1 Species diversity Ecological communities differ in species number and composition –tropics > temperate –remote islands < large islands –continents >

1

Species diversity

• Ecological communities differ in species number and composition– tropics > temperate– remote islands < large islands– continents > islands

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Species diversity

• Comprised of– species richness: number of species present– heterogeneity of species

• equitability or evenness• relative abundance of each species present in the

community

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Measurement of species diversity

• Species richness– number of species present in community– first and oldest concept of diversity– simplest estimate of diversity– only residents are counted– treats common and rare species with the

same weight

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• Heterogeneity of species– uses relative abundance to give more weight

to common species

– possibilities in a 2-species community:

Comm 1 Comm 2Species A 99 50Species B 1 50

100 100

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• Shannon-Wiener diversity function

H' = - (pi) [ln(pi)]

H’ = Shannon-Wiener index of species diversity

s = number of species in community

pi = proportion of total abundance represented by ith species

s

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Shannon-Wiener diversity index

Community 1

Species N pi ln(pi) pi[(ln(pi)]

A 99

B 1

Community 2


A 50

B 50

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Shannon-Wiener diversity index

Community 1


A 99 0.99 -0.010 -0.010

B 1 0.01 -4.605 -0.046

100 1.00 -0.056

H’ 0.056

Community 2


A 50 0.50 -0.693 -0.347

B 50 0.50 -0.693 -0.347

100 1.00 -0.694

H’ 0.694

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• Shannon-Wiener diversity function– values range from near zero to ???– increased values indicate increased diversity– index has no units; value only as comparison

between at least two communities

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Species diversity

• What increases species diversity (H’)?– increasing the number of species in the

community (s)– increasing the equitability of the abundances

of each species in the community

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Evenness

• Measurement of equitability among species in the community

• Pielou evenness

E = H’ / HmaxE = Pielou evenness

H’ = calculated Shannon-Wiener diversityHmax = ln(s) [species diversity under maximum equitability conditions]

– values range from near zero to 1

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Diversity and evenness

Community 1 Community 2

s 2 2

H’ 0.056 0.694

Hmax 0.693 0.693

E 0.081 1.000

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Community 1


A 62

B 97

C 110

D 84

E 16

Practice problem

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Community 2


A 88

B 10

C 0

D 211

E 27

Practice problem

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Practice problem

Community 1 Community 2

s

H’

Hmax

E

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Species diversity indices

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Commonness, rarity and dominance

• Preston’s log normal distribution model– a few common species with high

abundances– many rare species with low abundances

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• MacArthur’s broken stick model– random breaks in a stick log normal

distribution of pieces– results in a few large pieces and many small

pieces

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• Community organization– model 1

• a few very common species• many rare species

– model 2• a few very common and very rare species• most species of intermediate abundance

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Fig. 22.1, p. 435: Relative abundance of Lepidoptera captured in a light trap in England (6814 individuals representing 197 species).

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Biogeography

• Observations of relationships between– area and number of species– distance from source

• Island biogeography– E.O. Wilson and Robert MacArthur

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Island biogeography

• Island communities: well-defined, captive

• Variables– size– degree of remoteness– elevation

• Simple community structure

• Increase in area increase in number of species

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Island biogeography

• Habitats considered as “insular” because they are isolated from other communities– caves– mountain tops– some peninsulas– wildlife or game preserves

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Fig. 24.14, p. 502: Number of land-plant species on the Galapagos Islands in relation to the area of the island.

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Fig. 24.15, p. 503: Species-area curve for amphibians and reptiles of the West Indies.

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Island biogeography

• Relationship between remoteness and number of species– increase distance from mainland decrease

number of species– number of species present is dependent on

immigration from mainland• rate is a function of the number of species already

present on the island• number of species present = balance between

immigration and extinction

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Fig. 24.17, p. 504: Equilibrium model for biota on a single island.

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Fig. 24.18, p. 504: Equilibrium model for biota on several islands of different size and remoteness.

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Island biogeography

• Small species are found on more islands than are large species

• Number of herbivore species > carnivores

• Number of generalist herbivore species > specialist herbivores

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Island biogeography

• Species:area relationship– log : log relationship– 10-fold decrease in area 50% decrease in

number of species

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Island biogeography

• Species:area relationship

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Latitudinal diversity gradients

• Abundance and diversity patterns– latitude– elevation– mountainsides– peninsulas

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Fig. 22.5, p. 438: Number of tree species in Canada and U.S.

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Fig. 22.6, p. 439: Number of species of land birds in North and Central America.

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Fig. 22.7, p. 440: Number of species of calanoid copepods in top 50 m of transect from tropical Pacific to Arctic Ocean.

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Fig. 22.9, p. 440: Number of species of mammals in continental North America.

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Fig. 22.10, p. 440: Species richness of mammals in North and South America in relation to latitude.

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• Tree species– Malaysia (4

acres): 227– Michigan (4

acres): <15

• Ant species– Brazil: 222– Trinidad: 134– Cuba: 101– Utah: 63– Alaska: 7

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• Snake species– Mexico: 293– U.S.: 126– Canada: 22

• Fish species– Amazon R: >1000– Central American

rivers: 450– Great Lakes: 172

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Latitudinal gradient hypotheses

• History (time)

• Spatial heterogeneity

• Competition

• Predation

• Productivity

• Environmental stability (climate)

• Disturbance

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• History (time) hypothesis– tropical habitats older, more stable– support for

• geological past of temperate less constant than tropics due to glaciation

• all communities diversify with time

– argument against• as glaciers moved in, species moved south to

escape• history hypothesis can not be tested

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• Spatial heterogeneity hypothesis– higher diversity in tropics due to increase in

number of potential habitats environmental complexity moving away from

equator• macro level: e.g., topographic features• micro level: e.g., particle size, vegetation

complexity

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• Spatial heterogeneity hypothesis– Hutchinson’s n-dimensional niche

specialization– types of diversity defined by spatial

heterogeneity• within-habitats ( diversity)• between-habitats ( diversity)

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Diversity defined by spatial heterogeneity

Between habitat diversity ()

Temperate Tropical

No. species per habitat 10 10

No. different habitats 10 50

Within-habitat diversity ()

Temperate Tropical

No. species per habitat 10 50

No. different habitats 10 10

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• Competition hypothesis– less competition in temperate and polar

environments compared to tropics because these populations are more regulated by extreme environmental conditions than by biological factors

– populations maintained <K due to weather, etc. and major sources of mortality are abiotic

– since population sizes small, decreased competition for resources

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• Competition hypothesis– no weather extremes in tropics,

populations can increase to densities at which competition for resources is necessary

– promotes species diversity through specialization resource partitioning

and diversity higher in tropics due to organisms being more specialized to habitats

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Fig. 22.14a, p. 447. Niche breadth versus niche overlap determined by competition within the community.

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• Predation hypothesis– increased species diversity in tropics is

function of increased number of predators that regulate the prey species at low densities

– decreases competition among prey species– allows coexistence of prey species and

potential for new additions

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Fig. 22.16, p. 449. Janzen-Connell model for increased diversity of tropical rainforest trees: seed predation versus distance of seed from tree versus seed survival.

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• Predation hypothesis– there is more selective pressure on prey

evolving avoidance mechanisms than in becoming better competitors

– cropping principle• remove predators and prey start competing• predation increases diversity by reducing

intraspecific competition among prey species

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Community anchored by keystone starfish Heliaster in northern Gulf of California.

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• Predation hypothesis– cropping principle in lakes

• top predators (fish) feed on zooplankton• if fish are removed community diversity

decreases, becomes dominated by a few species of large, grazing zooplankton

• add fish diversity of small zooplankton and their invertebrate predators increases

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• Productivity hypothesis– tropics support a greater number of species

because more resources are available, allowing for more specialization

– in general: production diversity– exceptions

• marshes: high production, relatively low diversity• deserts: low production, high diversity

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• Environmental stability (climatic) hypothesis– annual climate in tropics more stable than

temperate or polar climates– constant climate finer specializations and

adaptations, shallower niches– tropical species number of broods / year

potential for evolutionary change rate of speciation

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• Environmental stability (climatic) hypothesis– high diversity habitats generally found in

stable climates; low diversity habitats associated with severe and/or unpredictable climates

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• Disturbance hypothesis– if community disturbance frequency is very

high local extinction of species species diversity

– if community disturbance frequency is very low competitive exclusion by dominant species species diversity

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• Disturbance hypothesis– intermediate disturbance hypothesis

• moderate disturbance maximizes diversity• leads to patches at local level

– intermediate disturbance high species diversity in some communities (not all)

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Fig. 22.20, p. 453. Model for intermediate disturbance hypothesis.

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Fig. 22.21, p. 453. Effect of periwinkle grazing on algae diversity.

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Fig. 22.21, p. 453. Effect of periwinkle grazing on algae diversity.

Community dominated by one algal species

Predator limits number of possible algal species

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Basic concepts related to energy flow and trophic structure

• Energy moves through community and is lost as heat

• Nutrients move through the community in cycles and are retained

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• Niche– sum of all parameters that enable an organism

to live in its biotic and abiotic environments• competition, food gathering, predator escape, mate

location, reproduction, etc.• temperature, moisture, nutrients, soil structure,

salinity, etc.

– Hutchinsonian niche: n-dimensional hypervolume

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• Trophic level– Lindeman (1942)

• classification of animals according to location in lake

• lake trophic groups– benthic– demersal– plankton– nekton

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• Trophic level– Lindeman (1942)

• described food chain with primary producers at base and other trophic levels of animals based on feeding relationships

• more accurately described as food web, since few organisms other than plants occupy only one feeding level

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Food webs and energy flow

• Trophic levels– ecosystem feeding levels– biomass and usable energy as level – most systems support only four trophic levels– aquatic communities have slightly longer food

chains than terrestrial communities– ultimate food chain length limited by inefficiency

of energy transfer from one trophic level to the next

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Food webs and energy flow

• Food chains– sequence of organisms where each is the

food source for the next

• Food webs– represent energy flow through ecosystem

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Trophic levels

Tertiary consumers (top carnivores)

Secondary consumers (carnivores)

Primary consumers (herbivores)

Primary producers (plants)

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Heat Heat Heat Heat

Heat

Heat

Heat

First TrophicLevel

Second TrophicLevel

Third TrophicLevel

Fourth TrophicLevel

Solarenergy

Producers(plants)

Primaryconsumers(herbivores)

Tertiaryconsumers

(top carnivores)

Secondaryconsumers(carnivores)

Detritivores(decomposers and detritus feeders)

Heat Heat

Food chain model

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Figure 23.6, p. 465. Hypothetical food web model.

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Food web terminology

• Top predators: species eaten by nothing else in the food web• Basal species: species that feed on nothing within the food web• Intermediate species: species that have both predators and prey

within the food web• Trophic species: groups of organisms that have identical sets of

predators and prey• Cycles within food web: which species eat which other species• Interaction: any feeding relationship within food web• Connectance: number of actual interactions in food web divided by

number of possible interactions• Linkage density: average number of interactions per species in the

food web• Omnivores: species that feed on more than one trophic level• Compartments: groups of species with strong linkages among group

members but weak linkages to other groups of species

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Figure 23.8, p. 467. Distribution of food chain lengths in the Ythan Estuary, NE Scotland.

95 species

5518 food chain lengths counted

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Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966).

Producers Producers Producers Producers

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Rocky intertidal community food web (Paine’s 1966 study)

• (Producer level omitted from original figure)• Level 1

– herbivorous gastropods and chitons– filter feeding bivalves– suspension feeding barnacles and

brachiopods

• Levels 2-4: carnivorous gastropods• Level 5: top carnivore

– Heliaster starfish

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Keystone species

• Usually the top carnivore

• Presence or absence determines community structure and composition

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Top carnivore

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Top carnivore

X XX

X

X

XSpecies outcompeted in absence of

keystone species

Space competitor

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Keystone species

• Paine (1974): Pacific rocky intertidal community– dominated by Pisaster starfish– remove starfish → mussel Mytilus californiensis

↑ → excludes all other invertebrate species– Mytilus becomes numerically dominant– Pisaster feeds on Mytilus → prevents Mytilus

domination of community → ↑ community diversity

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Figure 23.3, p. 462. Simplified Antarctic marine food web.

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Fig. 23.4, p. 464. Food web of boreal forest of northwest Canada.

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Generalizations about food webs

• Size of animal increases with increase in trophic level

• Abundance decreases with increase in trophic level

• Large animals can not exist on small animals as prey

• Small carnivores are limited to prey that can fit into their mouths

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Which trophic level is most important?

• Studies by Charles Elton in two square miles of Wytham Woods

• Which species could be removed without changing the community?– top carnivore, except keystone species– lower levels are food source for higher levels– importance of top carnivores <<< herbivores

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• Dependent on complexity of community– increased number of interconnections in

community → increased complexity of food web → increased stability of community structure → alternate food sources should one be removed

– redundancy model versus rivet model

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• Determining species importance– species with highest biomass– where nutrients accumulate– where energy accumulates

Documents

1 Species diversity Ecological communities differ in species number and composition –tropics > temperate –remote islands < large islands –continents >