ecology and evolution 15

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Community StructureLecture 15



What is an ecological community?

� A community is a group of species that occupy agiven area and interact directly or indirectly

� Communities can be measured in a number of ways:

Species richness

Evenness Relative abundance

Frequency

Diversity ± Shannon index

Dominance



Rank abundance curves� Rank abundance curves are a

simple way of graphically

depicting the relationship between species richness andevenness for a community

� The y-axis is relativeabundance, the x-axis isabundance rank

� Intepretation: steeper curvesmean fewer species, greaterdominance, reduced evenness



Dominance

� Dominance measures reflectthe degree to which one

species represents the majority of individuals in a community

� In high-elevation beech forestsin Great Smoky MountainsNational Park, beech canrepresent > 90% of woody stems



Foundation species

� Generally an autotroph thatcontrols energy flows and

nutrient cycles through theecosystem ± we would call thisspecies a foundation species

Ex: beech in beech forests



Keystone species

� Generally a top predator ± akeystone species is one that,

through its feeding actions,controls community structure



Loss of keystone or foundation species

� Studies show that the removal (accidental or

intentional) of keystone or foundation speciescan have cascading affects on ecosystems

The loss of sea otters devestated fishing stocks inthe Pacific

T

he loss of A

merican beech in high-elevationforests reduced diversity, alters animal use of these forests



Food webs

� Feeding relationships can have both direct and indirect effects

on organisms in a community

� A very important step in every trophic diagram is missingfrom this one ± what is it?



Functional groups

� Guilds are groups of species within a community

that exploit a resource in a similar fashion Example: plants are photosynthetic autotrophs

However, within a forest there are woody speciesthat dominants and others that are subcanopy

trees and shrubs Herbs may be annuals, wintergreen perennials,spring ephemerals, vernal herbs, etc...



Forest communities of G

RSM



Great Smoky Mountains elevationprofile



� The previous figure and this graphare the work of R.H. Whittaker

(1956) They represent two ways of

depicting the distribution of forest communities in GreatSmoky Mountains National Park

� Whittaker recognized that

elevation and aspect influencedprecipitation and temperature

Trees in the Smokies respondedto that to form characteristiccommunities



E

cotone� A n ecotone is a zone of transition between two communities orecosystems

Will often have some species from both communities in it

Ecotones are not always obvious



How are communities formedInteractive hypothesis

(Clements)Individualistic hypothesis

(Gleason)

� Communities are likesuperorganisms with species

locked in mandatory bioticinteractions

� The community is anintegrated whole

� Communities are chanceassemblages of species found

together because of similarabiotic requirements

� Communities are formed outof the independent responsesof species



Whittaker·s opinion

� In most cases the compositionof communities changes

continuously along anenvironmental gradient



How important is each species in a community?

R ivet model R edundancy model

� The rivet model suggests thateach species in a community isunique and that the loss of any species weakens thecommunity

� The redundancy modelsuggests that most species aresuperfluous and that there arenumerous species within eachguild

� The loss of any one species can

be compensated for by otherspecies in their guild



What influences community structure?

� Foundation and keystone species

�Niche constraints

� Indirect interactions

� Why is the world green?

Top-down vs. bottom-up control

� Species interactions� Environmental heterogeneity

� Resource availability

Paradox of enrichment



Foundation species

� Through their ability to control energy flows and

nutrient cycles foundation species can influencethe composition and structure of a community

� Foundation species that are allelopathic canexclude species that might compete with it

Why are spruce and fir not found in beech gaps?



Keystone species

� Robert Paine found thatcommunity diversity was

higher in areas with apredatory starfish, Pisaster ochraceous, than in those without it

Why do you think this is?



Niche constraints

� Communities are theconsequence of variations in

the environment and the setsof conditions under whichspecies can exist (their niches)



Shelford·s Law of Tolerance� The distribution of a species will be controlled by

the environmental factor for which the species has

the lowest tolerance



Diffuse species interactions

� Competition and predation do not usually occuronly between two species

Rather, in communities species of the same guildcompete with all or most other species of the sameguild (and some from other guilds)

Predators are capable of ³switching´ to alternativeprey and prey often have to worry about more than

one predatorx Removal of any one competitor or predator has little

effect on the community

x This implies that for most species in a community there issome functional redundancy



Indirect interactions

� K eystone predation influences the relative

successes of competing species� A pparent competition: when the abundances of prey species are lower when found together(implying interspecific competition) than when

they occur separately This is mediated by a predator who lowers the

abundances of each species in a way that makes itlook as if they compete with one another, when in

fact, they don¶t



Why is the world green?

� Proposed by Hairston, Smith, and Slobodkin

(HSS) (1960) it questioned why herbivores don¶tdefoliate plants regularly

The ³Green World´ model attempts to get atcommunity structure through understandingtrophic interactions

Implicit in this question is the idea that energy isthe currency of ecosystem function (EugeneOdum)



The HSS model

� Density-dependent factors like competition are

more important than density-independentfactors

� They argued that plants, detritivores, andcarnivores were resource limited but that

herbivores were controlled by predators



Top-down control

� Communities structured by the actions of top

predators reducing the numbers of herbivoresare considered to be controlled in a top-downmanner

The HSS model is a top-down model



Criticism of the HSS model

� HSS suggest that herbivores are not food

limited, overlooking the fact that plants are notdefenseless

� Herbivores can be limited by other factors besides predation



Bottom-up control

� Trophic diagrams suggest that the productivity of any trophic level is controlled by theproductivity of the level below it (remember the10% rule)

� A community with a foundation species would

be one controlled by bottom-up processes



Species interactions

� The birth, growth, and reproductive output of aspecies is influenced by its tolerance to the rangeof environmental conditions encountered in itshabitat (fundamental niche space)

Variations in environmental factors also influencethe outcome of competition



Environmental heterogeneity

� Environmental conditions are not homogenous within a community

� Resources like light, water, macronutrients, andmicronutrients vary at small spatial scales

� More heterogenous environments usually have

greater diversity Ex: the number of bird species in a prairie vs. in a

forest

x Forests have greater structural heterogeneity



Resource availability

� Soils with high fertility usually have fewer vascular plant

species than those with low fertility

³Paradox of enrichment´

� Enriching soils with importantlimiting nutrients changescompetitive interactions



Park grass experiment: the costs ofnitrogen enrichment

� A > 140 year-long experiment tounderstand the effect that

nitrogen fertilization has onplant diversity

� In all cases species richnessdeclined

� Only the unfertilized plotsretain the original plant species

and diversity



Resource availability depends on size

� Size symmetric competition occurs when anorganism gains resources in proportion to itssize

Belowground competition in plants

� Size asymmetric competition occurs when one

species takes more than its size-predicted shareof resources

A boveground competition in plants

ecology and evolution 15

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