Transcript Chapter 21: Community Structure - Eco

Chapter 21: Community
Structure
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 by W. H. Freeman and Company
 Ecologists have puzzled for almost a century over how to
What
is a community?
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
(c) 2001 by W. H. Freeman and Company
The Community View of Frederic E. Clements
 Clements
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 functioning
 that communities were discrete entities with recognizable
boundaries

(c) 2001 by W. H. Freeman and Company
The Community View of Henry A. Gleason
 Gleason
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

(c) 2001 by W. H. Freeman and Company
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
(c) 2001 by W. H. Freeman and Company
Diverse Concepts of Community
 The
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.
(c) 2001 by W. H. Freeman and Company
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

(c) 2001 by W. H. Freeman and Company
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
(c) 2001 by W. H. Freeman and Company
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.
(c) 2001 by W. H. Freeman and Company
“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
systems
 ecological and evolutionary effects of all populations upon one
another

(c) 2001 by W. H. Freeman and Company
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

(c) 2001 by W. H. Freeman and Company
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
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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)
(c) 2001 by W. H. Freeman and Company
Plants
contribute
tobroad-leaved
conditions maintaining
 Transitions
between
and needle-leaved
ecotones.
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
(c) 2001 by W. H. Freeman and Company
The Continuum Concept 1
 Ecotones
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
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The Continuum Concept 2
 The
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
(c) 2001 by W. H. Freeman and Company
represent a closed community

Gradient Analysis

A 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
conditions, with few cases of consistent association between
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W. H. Freeman and Company

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
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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!
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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
(c) 2001 by W. H. Freeman and Company
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.
(c) 2001 by W. H. Freeman and Company
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 time
the rules governing community structure depend on
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by perhaps
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 community
 the number of trophic levels and the number of guilds per
trophic level increase with community diversity

(c) 2001 by W. H. Freeman and Company
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
(c) 2001 by W. H. Freeman and Company
Top-Down versus Bottom-Up Control
 Ecologists
have debated the relative strengths of topdown 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 bottomup 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
(c) 2001 by W. H. Freeman and Company
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 individuals
 ecologists 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
(c) 2001 by W. H. Freeman and Company
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
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Species-Area Relationships
 Analyses
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
(c) 2001 by W. H. Freeman and Company
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
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 Larger
areas have greater habitat heterogeneity.
Why
do
larger
areas
have
more
species?
 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 diversity
 broader distributions over habitats
 numbers large enough to prevent stochastic extinction

(c) 2001 by W. H. Freeman and Company
Diversity Indices
 Although
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 inventoried
 species 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...
(c) 2001 by W. H. Freeman and Company
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
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Properties of Diversity Indices

Simpson’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
(c) 2001 by species,
W. H. Freeman and
Company it more comparable to Simpson’s
making

Rarefaction

Richness values from samples of unequal size cannot be compared:

rarefaction allows for comparisons, using a statistical procedure in which equalsized 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
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Summary 1
A
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.
(c) 2001 by W. H. Freeman and Company
Summary 2
 In
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.
(c) 2001 by W. H. Freeman and Company
Summary 3
 Community
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.
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Summary 4
 In
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
(c) 2001 by W. H. Freeman and Company