chapter 21: community structure robert e. ricklefs the economy of nature, fifth edition (c) 2001 by...

Chapter 21: Community Structure

Robert E. RicklefsThe Economy of Nature, Fifth Edition

(c) 2001 by W. H. Freeman and Company

What is a community?Ecologists have puzzled for almost a century over how to define a community, the assemblage of species that occur together in the same place.

Although ecologists agree that coexisting species interact strongly through consumer-resource and competitive interactions, they do not agree about what a community is.

two extreme views have dominated the debate over the nature of the community: F.E. Clements’s discrete unit H.A. Gleason’s loose assemblage of species


The Community View of Frederic E. ClementsClements saw the community as a superorganism in

which the functions of various species are connected like the parts of the body.

Clements’s view included the following ideas:that component species had coevolved so as to enhance their

interdependent functioningthat communities were discrete entities with recognizable

boundaries


The Community View of Henry A. GleasonGleason saw the community as a fortuitous association of

species whose adaptations and requirements enable them to live together under the particular conditions of a particular place.

Gleason’s view included the following ideas: that component species occurred together largely by coincidence that there was no distinct boundary where one community meets

another


Biological Communities every place on earth is shared by many coexisting organisms:

these plants, animals, and microbes are linked to one another by their feeding relationships and other interactions, forming a complex whole referred to as a biological community: ecologists are uncertain as to the factors that determine the number of species

that can coexist


Diverse Concepts of CommunityThe holistic concept of Clements and others recognizes

that we can only understand each species in terms of its contributions to the dynamics of the entire system.

The individualistic concept of Gleason and others recognizes that community structure and function simply express the interactions of individual species, and do not reflect any organization above the species level.


Community Concepts - A Middle Ground?An intermediate or mixed view of communities also exists, which:accepts the individualistic view that most interactions are

antagonistic and that communities are haphazard assemblages of species

accepts the holistic premise that some attributes of communities arise from interactions among species, reinforced through coevolution


Ecologists use several measures of community structure. One of the most widely used measures of community structure is the

number of species it includes, or species richness: this measure captures differences among tropical, temperate, and boreal regions:

16 km2 Barro Colorado Island in Panama supports 211 tree species, more than in all of Canada

plots of 1 hectare in Amazonian Peru and Ecuador support more than 300 tree species


Ecologists use several measures of community structure.Because biological communities contain large numbers of

species, it is helpful to partition diversity into numbers of species at each trophic level (such as herbivores):within trophic levels, method or location of foraging distinguishes

guilds of species (such as leaf eaters within the herbivore trophic level)

Patterns of relative abundance also permit ecologists to quantify structure of communities.


“Community” has many meanings.Community has a spatial definition:

assemblages of plants and animals occurring in a particular locality and dominated by one or more prominent species or some physical characteristic

Community has a functional definition focusing on interactions:migrations of animals and linkages between terrestrial and aquatic

systemsecological and evolutionary effects of all populations upon one

another


A Natural Unit of Ecological Organization? The holistic view of communities predicts a closed

community: the distributions of species are coincident the boundaries between communities (ecotones) are distinct

The individualistic view of communities predicts an open community: the distributions of species are independent the boundaries between communities are diffuse


When do communities have distinct boundaries? The concept of closed communities predicts discrete boundaries between

communities: such boundaries should be expected under two circumstances in nature:

when there is an abrupt transition in the physical environment when one species or life form dominates strongly, such that the edge of its

range determines the limits of many other species


Ecotones Ecotones represent boundaries between closed communities:

such boundaries occur when there are sharp discontinuities in the physical environment: the interface between terrestrial and aquatic communities the boundary between soil types with contrasting properties (such as the

boundary between serpentine and nonserpentine soils)


Plants contribute to conditions maintaining ecotones.Transitions between broad-leaved and needle-leaved

forests become more pronounced because of conditions created by the plants themselves:increased soil acidity and greater accumulation of

undecayed litter distinguishes the needle-leaved forest

Fire may sharpen the boundary between prairies and forests in the Midwestern U.S.perennial grasses resist fire damage, but fires cannot

penetrate deeply into forests


The Continuum Concept 1Ecotones are less likely to form along gradients of gradual

environmental change:the deciduous forest region of eastern North America does not fit the

concept of the closed community: few species have closely overlapping geographic ranges, tending to be

independently distributed sharp ecotones are not found

As ecologists sought to understand the ecology of the eastern forests, they turned to the continuum concept.


The Continuum Concept 2The continuum concept embodies several key concepts:

plants and animals replace one another continuously along environmental gradients

species have different geographic ranges, suggesting independent evolutionary backgrounds and ecological relationships

because few species have broadly overlapping ranges, the assemblage of species found in any particular place does not represent a closed community


Gradient AnalysisA gradient analysis is usually undertaken by measuring the

abundances of species and physical conditions at a number of locations within a landscape: the abundances of species are then plotted as a function of the value of any

physical condition

Studies by R.H. Whittaker in the Great Smoky Mountains revealed few cases of distinct ecotones between associations of species: species were distributed more or less independently over ranges of ecological

conditions, with few cases of consistent association between species


Feeding relationships organize communities in food webs. From an ecosystem perspective, species are usually combined into relatively

few trophic levels: a food web analysis emphasizes the diversity of feeding relationships within an

ecosystem: food web analysis thus has greater potential to differentiate community

structure however, community structure is difficult to define, so different analyses may

produce different results


Does food web complexity lead to increased community stability?Food web complexity should lead to stability:

when consumers have alternative resources, their populations depend less on fluctuations in any one resource

where energy can take many routes through the ecosystem, disruption of one pathway shunts more energy through another

But more diverse communities with many food web links may create pervasive, destabilizing time lags in population processes!


How does food web structure affect community stability? Robert Paine and others who have studied food webs in natural

communities have stressed the importance of consumer-resource relationships in community organization: populations of keystone predators are particularly important in maintaining

community stability and diversity


There are different ways to portray food webs.Connectedness webs emphasize feeding relationships as

links in a food web.

Energy flow webs represent an ecosystem viewpoint, in which connections between species are quantified by flux of energy.

Functional webs emphasize the importance of each population through its influence on growth rates of other populations.


How does food web structure affect community stability?Is one particular arrangement of feeding relationships more stable

than another?

How important is food web stability to the structure of natural communities?

These questions have proven difficult to answer:natural food webs vary tremendously, but each has persisted over long

periods of timeperhaps the rules governing community structure depend on particular

circumstances of each system(c) 2001 by W. H. Freeman and Company

Generalizations emerge from food web studies.Communities may be characterized by the number of

species (richness) and number of feeding links per species:the number of feeding links per species is independent of the

species richness of the communitythe number of trophic levels and the number of guilds per

trophic level increase with community diversity


Trophic levels are influenced by predation and production.Alternative views of the effects of various trophic

levels upon one another emphasize either top-down control or bottom-up control:Hairston, Smith, and Slobodkin argued in 1960 that the “earth

is green” because carnivores depress the populations of herbivores that would otherwise consume most of the vegetation: this is a top-down perspective emphasizing a trophic cascade


Top-Down versus Bottom-Up ControlEcologists have debated the relative strengths of top-

down versus bottom-up control mechanisms:is the earth green because plants resist consumption through

various digestion inhibitors and toxic substances?studies in lakes find evidence for both top-down and bottom-

up control of community structure: primary production generally determines the sizes of higher trophic

levels (bottom-up control), but top-down interactions adjust these sizes within a narrower range


Species vary in relative abundance.One of the important differences among species

within communities is their relative abundance:in most communities, a few species achieve dominance while

most are rare, represented by relatively few individualsecologists have portrayed relative abundances in rank-order

graphs, which reveal interesting patterns: although such patterns have been modeled, such models have been

most valuable as descriptive tools rather than elucidating processes that determine relative abundance


Number of species increases with area sampled. Arrhenius first formalized the species-area relationship as:

S = cAz

where: S = number of species encountered

A = area

c and z are fitted constants

After log transformation, the relationship is linear:

logS = logc + zlogA(c) 2001 by W. H. Freeman and Company

Species-Area RelationshipsAnalyses of many species-area relationships have

shown that values of the slope, z, fall within the range 0.20 - 0.35.

Are species-area relationships artifacts of larger sample size (more individuals) in larger areas?comparisons of species numbers in different areas where

samples of similar size were used still reveals a species-area relationship


Predictable Species-Area Relationships Slopes of species-area curves vary in predictable ways:

z-values are less for continental areas, greater for islands: rapid movement of individuals within continental areas prevents local extinction

within small areas


Why do larger areas have more species?Larger areas have greater habitat heterogeneity.

For islands, size per se makes the island a better target for potential immigrants from the mainland.

Larger islands support larger populations, which persist because they have:greater genetic diversitybroader distributions over habitatsnumbers large enough to prevent stochastic extinction


Diversity IndicesAlthough species richness is a useful measure of biological

diversity, it also has certain problems:the number of species encountered varies with the number of

individuals inventoriedspecies differ in abundance and thus in their functional roles in

communities

Diversity indices have addressed the second of these problems by weighting species by their relative abundance...


Common Diversity Indices Simpson’s index is:

D = 1/pi2

where: pi = the proportion of each species in the total sample

Shannon-Wiener index is:

H = - pilogepi


Properties of Diversity IndicesSimpson’s index, D, can vary from 1 to S, the number of species in a

sample: larger values of S indicate greater diversity when all species have equal abundances, D = S when species have unequal abundances, D < S rare species contribute less to the index than common ones

The Shannon-Wiener index, like Simpson’s, takes on larger values with greater diversity: expressing Shannon-Wiener as eH scales the index to the number of species,

making it more comparable to Simpson’s(c) 2001 by W. H. Freeman and Company

Rarefaction Richness values from samples of unequal size cannot be compared:

rarefaction allows for comparisons, using a statistical procedure in which equal-sized subsamples are drawn at random from the total sample: this portrays relationship of richness to sample size rarefaction was used by Howard Sanders to compare samples of benthic

organisms


Summary 1A biological community is an association of interacting

species.

Ecologists consider community diversity and organization of species into guilds and food webs.

Two competing concepts of community organization are holistic and individualistic, predicting closed and open communities, respectively.


Summary 2In general, ecologists find that communities do not form

discrete units. Species tend to distribute themselves independently along environmental gradients in a pattern more consistent with the open community concept.

Ecologists have devised techniques of gradient analysis to study distributions of species with respect to gradients of environmental conditions.


Summary 3Community structure can be summarized by means of

food webs that emphasize various relationships among species.

Consumers can depress abundances in trophic levels below them in a trophic cascade or top-down effect. Bottom-up effects occur when one trophic level affects productivity of higher trophic levels.


Summary 4In any community, some species are common and some are

rare. Patterns of relative abundance have been characterized, but their meanings are poorly understood.

The number of species increases with the area sampled, more strongly so on islands.

Various indices of diversity have been used to compare the number and relative abundances of species between communities.


chapter 21: community structure robert e. ricklefs the economy of nature, fifth edition (c) 2001 by...

Documents

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biological community

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component species

number of species

species richness

species level

functions of various