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Introduction to community ecology: lecture topics


Introduction to community ecology: lecture topics How do we describe communities? How do we characterize, compare, and ordinate (order, organize) communities? – PowerPoint PPT presentation

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Title: Introduction to community ecology: lecture topics

Introduction to community ecology lecture topics
  • How do we describe communities?
  • How do we characterize, compare, and ordinate
    (order, organize) communities?
  • What indices can we use to quantify species
    diversity of a community, and what are their
    strengths and weaknesses?
  • What, exactly, is a community?
  • Is it something real, easily delineated and
    highly structured biologically?
  • Or is it simply a collection of independent
    species, an arbitrary assemblage?
  • How can we use this information about

Communities are assemblages of species--here
fairly distinctive (California coast) (from
Rickleffs 2001)
Oak woodland
Lets look at bird species in 3 habitats
questions is how to compare habitats?
(Data from tin-mined areas in Indonesia from
Stiling text, Table 16.1)
Ecologists have devised a variety of indices to
species diversity of sites
  • One can use abundance of individuals, or other
    indices of importance (such as biomass, energy
  • Species richness no. species (simplest index!)
  • Dominance indices reflect concentration of
    importance by a few species, and thus emphasize
    the most abundant (or important) species
  • Simpsons index Ds Si1 to sni(ni-1)/(N(N-1)
  • ni number of individuals in species i, N
    total number of individuals in sample, s no.
  • Diversity indices like Shannons emphasize rare
  • Shannons index Hs -Si1 to spiloge(pi)
  • pi what proportion of sample is species i,
    i.e., ni/N

Other ways to quantify species diversity...
  • Evenness or equity of distribution Actual
    diversity/maximum possible diversity
  • Using Shannons index, Evenness Hs/Hmax
  • Hmax -loge(1/S), where S number of species in
    sample (this formula is derived easily from Hs
  • (One can also calculate Hmax simply by
    apportioning the species evenly--dividing total
    individuals by total number of species--and then
    calculating Hs)
  • Evenness expresses the extent to which resources
    (or energy, etc.) are divided equitably among
    species in the community

Now we can ask, for the actual communities given
in this extended example, what are values of
these various indices for species diversity?
What do diversity indices tell us about the bird
communities in the example?
  • First, the most species-rich habitats, by far,
    are the unmined sites (native vegetation)
  • What a surprise?
  • This pattern is partly function of the number of
    individuals (diversity is often related to
  • Second, the mined site shows the greatest
    dominance by a few species (presumably those
    species tolerant of the disturbance!)
  • The greatest species diversity (Shannon index)
    is in the unmined site
  • This diversity is not due to evenness
  • Therefore it must be due to the richness
  • Note Rank order of communities differs by index!

We can also ask what are the similarity indices
among the three habitats containing Indonesian
bird species?
  • Here, we use the Jaccard Coefficient (Cj), in a
    matrix of between-habitat comparisons
  • Cj a/(abc), where a no. species shared
    between two samples, b no. species in sample 1
    but not 2, and c no. species in sample 2 but
    not 1
  • This analysis shows mined (unrestored) site and
    2nd-yr. Restored sites are most similar, unmined
    site most distinctive

One application of similarity (or distance)
indices such as weve calculated is to ordinate
the sites (arrange graphically)
Unmined (native) site
Distance 1- similarity 0.68
Mined (unrestored) site
2nd-yr. Restored site
Such an analysis suggests here that the 2nd-yr.
restored site has moved in the direction of the
unmined site (a step in right direction), but not
very far after only two years.
Applicability of some of these species diversity
  • Conservation biologists, for example, tend to be
    interested more in species richness, rather than
    species diversity components
  • One can weight the species in an assemblage when
    calculating overall community metrics
  • Weight by rarity of species
  • Weight by taxonomic (genetic) distinctiveness of

Yet another way that ecologists have looked at
communities is in terms of Species-abundance
  • Method Plot species abundance on logarithmic
    y-axis, versus rank on x-axis
  • Best known types of species-abundance curve
  • Lognormal model--arises in diverse communities
    with many ecological processes influencing
  • Broken stick model--arises when species are
    fairly equitable in abundance dont dominate
    each other, e.g., due to simultaneous arrival in
    community equal competitive abilities
  • Geometric series model--corresponds with
    competitive preemption of resources by first
    arriving species, or stressful effects of
    pollutants such that only a few species are well

Hypothetical lognormal (left), broken stick
(right) species-abundance curves
Note normal distribution, log axis
Geometric series model of species abundance
Some examples of species-abundance curves (from
Rickleffs 2001) ...
Forest birds
Vascular plants in Eastern (U.S.) deciduous forest
Vascular plants in alpine fir forest
What is community ecology (based on what weve
seen so far this lecture)?
  • New level of organization (from populations)
  • Define community a group of species populations
    (e.g., plants, animals, fungi) in a given place
    also, an ecological unit or assemblage of species
    at a particular spatial scale
  • Emergent properties of communities lead to new
    set of questions (that make no sense at
    population level of organization)
  • How many species are there in a community, and
  • What are relative abundances of species, and why?
  • How are communities structured (e.g., ordinated)?
  • How interdependent are the species in their

What is the nature of communities?
  • One extreme view Community as superorganism
  • Concept most closely associated with Clements
    (20th Century), Forbes (1883)
  • Local communities hypothesized to be closed,
    discrete units, separated by sharp boundaries (of
    coincident species), and structured by strongly
    interdependent ecological (and co-evolutionary)
    relationships among component species
  • Gaia Hypothesis is related conceptidea of entire
    Earth system as a superorganism, with homeostatic
    emergent properties (e.g., ability to regulate
    atmospheric composition of gases), and different
    species have particular functional roles
  • One impetus for this Clementsian viewto allow
    classification of local communities based on
    dominant spp.

Are communities real, highly structured by
strongly interdependent species interactions?
Or are communities simply assemblages of species
that happen to co-occur in some locales?
Are communities closed or open?
What is the nature of communities?
  • Second, extreme view open communities
  • Community viewed as loose assemblage (continuum)
    of individualistic (independent) species, each
    responding to slightly different ecological
  • Concept most closely associated with Gleason, who
    challenged Clementsian view
  • If Gleasonian view correct, then few natural
    boundaries exist between communities, and precise
    classification of communities at best arbitrary
    each species best viewed as distributed
    independently of others with respect to
    competition, predation, other factors that
    affect distribution
  • Ecotones are natural, relatively abrupt, changes
    in community caused by physical environment, and
    are not support for Clementsian view

Most ecologists today accept Gleasonian view,
based on data such as these tree data of R.H.
Whittaker (from Rickleffs 2001)
Geographic ranges, e.g. these independent
distributions of Kentucky tree species, also
support Gleasonian view (Rickleffs 2001)
Plant communities (Gt. Smoky Mts.) again support
Gleasonian view independent distributions,
each species occurs widely outside the forest
type bearing its name (communities arbitrary)
(from Rickleffs 2001)
Ecotones are important, and do create relatively
abrupt boundaries (from Rickleffs 2001)...
Plant distributions in southwestern Oregon see
also next slide showing soil data for same sites
But these ecotonal boundaries generally result
from physical environment, not from tight species
interactions (from Rickleffs 2001)
This debate about the tightness of community
structuring continued into latter part of 20th
  • E.g., debate about the prevalence of competition
    structuring communities of birds, and other
  • Cody, MacArthur, others believed that
    competitive interactions are constant, strong,
    the norm among species
  • Wiens, Simberloff, others argued that
    competition not particularly strong
  • European community ecologists still emphasize
    plant community classifications (termed
  • Recent evidence on invasive species suggests that
    more structuring may be going on in some
    communities than previously thought
  • The example given involves Centaurea grass
    allelochemicals, and coevolved plants (discussed
    early in semester)

  • Communities have emergent properties such as
    diversity of species, and distributions of
    species abundances
  • Ecologists have devised diverse metrics to
    quantify community characteristics and
    relationships, and to characterize community
  • These metrics can be useful for conservation
    efforts, which emphasize particular species, rare
    threatened species
  • Most communities are probably not tightly
    structured, not real entities, but rather local
    assemblages of independently distributed species
    occupying particular area

Acknowledgements Some illustrations for this
lecture from R.E. Ricklefs. 2001. The Economy
of Nature, 5th Edition. W.H. Freeman and
Company, New York.