Recap: Ecological Succession
Succession concepts,
type of succession
Mechanisms of succession
Climax community
16.3 Succession becomes self-limiting as it
approaches the climax
Succession continues until the
addition of new species to the
sere and the exclusion of
established species no longer
change the environment of the
developing community.
The progression from small to
large growth form modifies the
conditions of light, temperature,
moisture and soil nutrients.
Conditions change slowly after
the vegetations achieves the
largest growth form that the
environment can support.
Final dimensions of a climax
community are limited by climate
independently of events during
succession.
Succession becomes self-limiting as it approaches
the climax
Time required for succession from a new or disturbed habitat to a climax
community depends on nature of climax and initial quality of habitat
Mature oak-hickory climax forest from old field in North Carolina: 150 yrs
Climax stage of grasslands in western North America: 20-40 years
Humid tropics, reach climax within 100 years from clear cut, but may take
a few more centuries to achieves a fully mature structure and species
composition.
Sand dune  beech-maple climax, up to 1,000 years
Climax is an elusive concept:
Communities also change in response to climate change, hunting, fire, and
logging, disappearance of keystone consumers (wolf, passenger pigeon)
and trees (chestnuts, eastern hemlock)
Climax communities under extreme environmental
conditions
Fire is an important
feature of many climax
communities, favoring
fire-resistant species
and excluding species
that would otherwise
dominate.
Longleaf pine after a
fire
Seedling may be badly
burned, but the
growing shoot is
protected by the long,
dense needles.
Grazing pressure also modify a climax community
Grassland can be turned into
shrubland by intense grazing
Herbivivores may kill or severely
damage perennial grasses and allow
shrubs and cacti that are unsuitable
for forage to invade.
Selective grazing
Some species prefer to feed on areas
previously grazed by others. Both
zebras and Thompson’s gazelles feed
on Serengeti ecosystem of east
Africa, but eating different plants.
In North America, cattle grazing may
lead to invasion by alien cheatgrass,
which promote fire.
Transient and cyclic climaxes
Succession is a series of changes leading to a stable climax, whose
character is determined by local environment. Once established, a
beech-maple forest perpetuates itself, and its general appearance
changes little despite constant replacement of individuals within the
community.
Transient climaxes: such
as communities in
seasonal ponds – small
bodies of water that
either dry up in summer,
or freeze solid in winter.
The extreme seasonal
changes regularly
destroy the communities
that become established
in the ponds each year.
On African savannas, carcasses of large mammals are devoured by a
succession of vultures including: large, aggressive species  smaller
species that glen smaller bits of meat from bone  species that cracks
open bone to feed on marrow.
Cyclic climax:
Suppose, for example, species A can only germinate under species B, B only
under C, and C only under A. The relationships create a regular cycle of
species dominance in the order of A, C, B, A, C, B, A, …, in which the length
of each stage is determined by the life span of the dominant species.
Cyclic succession is usually driven by stressful environmental conditions.
When high winds damage
heaths and other types of
vegetation in northern
Scotland, shredded foliage and
broken twigs create openings
for further damage, and soon a
wide swath is opened in the
vegetation.
Regeneration occurs on the
protected side of damaged area
while wind damage further
encroaches on exposed
vegetation.
Temporal: wind damage and
regenerate, cycling
Spatial: mosaic patches
BIOL 4120: Principles of Ecology
Lecture 17: Biodiversity
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Species richness varies over the surface
of the earth
Why so may species in
tropics and few toward
the poles?
1 hectare of forest
Boreal: <5
Temperate : 10-30
Tropical: 100-300
Outline (Chapter 20)
17.1 Variation in the relative abundance of species influences
concepts of biodiversity
17.2 The number of species increase with the area sampled
17.3 Large-scale patterns of diversity reflect latitude,
environmental heterogeneity, and productivity
17.4 Diversity has both regional and local components
17.5 Diversity can be understood in terms of niche relationships
17.6 Equilibrium theories of diversity balance factors that add and
remove species
17.7 Explanations for high tree species richness in the tropics
focus on forest dynamics
17.1 Variation in the relative abundance of
species influences concepts of biodiversity
Biodiversity: variation among organisms and ecological systems
at all levels, including genetic variation within populations,
morphological and functional differences between species, and
variation in biome structure and ecosystem process in both
terrestrial and aquatic systems.
Indices of biodiversity
1. Species richness: number of species that occur within the
community (simple and most general one).
2. Relative abundance: counting all individuals of each species in
a number of sample plots within a community and determining
what percentage each contributes to the total number of
individual of all species.
Can also be quantified by density, or biomass of individuals
within a sample area, by the frequency of sample plots in
which a specific species is recorded, or by the cover
(proportion of the area of habitat covered by a species)
24
species,
256
trees
10 species
Compared to 1st forest stand, this stand has less species. Also, two
species make up 83.5% of the total tree density.
Species diversity
Graphical display of attributes of community structure
1. Rank-abundance diagram: graphical way to show relative
abundance
2. Species evenness: equitable distribution of individuals among
species
Gradual slope in the rank-abundance diagram
Length shows species
richness, slope shows
species eveness.
Rank-abundance plot
Birds in a deciduous forest
in W. Virginia; vascular
plants in a subalpine fir
forest in Great Smoky
Mountains, TN; and
vascular plants in a
deciduous cove forest in
Great Smoky Mountains.
Abundance is represented
by number of species for
birds and by NPP for plants
(Whittaker 1975).
Species diversity
Diversity indexes: an index considering only species
richness
Margalef’s index
D=(S-1)/ln(N)
S is species richness, N is sample size
Menhinick’s index
D=S/Sqrt(N)
Both indexes normalize species richness in relation
to the size of the sample.
Simpson’s Species diversity
Diversity indexes: an index considering both the number and
relative abundance of species within the community.
Simpson’s index (D): three definitions
Definition:
Where ni is the number of individuals in species i; n is the
total number of individuals of all species
D measures the probability that two individuals randomly
selected from a sample will belong to the same species
Range of D: 0 to 1 (1 is no diversity)
Species diversity
Simpson’s index of diversity=1-D
Simpson’s reciprocal index =1/D
also called Simpson’s diversity index
Calculate Simpson’s Index using Tables 16.1 and 16.2
Stand 1: D=0.13 , index of diversity 0.87, diversity index 7.7
Stand 2: D=0.36
0.64
2.8
Shannon's diversity index
introduced by Claude Shannon:
pi is the fraction of individuals belonging to the i-th species,
relative abundance.
The most widely used
17.2 The number of species increases
with the area sampled
Species-area relationship
Olaf Arrhenius, 1921
S=cA^z
Power function
log (S) = log(c) + z log (A)
Z=0.20-0.35
The number of species
increase with the area sampled
More species are found within
large areas than within small
areas.
The slope of S-A
relationship is
influenced by different
processes on different
scales
Three grasslands in
North Carolina, USA,
the Netherlands, and
Sweden
Small sample: 10 cm2
Specie richness
increase with sample
size over small areas.
S-As are nearly
superimposable.
Species richness-area relationships on
islands
The fourth process: extinction
Slope of S-A is higher when islands of different sizes are
compared than it is across continental areas over a comparable
size range.
1. The range of endemic island species (a species found only on
that island and nowhere else) can be no larger than the island,
but the range of continental species generally exceeds the
particular sampling area.
2. Dispersal in island is limited and easily to go extinction, thus
smaller and larger areas within continents have more similar
complements of species than do smaller and larger islands, and
the slope of S-A is consequently lower.
3. Habitat diversity also influence S-A relationship:
Large island has more diverse habitats
Recap:
Biodiversity
Biodiversity
Concept
Components of biodiversity
Species richness and relative abundance
Different species diversity indices
Margalef’s index
Menhinick’s index
Simpson’s indices
Species and area (S-A) relationship
17.3 Large-scale patterns of diversity reflect
latitude, environmental heterogeneity, and
productivity
Species richness increases from
north to south in the Northern
Hemisphere
Mammal species richness increases
toward the equator and in regions
of high habitat diversity
Numbers are species richness in
sample blocks 150 miles on a side
Simpson 1964.
Land birds show similar pattern,
but trees and reptiles, and
amphibians present strikingly
different patterns.
Land birds show similar pattern, but trees and reptiles, and
amphibians present strikingly different patterns.
Ecological heterogeneity and habitat
productivity
Diversity is higher in structurally complex habitats. Among several
habitats in temperate regions, the average of bird species tends to
increase with habitat productivity (Whittaker 1975)
Vegetation structure may be more
important than productivity in
determining diversity
Sonoran Desert, CA
and salt marsh, MI
Very productive, but structure simple
salt marsh has fewer plant species
than the desert.
Bird species diversity is correlated with foliage
height diversity
Robert and John
MacArthus, 1981
13 communities in
northeastern US
Deciduous forest, old
fields, and regenerating
forest habitats in
eastern North America
Solar energy input and precipitation
Two factors predict
species richness well:
Energy input from the
sun and water input
from precipitation
Brad Hawkins, UC
Irvine, 2003
Published data
Bars indicate number
of studies show
significant influences
Precipitation is more
important in tropics
and south-temperate
regions
Species richness is correlated with energy input
into the environment
PET: potential
evapotranspiration,
amount of water that
could be evaporated from
the soil and transpired by
plants, given the average
T and humidity.
The relationship leads to
energy-diversity
hypothesis
Large amount of energy in
an ecosystem can be
shared by a large number
of species.
Great input also support
higher productivity,
support large population
size, reduce extinction.
High energy increase rate
of speciation.
17.5 Diversity has both regional and local
components
Local diversity (alpha)
Number of species in a small area of homogeneous
habitat
Regional diversity ( gamma)
Total number of species observed in all habitats
within a geographic area that includes no
significant barriers to the dispersal of organisms.
Gamma diversity=alpha (each habit has all) or sum
of alpha diversity (no overlap of species)
Beta diversity: difference (or turn over) in species
from one habitat to another.
Beta diversity
Sorensen’s similarity (or coefficient of community)
Based on the species presence or absence
An Example: s1=24, s2=10, c=9; CC=2*9/(24+10)=
0.529
CC ranges from 0 to 1, (Jaccard Similarity J=
C/(C+U1+U2)
Change of beta diversity
Rate of similarity
decreases with the
distance between two
samples
Beta diversity is high in
Asia with respect to both
latitude and longitude.
Regional diversity in also
higher in Asia.
State level flora species
Beta diversity of plants decrease from
south to north in North America
Beta diversity decreases from south to
north in North America
Whittaker’s beta diversity index: beta=S_total/mean(alpha)
S_total: regional species richness; alpha is alpha diversity:
r=alpha_bar*beta.
Local communities and regional species pool
Species pool: species occur within a region.
Whether a species occurs in a local
community depends both on its adaptations
to local conditions and on its interactions with
other species
Species sorting: Species present within the
regional species pool are sorted into different
communities based on their adaptations and
interactions.
Species
sorting
8
5
12
Local communities and regional species pool
Mark Bertness,
Brown 2006
Species pool: mussels, algae species
Species sorting: exposed area, mussels; protected area, algae or
mussels.
Species interactions and ecological release
Competitive interactions between species also play an
important role in species sorting
Species sorting should be greatest where the regional
pool contains the most species.
In such a situation, each species should be able to
maintain itself over only a narrow range of habitats –
those to which it is best adapted – and beta diversity
should be high.
Species interactions and ecological release
Island and neighboring continental regions.
Comparing species richness among regions with
similar climate and range of habitats, but different
degrees of geographic isolation.
Island usually have few species than comparable
mainland areas, but island species often attain
greater densities than their mainland counterparts. In
addition, they expand into habitats that would be
filled by other species on the mainland: Collectively,
these phenomena are referred to as ecological
release.
Example of bird communities in
two continental regions and five
islands of various sizes within the
Caribbean basin
Panama: a large region with a
large species pool
St. Kitts: a small region with a
small species pool
When few species occur, each is
likely to be more abundant and to
live in more habitats
Thus, as the size of regional
species pool decrease, the realized
niche of each species becomes
broader.
17.4 Diversity can be understood in
terms of niche relationships





Fundamental niche
Boundaries of a particular species’s niche might extend
between T of 10oC and 30oC, prey sizes of 4 and 12
mm, perches on branches with diameters between 5
and 20cm, or day-time light levels between 10 and 50
W m-2
Niche overlap means competition
Niche relationships of species provide informative
measure of community structure
Three factors: total community niche space, niche
overlap among species, and niche breadth of individual
species
Competition, diversity and niche


Variation in local specie richness represent
difference in total niche space
In a particular location with a fixed volume of
total niche space, species can be added only by
increasing niche overlap (sharing of niche
space) or by decreasing niche breadth
(partitioning total niche space more finely).
Competition, diversity and niche




Intensive competition  species exclusion
High diversity is associated with weaker competition
between species
Mechanisms: competitors have to either avoid
competition through ecological specialization or be
limited by predators rather than by resources
High species richness in tropics results at least in part
from the presence of a great variety of ecological
resources
 More fruit eating bird species, nectar-feeding, and insectivorous
species
 More mammals (bat) feed on fruit, nectar and night-flying
insects (non-flying mammal species are similar to temperate)
 Herbivorous insects diversified to take advantage of immense
variety of plant species
Species diversity and niche diversity

Species diversity is paralleled by the functional
diversity or niche diversity of the species in a
community
Niche breadths /species are the same:
Morphological analyses have revealed that number
of species packed into a certain amount of
morphologically defined niche space is relatively
constant – average niche breadth remains the same
in communities with different number of species.
Therefore, as species diversity increases, so does
the niche diversity.
Bat’s communities in temperate
and tropical localities
Morphological space: two axes,
ratio of ear-forearm and ratio of
digit length
First: relate to type of prey can
locate
Second: flies ability to purse and
capture prey
Ontario, Canada and Cameroon,
West Africa
A tropical bat fauna occupies
more morphologically defined
niche space than a temperate
bat fauna
Fish exhibit more ecological roles in
more diverse communities
Fish are sampled
at four locations,
Mexico
From headwater
spring to a
downstream
community at
river mouth
Recap
Diversity patterns
Latitude, habitat heterogeneity, productivity
Local and regional diversity, beta diversity
Species interaction and ecological release
Species pool and species sorting
Species diversity and niche diversity
17.5 Equilibrium theories of diversity
balance factors that add and remove
species

Diversity patters:
 At global scale, species richness increases from high latitude
toward equator
 Within latitude belts, diversity appears to be correlated with
temperature, productivity, topographic heterogeneity within a
region and structure complexity of local habitats
 Isolated islands exhibit species impoverishment
 High diversity is associated with greater ecological variety
The ultimate source of diversity is speciation. Change in species
diversity is determined by speciation and extinction.
Steady-state (equilibrium) model




Similar to density-dependent regulation of
population size
Births – formation of new species or colonization by
species from elsewhere
Death – local extinction of species
Carrying capacity – steady-state number of species
Species richness on islands
Robert MarArthur and E.O
Wilson, 1960s
Equilibrium theory of
island biogeography:
The number of species on an
island balances regional
processes governing
immigration against local
processes governing
extinction.
A small offshore island
The steady-state number of species is
determined by the intersection of
colonization and extinction curves
Too small, only way to add
species is from mainland.
S: stead-state.
Smaller island support few species
Islands close to the mainland support
more species because of higher
immigration rates
Disturbance study by Simberloff and
Wilson
Red mangrove
Experiment by Daniel Simberloff and E.O.
Wilson
Disturbance
influence
species
If some
diseaster
exterminated a
part of an
island’s biota or
all of it, new
colonists would,
over time,
restore diversity
to its predisturbance
equilibrium.
Erecting metal scaffolds covered with plastic sheeting over entire red
mangrove trees and resampled at regular intervals for a year.
Equilibrium theory in continental community
At the continental scale,
new species are added to
the regional pool by
evolutionary process of
speciation as well as
immigration from
elsewhere (MacArthur
1969)
17.6 Explanations for high tree species
richness in the tropics focus on forest
dynamics
Why are there so many different kinds of trees in
the tropics?
Plausible mechanisms include:
 1. Environmental heterogeneity
 2. Disturbance such as tree falls
 3. Herbivores and pathogens allow rare species to
coexist
 4. Tree species are closely matched ecologically,
competitive exclusion takes a long time, and the
resulting rare species advantage allows many species
to coexist (neutral theory)

1. Environmental heterogeneity



Number of tree species varies in proportion to the
heterogeneity of the environment
Soil and climate
Could great variation in the physical environment in the
tropics account for the tenfold (or more) greater
diversity of trees in tropical than in the temperate
forest?
Environmental heterogeneity
It seems unlikely, unless trees recognize much
finer habitat differences in the tropics than they
do in temperate regions, especially considering
that temperate regions contain greater
heterogeneity in some climatic factors.
2. Disturbance and gap dynamics





Joseph Connell, UC Santa Barbara and others
High species richness of tropical rain forests is related
to habitat diversity created by disturbance
Disturbances open up space for colonization and
initiates a cycle of succession by species adapted for
colonizing disturbed sites.
Intermediate disturbance hypothesis: specie richness is
high when the intensity of disturbance is not too high
or low.
Death rate of trees due to disturbance showed no
difference in temperature and tropic regions (0.5-2%
died). It is doubt that gap formation could contribute
to 10 fold difference in species richness.
3. Herbivore and pathogen pressure



Consumers reduce the competition and promote
coexistence of many resource species (sea star
example)
Daniel Janzen, Uni. Penn suggests herbivory could be
responsible for the high species richness in tropical
forests.
Herbivores feed on buds, seeds and seedling of
abundant species and allow others to grow.
Herbivore and pathogen pressure

Several evidence support his “pest pressure”
hypothesis
 Monoculture may fail due to infestations of
herbivores
 Rubber plantation in Amazon grow poorly and in
Malaysia (lack specialist herbivores, and grow well)
 Prediction: seedlings should be less likely to become
established close to adult of same species than at a
distance from them (adults have herbivores)
Seedling survival varies with distance from the parent
trees
Fungicide treatment
improves survival in
seedlings of a
tropical tree
Thomas Bell, Uni. Of
Oxford
0.24 m2 plots
Two treatments
Applied fungicide and
control
Thinned half of plots
to create low density
4. Random ecological drift
Steve Hubbell: Neutral theory
Under model of specieation, the equilibrium species richness in the
metacommunity (regional diversity) would be S=Jm*nu
Jm is the number of individuals in the metacommunity
Nu is the rate of speciation expressed per individual.
THE End
Response of Stipa neomexicana plants
Jessica
Gurevitch
University of
New York at
Stony Brook
Stipa: C3
perennial
grass
Semi-arid
grassland in
Arizona
Competition release


Competitive release occurs when a
species expands its niche in response
to the removal of a competitor or
when a species invades an island and
expands into unoccupied habitats
Examples of competitive release
• Response of Stipa neomexicana plants
• Decline in baleen whales has allowed for
an increase in other krill-dependent
predators (penguins, seals)
Commercial whaling in Antarctic Ocean




Baleen whales: 1
million a century ago
eat Antarctic krill (4%
of body weight)
Now, less than
200,000
Other krill-dependent
predators such as
seals and penguins
have been found
greatly increased in
abundance
Competition release due to the
dramatic decrease in baleen whale
population
Recap
Diversity and sampling area
S-A relationship, scales and island
Diversity patterns
Latitude, habitat heterogeneity, productivity
Local and regional diversity, beta diversity
Species interaction and ecological release
Species pool and species sorting