Transcript What`s a community

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?
Communities are assemblages of species-here fairly distinctive (California coast) (from
Rickleffs 2001)
Chaparral
Grassland
Oak
woodland
Let’s look at bird species in 3 habitats:
questions is how to compare habitats?
HABITAT:
BIRD SPECIES
UNRESTORED 2ND-YR. RESTORED UNMINED
Yel low-vente d bu lbul
Olive-ba cked s unbi rd
Ash y tail orbird
Spotted dove
Orange -bell ied flower-p ecker
7
5
2
1
1
11
10
3
4
3
44
12
20
8
8
Blue -throa ted bee-e ater
Cuckoo doves
Magpi e rob in
Les s Coucal
Plai ntive Cu ckoo
Savann a ni ghtj ar
Whi te-he aded mun ia
Sca ly-breas ted muni a
Rich ard's pi pit
Red-eyed bul bul
Woo dpecke r
Black-s houl dered ki te
Strip ed titba bble r
Larg e-tai led nigh tjar
Eastern m arsh harrie r
Sca rlet-b acked flower pecker
0
2
0
0
0
3
3
0
2
0
0
0
0
0
0
0
4
0
0
1
0
4
4
0
0
0
0
0
0
0
0
0
5
4
13
5
7
0
0
0
0
6
4
4
4
0
1
1
(Data from tinmined areas in
Indonesia; from
Stiling text,
Table 16.1)
Ecologists have devised a variety of
indices to species diversity of sites

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One can use abundance of individuals, or other
indices of “importance” (such as biomass, energy
consumed)
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
index: Ds = Si=1 to s{ni*(ni-1)/(N*(N-1))}
 ni = number of individuals in species i, N = total number
of individuals in sample, s = no. species
 Simpson’s

Diversity indices like Shannon’s emphasize rare spp.
 Shannon’s
index: Hs = -Si=1 to s{pi*loge(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
Shannon’s index, Evenness = Hs/Hmax
 Hmax = -loge(1/S), where S = number of species in
sample (this formula is derived easily from Hs index)
 (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?
BIRD SPECIES
UNRESTORED
2ND-YR. RESTORED
UNMINED
TOTAL NUMBER SPECIES
9
9
16
TOTAL NUMBER INDIVIDUALS
26
44
146
SIMPSON INDEX
8.13
7.28
7.48
SHANNON INDEX
2.01
2.01
2.34
H-MAX
2.20
2.20
2.77
EVENNESS INDEX
0.92
0.91
0.85
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 productivity!)
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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 component

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?
UNRESTORED
2ND-YR. RESTORED
UNMINED
0.70
0.32
2ND-YR. RESTORED
0.39
•Here, we use the Jaccard Coefficient (Cj), in a matrix of
between-habitat comparisons
•Cj = a/(a+b+c), 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 we’ve
calculated is to “ordinate” the sites
(arrange graphically):
“Distance” =
1- similarity =
0.68
Mined
(unrestored)
site
0.30
Unmined (native) site
0.61
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 indices?


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
species
Yet another way that ecologists have
looked at communities is in terms of
Species-abundance curves


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 abundance
 Broken stick model--arises when species are fairly
equitable in abundance & don’t 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 adapted
Hypothetical lognormal (left), broken
stick (right) species-abundance curves
Note: “normal” distribution, log axis
Geometric series model
of species abundance curve
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 we’ve seen so far this lecture)?
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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 why?
 What are relative abundances of species, and why?
 How are communities structured (e.g., ordinated)?
 How interdependent are the species in their distributions?
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 coevolutionary) relationships among component species
 Gaia Hypothesis is related concept—idea 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 view—to 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 factors
 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 Century

E.g., debate about the prevalence of competition structuring
communities of birds, and other organisms
 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 biocoenoses)
 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)
Conclusions:
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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 types
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.