- Similar resource requirements

F.E. Clements (1916, 1936) idea of

succession F.E. Clements (1916, 1936) idea of

succession • Succession• Sere• Climax• Ecosystem =

superorganism

Types of SpeciesTypes of Species

Early successional- good colonizers- rapid growth- short lived (r-selected)

Early successional- good colonizers- rapid growth- short lived (r-selected)

Late Successional-poor colonizers- slow growth- long lived (k-selected)

Late Successional-poor colonizers- slow growth- long lived (k-selected)

Correlation between life history traits necessaryCorrelation between life history traits necessary

EE LL

SuccessionSuccession

bb bbAA cc

BBcc cc cc

CC

Early Colonizing

Early Colonizing

MidMixedMid

MixedLate

ClimaxLate

Climax

TimeTime

Under Equilibrium ModelsUnder Equilibrium Models

• Community returns to same position after disturbance

• At equilibrium, processes that structure the community produce no net change

• Community returns to same position after disturbance

• At equilibrium, processes that structure the community produce no net change

Equilibrium TheoryEquilibrium Theory

Single stable stateSingle stable state

Multiple stable states

Multiple stable states

Outcomes of integrated viewOutcomes of integrated view

• Equilibrium assumed (not tested)

• Explained succession

• Super-organism concept widely accepted

• Dominated community ecology until the 1950’s and beyond

• Equilibrium assumed (not tested)

• Explained succession

• Super-organism concept widely accepted

• Dominated community ecology until the 1950’s and beyond

Non-equilibrium models

• Disturbance is the norm rather than the exception

• Disturbed patches provide opportunities for colonization by dispersive species

• Patchiness promotes diversity on a larger scale

Evidence for each view:• Superorganism: • remove plants or autotrophs, the community will

disappear • mutualisms and symbiotic relationships are common • example: herbivore gut bacteria• Non-equilibrium • high-level consumers can be removed without major

effects on community • disturbances often play a role in determining

community structure; these are random

Alternative succession modelsAlternative succession models

Connell and Slatyer (1977) – outlined 3 models:

1. Facilitation – Clementsian succession

2. Tolerance

3. Inhibition

Based on effect of initial spp. on subsequent spp.

Connell and Slatyer (1977) – outlined 3 models:

1. Facilitation – Clementsian succession

2. Tolerance

3. Inhibition

Based on effect of initial spp. on subsequent spp.

Facilitation ModelFacilitation Model

EE LL

RecruitmentRecruitment

GrowthGrowth

RecruitmentRecruitment

EE LL

FacilitationFacilitation

MortalityMortality

DisturbanceDisturbance

EEEE

EEEE

Early StandEarly Stand

EEEE

LLLLLL

Mixed StandMixed Stand

LLLL

LL LLLL

Late Successionals onlyLate Successionals only

Tolerance ModelTolerance Model

EE LL

RecruitmentRecruitment

GrowthGrowth

RecruitmentRecruitment

EE LL

ToleranceTolerance

MortalityMortality

DisturbanceDisturbance

EEEE

LLLLLL

Mixed StandMixed Stand

LLLL

LL LLLL

Late Successionals onlyLate Successionals only

EELLLL

LL

EE

Inhibition ModelInhibition Model

EE LL

RecruitmentRecruitment

GrowthGrowth

RecruitmentRecruitment

EE LL

InhibitionInhibition

MortalityMortality

DisturbanceDisturbance

EEEE

LLLLLL

Mixed StandMixed Stand

LLLL

LL LLLL

Late Successionals onlyLate Successionals only

LL EELLEE

Time 0Time 0 Disturbance opens space; slate wiped cleanDisturbance opens space; slate wiped clean

Time 1Time 1 Only certain species can establish themselves in open space; Opportunists, Fugitives, Weeds

Only certain species can establish themselves in open space; Opportunists, Fugitives, Weeds

No special requirements for first colonizersNo special requirements for first colonizers

Time 2Time 2 First colonists modify environment so it becomes less suitable for their further recruitment but more suitable for other species

First colonists modify environment so it becomes less suitable for their further recruitment but more suitable for other species

First colonists make environment less suitable for their own further recruitment, but this has little or no effect on other species

First colonists make environment less suitable for their own further recruitment, but this has little or no effect on other species

First colonists make environment less suitable for all subsequent species

First colonists make environment less suitable for all subsequent species

Time 3Time 3 Process continues until residents no longer facilitate recruitment of other species

Process continues until residents no longer facilitate recruitment of other species

Process continues until no species can invade and grow in presence of residents

Process continues until no species can invade and grow in presence of residents

First colonists continue to hold space and exclude all others (First Come, First Served)

First colonists continue to hold space and exclude all others (First Come, First Served)

ModelModel FACILITATIONFACILITATION TOLERANCETOLERANCE INHIBITIONINHIBITION

SuccessionSuccession

•Can occur without invoking the existence of a “Super-organism”

•Sequential replacement a consequence of individual species properties

•Can occur without invoking the existence of a “Super-organism”

•Sequential replacement a consequence of individual species properties

Physical disturbance

What are the components of disturbance?

• The frequency of a disturbance

• The intensity of the disturbance

• The timing of the disturbance

– Influences the availability of larvae to recolonize the disturbed area

Disturbance simplified

• The greater the disturbance the more habitat that will be opened up

Intermediate Disturbance Hypothesis (Connell 1972)

• Disturbance (eg, tree falls, storms) creates patchinessand new space to be colonized

•Patchwork is created across the landscape with- early and late successional species- inferior and superior competitors

This theory is considered a non-equilibrium view of how natural communities are structured because landscape is a

patchwork of different stages of succession.

Intermediate Disturbance Hypothesis (2)

Frequency and intensity of various kinds of abiotic disturbances affect patterns of diversity.

Disturbance is critically important in structuring communities because it can prevent competitively dominant species from excluding others.

Weak/infrequent disturbances insufficient to prevent competitive exclusion

Intense/frequent disturbances exclude species sensitive to disturbance

Highest diversity might therefore be expected at intermediate frequency or intensities of disturbance

Intermediate Disturbance Hypothesis (Connell)

Community structure could be controlled Community structure could be controlled bottom-upbottom-up by nutrients: by nutrients:

herbivoresherbivores

predatorspredators

nutrientsnutrients

autotrophsautotrophs numbers of numbers of autotrophs autotrophs are limited are limited by mineral by mineral nutrientsnutrients

community community structure can be structure can be changed by changed by manipulating the manipulating the lower levelslower levels

Top-down vs. bottom-up control

Community structure could be controlled Community structure could be controlled top-downtop-down by predators (trophic cascade model) by predators (trophic cascade model)

autotrophsautotrophs

nutrientsnutrients

herbivoresherbivores

predatorspredators numbers of numbers of herbivores herbivores are controlled are controlled by predatorsby predators

predicts a series predicts a series of +/- effects if of +/- effects if upper levels are upper levels are manipulatedmanipulated

Reintroduction Reintroduction and protection and protection of otters has of otters has

reduced urchin reduced urchin barrensbarrens

Predation by Predation by orcas has orcas has increased increased

urchin urchin barrensbarrens

Trophic cascades

Species-area Relationships

Known for a long time that there is a relationship between the size of an island and the number of species present on the island.

This relationship, which exists for all taxa studied to date, whether on land or in the sea, is known as the Species-area Relationship.

Species-area relationships Species-area relationships

• species-area curve - the larger the species-area curve - the larger the geographic area, the greater the number of geographic area, the greater the number of speciesspecies

fig 53.25

• larger areas have larger areas have more diverse more diverse habitathabitat

• this can be used this can be used to predict how to predict how habitat loss may habitat loss may affect key speciesaffect key species

Species Area Relationships

• As a rule of thumb for every 10x increase in habitat area you can expect a doubling in species abundance

• this relationship is best described by the regression formula S=cAz

– where: S = the number of species, c= a constant measuring the number of species/unit area, A= habitat area, and z is another constant measuring the shape of the line relating S & A

Often linearized• ln (S ) and ln (A )

• ln (S ) = ln (c ) + z ln (A )– z is now the slope– ln (c ) is now the intercept

ln (S )

ln (A )

Top: Species-area curve for corals in coral reefs on Rasdu Atoll, Maldives, and on Heron Island, Great Barrier Reef. Adapted from Scheer (1978). Bottom: Relation of number of species and number of individuals in a sample, based on twenty samples of benthic invertebrates collected from Buzzards Bay,

Why do Species-Area Relationships Exist?

Habitat heterogeneity - as area increases so will habitat number, and species number

Area per se - extinction rates will go down with increasing area as populations increase

Passive sampling - as area increases there is a larger “target for immigrants to “hit” –

Disturbance - smaller areas will be subject to more disturbance (DI mortality) and species number will be frequently “set back”

Importance of Islands in Ecology

Islands can provide opportunities for natural experiments because different islands in an archipelago can have different species of potential competitors, or lack certain predators. Thus, the effects of processes such as competition and predation can be easily studied on islands.

Islands are also widespread, even on land, because any isolated patch of habitat is effectively an island (e.g., lakes, coral reefs, kelp beds) for the species living there.

Island Biogeography

Because of the generality of the species-area relationship, Preston (1962) and MacArthur & Wilson (1963, 1967) proposed that islands were supporting as many species as possible.

Since islands continuously receive immigrants, yet species number stays constant, there must be a balance between immigration and extinction.

Preston and MacArthur & Wilson proposed that the number of species on an island is in a dynamic equilibrium between immigration and extinction.

Island Biogeography (Island Biogeography (MacArthur and MacArthur and Wilson, 1960’s)Wilson, 1960’s)

fig 53.26a

• immigration rate decreases with Sp. N immigration rate decreases with Sp. N since it becomes more likely that since it becomes more likely that immigrants will not be new speciesimmigrants will not be new species

• extinction rate increases with Sp. N extinction rate increases with Sp. N because of a greater likelihood of because of a greater likelihood of competitive exclusioncompetitive exclusion

• equilibrium reached when immigration equilibrium reached when immigration and extinction rates are equaland extinction rates are equal

• equilibrium number is correlated with equilibrium number is correlated with area and distance area and distance from mainlandfrom mainland

The number of species on an island is in a dynamic The number of species on an island is in a dynamic equilibrium determined by imm. and ext. ratesequilibrium determined by imm. and ext. rates

Island Biogeography (2)

This “dynamic equilibrium” between immigration and extinction was developed into a quantitative theory that was termed The Theory of Island Biogeography.

The theory of Island Biogeography has two major points: the area and distance effects.

A

B

Mainland

Area effect

Area EffectArea Effect

fig 53.26b

• larger islands are more likely to be larger islands are more likely to be found by immigrants which found by immigrants which increases immigration rateincreases immigration rate

• organisms are less likely to go organisms are less likely to go extinct on larger islands because extinct on larger islands because there is more available habitatthere is more available habitat

• equilibrium number is higher on equilibrium number is higher on larger islands because of both larger islands because of both higher immigration and lower higher immigration and lower extinctionextinction

Island size influences immigration and extinction rates Island size influences immigration and extinction rates because……because……

A

B

Mainland

Distance effect

Distance EffectDistance Effect

fig 53.26c

• given islands of the same size, given islands of the same size, immigration will be higher on immigration will be higher on near islands since they are more near islands since they are more likely to be found by likely to be found by immigrantsimmigrants

• extinction rates the same (same extinction rates the same (same size islands)size islands)

• equilibrium number is higher equilibrium number is higher on near islands because of on near islands because of higher immigrationhigher immigration

Distance from the mainland influences immigration and Distance from the mainland influences immigration and extinction ratesextinction rates

Tests of the Island

Biogeographic Theory

Lots of small scale colonization studies were consistent with the Theory

Best know test is the “million dollar experiment” of Simberloff and Wilson

Although the results of this study continue to be cited in support of the Theory, Simberloff says they only provide weak support.

Tests of Island Biogeographic Theory (2)

This was because many of his extinctions were found to be transients that could not survive on his mangrove islands, or species that visited the islands as part of a larger range (e.g., wasps). Thus, much of the measured turnover was “pseudoturnover”.

He concluded that the Theory still needed verification, as have others since then.

Insect recolonization of four defaunated mangrove islands. The y axis indicates the predefaunation species richness of each island. Most of the islands reached an equilibrium species number after 250 days that was approximately the same as the initial richness. (From Simberloff and Wilson 1969.)

Island biogeography is a simple Island biogeography is a simple model and we must take into model and we must take into account abiotic disturbance, account abiotic disturbance,

adaptive changes, and speciation adaptive changes, and speciation eventsevents

Latitudinal species richness gradientsLatitudinal species richness gradients• species richness of many taxa declines from species richness of many taxa declines from

equator to polesequator to poles

• Why? NOT KNOWNWhy? NOT KNOWN

fig 53.23

Land birds

Could be evolutionary or Could be evolutionary or ecological factors, or both?ecological factors, or both?

Diversity along geographical gradients. Corals from the Great Barrier Reef; copepods from the Pacific; remaining data from all oceans. After Thorson (1957) and Fischer (1970).

Factors Hypothesized to Influence Biodiversity (Factor/ Rationale) Factor Rationale

1. History 2. Spatial heterogeneity 3. Competition 4. Predation 5. Climate 6. Climatic variability 7. Productivity 8. Disturbance

More time permits more complete colonization and the evolution of new species Physically or biologically complex habitats furnish more niches a. Competition favors reduced niche breadth b. Competitive exclusion eliminates species Predation retards competitive exclusion Climatically favorable conditions permit more species Stability permits specialization Richness is limited by the partitioning of production among species Moderate disturbance retards competitive exclusion

Source: Modified after Pianka (1988) and Currie (1991).

Recent Explanations for Latitudinal Diversity Gradients

increased area of the tropics

increased effective evolutionary time due to shorter generation times in the tropics

The world’s tropical lands cover about four times the area s the world’s second largest biome, the tundra. Tropical oceans also cover more surface than oceans in other climate zones. From Rosenzweig (1992).

Island Biogeography and Conservation

In many areas, (1) the total area of natural habitats is shrinking, and (2) formerly contiguous habitats are being fragmented.

In island biogeographic terms, this means that island areas are shrinking and large islands are being broken into archipelagos.

Island Biogeography and Conservation (2)

Island biogeographic theory allows predictions to be made about the effects of reducing and fragmenting habitats, and to make recommendations for conservation

Areas of application: (1) How large should preserves be? (2)How does isolation affect species number in reserves? (3) What kinds of species will survive if area is reduced?

Application of biogeographic principles to the design of nature preserves. In each pair of figures the design on the left is preferred over that on the right, even though both incorporate the same area.

The concepts are: A, a continuous reserve is better than a fragmented one; B, the ratio of area to perimeter should be maximized; C, distance between refuges should be minimized; and D, dispersal corridors should be provided between fragments.

(from Ecology and Evolution of Communities, ed. M. L.

Cody and J. M. Diamond, 1975 .

What is a Food Web?

• Describes which kinds of organisms in communities eat other kinds

• Community food web is a description of feeding habits of a set of organisms based on taxonomy, location or other criteria

• Webs were derived from natural History approaches to describing community structure

Food Webs: trophic assignments

• Primary Producers – Phytoplankton and some macrophytes

• Consumers: heterotrophic animals– Primary consumers: herbivores– Secondary and higher-level consumers:

carnivores, predators

• Decomposers: bacteria and fungi

Food webs: trophic assignments cont.

Trophic level Name 4 Top carnivore

3 Carnivore

2 Herbivore

1 Autotroph

What is a Food Web (2)?

• Food webs portray flows of matter and energy within the community

• If community is like a city, then Food Web is like a street map of a city

• Web omits some information about community properties – e.g., minor energy flows, constraints on

predation, population dynamics

Food Webs: Methods

1. Identify component species

2. Sample to determine who is eating whom

3. Sampling and gut analysis to quantify frequency of encounters

4. Exclosures and removals of species to determine net effects

5. Stable isotopes

6. Mathematical models

Descriptive Food Webs

Interaction or functional food webs depict the most influential link or dynamic in the

community

What is a Food Web (cont.): Complexity meets reality

• Fallacy of linear food chains as a adequate description of natural food webs – Food webs are reticulate– Discrete homogeneous trophic levels an abstraction or

an idealism– omnivory is rampant– ontogenetic diet shifts (sometimes called life history

omnivory)– environmental diet shifts – spatial & temporal heterogeneity in diet

What is a food web (cont.)?

• Modern Approaches to Food Web Analysis– Connectivity relationships

– Importance of predators and interaction strength in altering community composition and dynamics

– Identification of trophic pathways via isotope analysis.

• Weakness of above: no quantitative measure of food web linkages.

Trophic Basis of Production• Assimilation efficiency varies with resource

– 10% for vascular plant detritus– 30% for diatoms and filamentous algae– 50% for fungi– 70% for animals– 50% for microbes (bacteria and protozoans)– 27% for amorphous detritus

• Net Production Efficiency production/assimilation ~ 40%

Energy flow through ecosystems

• Energy transfer between trophic levels is not 100% efficient, and energy is lost as it passes up a food chain.

• Herbivores eat a small proportion of total plant biomass

• They use a small proportion of plant material consumed for their growth. The rest is lost in feces or respiration

• Less energy is available at the next trophic level.

Marine Ecology: Food Webs

• Ecological efficiency is defined as the energy supply available to trophic level N + 1, divided by the energy consumed by trophic level N. You might think of it as the efficiency of copepods at converting plants into fish food.

• In general, only about 10% of the energy consumed by one level is available to the next.

• Difficult to measure so food web scientists focus on measure of transfer efficiency

Food Webs

• A pyramid of biomass represents the amount of energy, fixed in biomass, at different trophic levels for a given point in time

• The amount of energy available to any trophic level is limited by the amount stored by the level below.

• Because energy is lost in the transfer from one level to the next, there is successively less total energy as higher trophic levels.

Food Webs in the Ocean• The oceans can be an exception, because at

any time the total amount of biomass in microscopic algae is small. A pyramid of biomass for the oceans can appear inverted

• However, a pyramid of energy, which shows rates of production rather than biomass, must have the characteristic pyramid shape. Algae can double in days, whereas zooplankton that feed on them might double in months, while fish feeding on zooplankton might only reproduce once a year. Thus, a pyramid of energy takes into account turnover rate, and can never be inverted.