This document discusses key concepts in community ecology, including definitions of communities and their structure. It describes how communities are classified based on factors like the biological and physical structure of the plant community. Vertical structure is influenced by plant life forms, while horizontal structure relates to species distribution across landscapes. Species interactions, food webs, and historical assembly processes also shape community organization.

1. Presented By
RAKESH KR. MEENA
ID-15MSENT014
SAM HIGGINBOTTOM INSTITUTE OF AGRICULTURE, TECHNOLOGY
& SCIENCES
(Deemed to be University)
ALLAHABAD-211007,U.P., INDIA

3. Community ecology [Chpt 20]
a) Definition of community
b) Views on community
organization
c) Community structure and
organization

4. Community Structure [Chpt 20]
Any definition of a community is necessarily VAGUE, but the
relevant features of a community require that an assemblage of
species coexist together in a habitat and they interact.
Therefore, a working definition of a COMMUNITY is:
A collection of plant and animal populations that are INTERACTING
directly or indirectly.
An ASSOCIATION is a plant community possessing a DEFINITIVE
SPECIES COMPOSITION.

5.  Autotrophic communities require only the energy from the
sun to drive the process of photosynthesis, such as forests
and grasslands.
 Heterotrophic communities, such as organisms that inhabit a
fallen log, depend on the autotrophic community for their
energy source.
 All communities have certain characteristics that define their
biological and physical structure, but these characteristics
vary in both SPACE and TIME.
 A GUILD is a GROUP OF SPECIES within a community that
interact MORE STRONGLY AMONG THEMSELVES than with
others, utilizing HABITAT or FOOD resources in a SIMILAR
MANNER.
·

6. Classification of most communities is in large part based on the
structure of the PLANT COMMUNITY or association.
Plants represent the MOST STABLE living component of the
community. Because they LACK MOBILITY, they must cope
with the prevailing environment and, therefore, are excellent
indicators.
Characterization of a community is based on both the:
• BIOLOGICAL STRUCTURE (the mix of species) and the
• PHYSICAL STRUCTURE (the physical features of the biotic and
abiotic environment).

7. The form and structure of terrestrial communities are largely
defined by the vegetation including growth form, vertical,
horizontal, and biological structure:
o Raunkiaer’s life forms - classification of plant life in
relation to the HEIGHT of PERENNATING tissue
(embryonic or meristemic tissues that remain inactive over
winter or prolonged dry periods) above ground.
Perennating tissues include buds, bulbs, tubers, roots, and
seeds.
Raunkiaer recognized six life forms:

8.  Phanerophytes - trees and shrubs greater than 25 cm in
height that have their leaf-producing buds elevated above
ground on stems.
 Cryptophytes (geophytes)-grasses which have above-
ground tissues that DIE BACK IN WINTER or during
prolonged dry periods and survive unfavorable periods as
BUDS buried in the ground on a BULB or RHIZOME.
 Therophytes-annuals that survive unfavorable periods as
SEEDS and complete their life cycle from seed to seed in one
season.
 Chamaephytes-perennial shoots or buds are on the
SURFACE of the ground to about 25 cm ABOVE the surface.
 Hemicryptophytes-perennial shoots or buds are CLOSE TO
THE SURFACE of the ground, often COVERED WITH LITTER.
Epiphytes-plants that GROW ON OTHER PLANTS with their
roots up in the air.

10.  A community with a high percentage of perennating tissue
well above ground (phanerophytes) would be characteristic
of WARMER, WETTER climates;
A community where most of the plants are classified as
cryptophytes and hemicryptophytes would be characteristic
of COLDER or DRIER environments.
Therophytes are very common in FIRE-DOMINATED
habitats.
 When species within a community are classified into life
forms and each life form is expressed as a PERCENTAGE, the
result is a LIFE FORM SPECTRUM that reflects the plants’
adaptations to the environment, especially climate, and
provides a standard means for describing community structure.
NB Caution should be exercised when using these life forms as a
basis for community classification. Overlap does occur among the
Raunkiaer life form spectra of various communities, with NO ONE
community represented by a SPECIFIC LIFE FORM and NO LIFE FORM
occurring in ONLY ONE COMMUNITY.

12. • Vertical structure is determined largely by life form of the plants,
including their size, branching, and leaves, which influences and is
influenced by the vertical GRADIENT OF LIGHT.
The structure of a well-developed forest has up to 5 LAYERS,
6 in a TROPICAL FOREST:
o Canopy - the primary site of energy fixation through
PHOTOSYNTHESIS which has a major influence on the rest of the
forest depending on the amount of sunlight that penetrates to lower
layers.
o Understory - generally consists of TALL SHRUBS and understory
TREES, and younger trees, some of which are the same species as
those in the canopy. Species that are unable to TOLERATE SHADE
will die; others will eventually grow to reach maturity after some of
the older trees in the canopy die or are harvested.
o Shrub layer - a layer of small to medium SHADE TOLERANT
SHRUBS.
o Herb or ground layer- the nature of this layer will depend on the
soil moisture and nutrient conditions, the slope position, the density
of the canopy and understory, and the aspect of the slope, all of
which vary from place to place throughout the forest.

13. o Forest floor - the site where DECOMPOSITION takes place
and where nutrients are released from DECAYING ORGANIC
MATTER for reuse by the forest plants.
Tropical rain forest also may have a sixth layer:
o Emergents - emergents are trees that rise above the general
canopy.

14. • The vertical structure of aquatic (freshwater) ecosystems is
determined by light penetration and profiles of temperature and
oxygen.
Four general layers are recognized:
o Epilimnion - a layer of FREELY CIRCULATING water, found in
the summer in well-stratified lakes.
o Metalimnion - a layer characterized by a thermocline—a very
steep and REAPID DECLINE IN TEMPERATURE.
o Hypolimnion- a deep, COLD layer of dense water about 4o
C,
often LOW IN OXYGEN.
o Bottom mud - a layer of bottom mud.
The following two structural layers are ROUGHLY based on light
penetration:
 Photic zone – (~LIGHT ZONE) an upper layer roughly
corresponding to the epilimnion which is dominated by
autotrophic phytoplankton and is the site of PHOTOSYNTHESIS.
 Benthic zone - (~BOTTOM ZONE) a lower layer roughly
corresponding to the hypolimnion and bottom mud where
DECOMPOSITION IS MOST ACTIVE.

16. • Horizontal structure is the pattern of vegetation ACROSS the
landscape.
It is LESS PREDICTABLE than vertical structure due to the
VARIABILITY in the PHYSICAL ENVIRONMENT, and DISPERSAL
and other biotic factors that interact to affect the distribution of
organisms.
Communities exist in large and small PATCHES separated from
one another producing a horizontal pattern that increases the
physical and ecological COMPLEXITY of the environment.
This patchiness exists on different scales across the landscape
including WITHIN PATCH heterogeneity, BETWEEN PATCH
heterogeneity, and heterogeneously patterned LANDSCAPES.
At all levels, the size, shapes, and dispersion of patches affect
their COLONISATION by individuals, the PERSISTENCE of these
individuals on the patch, the population dynamics, and the
NUMBER OF SPECIES in an area.

17. The MIX of species, including their NUMBER and relative DOMINANCE,
define the BIOLOGICAL STRUCTURE of a community.
Dominants in a community may be the MOST NUMEROUS, possess
the HIGHEST BIOMASS, preempt the MOST SPACE, make the largest
CONTRIBUTION OF ENERGY FLOW or mineral cycling, or by some
other means control or influence the rest of the community.
Keystone species are those whose presence is CRITICAL TO THE
INTEGRITY of the community.
Three features define biological structure:
• species dominance,
• species diversity, and
• species abundance.

18. Dominance- several measures are used to determine dominance:
Relative dominance - the ratio of basal AREA or aerial
COVERAGE or BIOMASS of a species to the TOTAL basal
AREA or coverage or biomass of all species in the community.
Relative abundance - the numerical abundance of ONE
SPECIES relative to the TOTAL ABUNDANCE of all species.
Relative frequency - an index based on the NUMBER OF
sample POINTS OR PLOTS in which a species is found to
occur relative to the TOTAL NUMBER OF SAMPLES taken.
This index is best used when the species are very DIFFERENT
IN SIZE (as large species may skew results, being big, but few individuals)

19. Importance value - the combination of relative dominance,
relative abundance, and relative frequency. Most species do
not achieve a high level of importance in the community, but
those that do serve as INDEX SPECIES.
Species diversity is usually determined by some index that
combines species RICHNESS (the number of species within a
community) with a measure of species evenness or equitability
(the relative abundance of individuals among the species).
The more equitable the distribution, the greater is evenness.
Species diversity increases as the numbers of individuals in the
total population are more equitably distributed among the
species.
Communities with low evenness are dominated numerically by
few species (lots of a small number of trees for example).

20.  Shannon index of diversity - determines the uncertainty
associated with two individuals drawn at random from the
SAME COMMUNITY belonging to the SAME SPECIES.
It considers both richness and evenness.
Uncertainty is GREATEST when species richness and species
evenness are HIGH.
In communities composed of organisms with a wide range of
size, an index may lead us to UNDERESTIMATE THE
IMPORTANCE of FEWER BUT LARGER individuals and
overestimate that of more common species.
The indices also FAILS TO DISTINGUISH between the
ABUNDANT AND RARER species, which contribute little to the
index.

21. The Shannon index of diversity is used as one measure of
the biological structure of a community for making
comparisons.
Comparisons are made of species diversity:
 within a community (alpha or α diversity),
 between communities or habitats (beta or β diversity), or
 among communities over a geographic area
(gamma or γ diversity).
Shannon index of richness - derived by dividing a
community’s ACTUAL species diversity by its MAXIMUM
POSSIBLE species diversity.

22. Species abundance - two communities with the same diversity
index value do not necessarily have the same exact species
richness and evenness.
A complete picture of the distribution of species abundances in
a community must involve an examination of the relative
abundance of each species against RANK,
where rank is defined by relative abundance (compared to other
species)
The most abundant species is plotted first along the x axis, with
the corresponding value of the y axis being the value of relative
abundance.
This process is continued until all species are plotted. The
resulting graph is a rank-abundance diagram.

23. Theoretically, the relative abundances of the species exhibited
in the rank-abundance diagram represent the manner in which
species DIVIDE THE RESOURCES or ENVIRONMENTAL SPACE.
In a community where the organisms are of SIMILAR SIZE,
ABUNDANT species preempt a LARGER PROPORTION of
space than do less abundant species.
Using the model of niche breadth and overlap among
competing species, the rank abundance curves provide a model
of niche partitioning.

24. Species abundance
Rank-abundance curves for two forest communities. Rank
abundance is the species ranking based on relative abundance.
Relative abundance (y axis) is expressed on a log10 axis. Note
that one forest community has a higher species richness and
evenness than the other.
WV mature hardwood forest
VA young hardwood forest

25. Three statistical models have been developed to describe the
patterns of relative abundance, all based on resource
partitioning:
o (A) Random niche (broken stick),
o (B) Niche preemption (geometric distribution hypothesis),
and
o (C) The log-normal hypothesis.

29. These models describe patterns of species abundance, but
they are of LITTLE VALUE in determining the UNDERLYING
CAUSES for the observed abundance relationships.
Any conceptual model of how communities are structured—
the patterns of species distribution and abundance—must
explicitly address the INFLUENCE OF SPECIES
INTERACTIONS.

30. Species interactions
There are no clear-cut generalizations about the role of
competition, predation, parasitism, and mutualism in shaping
the structure of all communities, but there are examples
where particular interactions play a significant role.
COMPETITION has historically been assumed to be the
DOMINANT INTERACTION, especially COMPETITIVE
EXCLUSION, but examples of competition are often less
definitive than for predation and parasitism.
The importance of competition in community structure likely
varies from community to community.

31. Within any given community, COMPETITION is most
pronounced among SESSILE organisms such as plants, or
among members of the SAME GUILD.
Determining the degree to which COMPETITION
INFLUENCES the RELATIVE ABUNDANCE of species is
DIFFICULT because so many OTHER FACTORS that have a
direct influence on population dynamics.
MUTUALISM has been studied less but COULD well be one of
the MOST IMPORTANT determinants of community structure.

32. Food webs and community structure - one of the most fundamental
processes in nature is the acquisition of food to supply the energy
and nutrients needed for assimilation.
The variety of species interactions—predation, parasitism,
competition, and mutualism—are all involved in the acquisition of
food resources.
A FOOD CHAIN is a descriptive diagram with a series of arrows,
each pointing from one species to another from which it is a source
of food.
Feeding relationships are virtually NEVER LINEAR and involve the
meshing of NUMEROUS FOOD CHAINS and organisms feeding at
various levels and in various food chains (=FOOD WEB)

33. o Physically STABLE environments tend to have food webs
consisting of LONGER FOOD CHAINS than do fluctuating
environments.
o Vertical stratification and herbivores: top carnivore ratios also
influence food web structure.
Two views of how community structure is regulated through
food webs have been developed:
o Bottom-up regulation and
o Top-down regulation.

35.  Bottom-up regulation - emphasizes the limitations imposed
by the AVAILABILITY OF FOOD resources (species
populations) at the NEXT LOWER LEVEL and the role of
competition among species that draw on those food
resources.
The level above is influenced by the resources provided by the
level below (autotrophs limit herbivores, herbivores limit
carnivores, etc.).
 Top-down regulation - the abundance at each level is
controlled by CONSUMERS (PREDATORS) at the TOP of the
food chain.
e.g. When carnivores suppress the number of herbivores,
plants experience a release from grazing and flourish.
These views are fundamentally different and are still the
subject of much debate.

36. Assembly rules - the removal or addition of species can have
profound effects on the structure and function of the
community.
Given the importance of interactions and the interdependence
among species within a community, how do species become
assembled to form a community?
Two approaches have been used to investigate community
development and structure:
1. Remove keystone species (a species whose activities
have a SIGNIFICANT ROLE in determining community
structure) and study how the community restructures itself
—what species become the new dominants and what species
increase or decrease.

37. 2. Reconstruct the sequence in which SPECIES WERE
ADDED when the community was formed and attempt to
determine what COLONIZATION SEQUENCES may or may not
be possible.
This approach attempts to establish the rules that govern the
assembly of species to form communities.
Perhaps the most important insight gained is the importance of
understanding the HISTORICAL CONTEXT in which a
community arises.
Although the SAME SPECIES may be found in SIMILAR
COMMUNITIES, each may have colonized the area at
DIFFERENT TIMES and in DIFFERENT SEQUENCES, which
would change species relationships. Each community
develops within a historical context that will influence species
interactions.
Thus different assembly routes will produce differences in
community organization.
Understanding of the development of communities is essential
to habitat restoration and conservation.

38. • Community classification - while communities may be discussed
as discrete units, the actual physical delineation of a community is
difficult.
Communities often BLEND INTO ONE ANOTHER, with differences
becoming more pronounced with distance.
Typically, community classification schemes include a consideration
of community “physiognomy”
(essentially the appearance of the community, the general
appearance, vertical structure, and growth form of vegetation).

39. • Because animal distribution appears to correlate with
VEGETATIONAL COMMUNITIES, classification of physiognomy will
relate to both plant and animal life.
• Communities are often named after the DOMINANT FORM OF LIFE,
usually plants, such as deciduous or coniferous forest, sagebrush,
shortgrass prairie.
• They may be further identified by a few CHARACTERISTIC or
DOMINANT SPECIES that represent a shorthand method of naming
the community e.g. kelp forest, maerl beds
• In the marine environment, animals often can be the dominant form
of life e.g. mussel beds, gorgonian reefs, coral reefs.
However, each community should be described by providing a
COMPLETE LIST of species and their RELATIVE POPULATION SIZES
and CONTRIBUTIONS TO TOTAL BIOMASS.

40. o Where HABITAT BOUNDARIES are WELL-DEFINED,
communities may be classified by PHYSICAL FEATURES such
as tidal flats, sand dunes, cliffs, ponds, and streams.
o Other communities may be further classified based on
prevailing ENVIRONMENTAL REGIMES (e.g. temperate/ fast
flowing etc)
or SPECIES COMPOSITION including frequency, dominance,
constancy, presence, and fidelity
Areas with similar combinations of species may be classified as
the same community type. The type is named after the dominant
organisms or the ones with the highest frequency (e.g. oak-
hickory forest).

41. Species may be grouped as EXCLUSIVE, those completely or
nearly completely confined to one type of community;
CHARACTERISTIC, those most closely identified with a certain
community;
or UBIQUITOUS, those with no particular affinity for any
community.
o ORDINATION is a technique for arranging and comparing
communities along a linear axis according to their SIMILARITY IN
SPECIES COMPOSITION. It is an exploratory data analysis
technique designed to seek patterns or trends.
Various statistical methods have been developed to analyse
ordinate communities, looking at large complex sets of variables:
e.g. principle component analysis, cluster analysis, discriminate
analysis, correspondence analysis etc. etc.

42. Moving across the landscape, we notice that the nature of the
physical and biological structure of the community changes.
Often those changes are small, subtle ones in the species
composition or height of the vegetation. However, as we
travel further and further, these changes become more
pronounced.
These changes in the physical and biological structure of
communities as we move across spatial gradients (the
landscape) are referred to as ZONATION.
If the transition between two communities is abrupt and
distinct, there may be no problem in defining the community
boundaries.
However, the differences in the species composition and
patterns of dominance observed in the two communities may
occur gradually over the distance in the two communities
may occur gradually over the distance from hilltop to stream.
In this case, the boundary is not so clear.

44. .

45. and rest….

46. Community dynamics [Chpt 21]
a) Succession
i) Models of succession
a) Views on community organization
i) Clements approach
ii) Gleason approach
iii) Fundamental nice
a) Plants, environment & community dynamics
i) Models of plant community dynamics
ii) Allogenic & autogenic changes

47. Community Dynamics
• As environmental conditions change in time and space, the
structure of the community, both physical and biological, also
changes. The result is a dynamic mosaic of communities on the
landscape. It is this changing pattern of community structure that is
the focus of community ecology.
• Zonation is a SPATIAL pattern related to gradients in
environmental conditions often associated with elevation, slope
position, and aspect.
• Succession is the TEMPORAL change in community structure
through time.
In contrast to zonation, succession refers to a given point in space
—a single location.
The sequence of communities is called a SERE and each of the
changes is a SERAL STAGE.
o Although each seral stage is a point in a continuum of vegetation
through time, it is recognizable as a distinct community with its own
characteristic structure and species composition.

48. SUCCESSION: Temporal variation in community structure in
one place.
oThe initial or early successional species, often referred to as
pioneer species, are usually characterized by high growth rates,
smaller size, high degree of dispersal, and high rates of
population growth (r-selected species).
o In contrast, late successional species generally have lower
rates of dispersal and colonization, slower growth rates, and
are larger and longer-lived (K-selected species).
o Two types of successional patterns are identified:
 Primary succession occurs on a site previously
unoccupied by a community.
 Secondary succession occurs on previously occupied
(vegetated) sites following disturbance.

49. oThe concept of succession was introduced by Henry Cowles in
1899 and expanded by Frederick Clements.
 They viewed succession as a predictable, directional,
inevitable process driven by the action of plants on their
environment (termed facilitation)
 that concluded with a stable end point determined by the
prevailing climate (the climatic climax community).

50. oThe climax represented a community at some equilibrium or
STEADY STATE with the physical and biotic environment that
continued to reproduce itself in the absence of disturbance.
 Clements further recognized only one climax for a region
whose characteristics were determined solely by climate.
Successional processes and modifications of the
environment overcome the effects of differences in
topography, soil parent material, and other factors.

51. Five models of succession:
1. Reciprocal replacement (A. S. Watt 1947)-succession is
viewed as a cyclic rather than a linear process leading to
some defined endpoint.

52. 2. Shifting-mosaic steady state (Bormann and Likens 1979)-an
outgrowth of the reciprocal replacement model where the
community is viewed as being composed of a mosaic of
patches, each in a phase of successional development.
Representation of a
forested landscape –
Although each patch
is continuously
changing, the
average
characteristics of the
forest remain
relatively constant—
in a steady state
condition.

53. 3. Autosuccession (Hanes 1971) - some communities, especially
those found in extreme environments, are often characterized
by an absence of temporal shifts in species composition
following disturbance
(The community replaces itself following disturbance rather
than going through a series of successional stages)

54. 4. Nonsuccessional dynamics (fluctuations)-fluctuations differ from
succession in that although the relative abundance of the
species making up the community may change over time, the
species composing the community remain the same.
(The species found within a community remain unchanged
through time, but species abundance, or age class, or
dominance does change)
No new species invade the site and changes in dominants may
be reversible.
These changes in species abundance result from seasonal or
annual variations in environmental conditions such as soil
moisture or temperature, or preferential selection of one
species over another by grazers.

55. 5. Degradative succession-succession in heterotrophic communities
involving the decomposition of dead organic material.
• Succession is characterized by early dominance of fungi and
invertebrates that feed on dead organic matter.
• The organisms that first colonize the site are ones that can feed
on fresh organic matter. Their feeding activities bring about
physical and chemical changes in the substrate.
• After they have exploited the energy and nutrients accessible to
them, they disappear and are replaced by a group of organisms
able to extract nutrients and energy left in a less accessible form.
• Each group changes the substrate to a point that it can no longer
survive there and is replaced by the next group of organisms
until the organic matter is degraded.

56. • As the biological and physical structure of vegetation changes
during the process of succession, animal life associated with those
stages also changes.
• Animals are influenced more by structural characteristics of
vegetation than by species composition.
Therefore, successional stages of animals may not correspond to
the successional stages of plants.
• Therefore, the key to diversity of wildlife in a given area
is the maintenance of a heterogeneous landscape with
habitat patches of various successional stages and of
adequate sizes and connectiveness to meet the animals
needs and promote gene exchange and dispersal.

58. Processes Controlling Community Dynamics
• Community structure does not vary randomly across the
landscape (zonation) or through time (succession); rather, it
exhibits repeatable, often predictable patterns.
•This suggests a common mechanism or mechanisms influencing
the process of succession.
• An association is a type of community with:
(1) relatively consistent species composition,
(2) a uniform physical (physiognomic) structure, and
(3) a distribution that is characteristic of a particular habitat, such
as a hilltop or valley.

59. Views of communities and succession
The Clements vs Gleason views of communities and succession:
o F. E. Clements (1916) developed a descriptive theory of
succession based on his view of the community as an
association or superorganism. The logic was that if clusters or
groups of species repeatedly associated together, that is
evidence for either positive or neutral interactions among them,
favoring the view of communities as integrated units.
o Based on this logic, Clements developed the organismal
concept of communities. He viewed species in an association
as having similar environmental requirements and therefore
similar distributional limits along important environmental
gradients. The boundaries between adjacent associations are
narrow, with very few species in common.
o This view suggests a common evolutionary history and similar
fundamental responses and tolerances for the component
species.

60. F.E. Clements (continued)
o Mutualism and coevolution play an important role in the
evolution of species making up the association.
The community developed as an integrated whole which
Clements considered as a superorganism, the ultimate
expression of which was the climax.
o The climax was an assemblage of vegetation that belonged
to the highest type of vegetation community possible under
the prevailing climate.
This climax can, according to Clements, reproduce itself,
“repeating with essential fidelity the stages of its
development.”
(i.e. the climax is the ‘mature adult’ association)

61.  H. A. Gleason (1917, 1926) regarded the community as
consisting of individual species that respond independently to
environmental conditions:
“the vegetation of an area is merely the resultant of two factors,
the fluctuation and fortuitous immigration of plants and an equally
fluctuating and variable environment.”
He emphasized species rather than communities as the essential
unit.
o Succession results from the individual responses of different
species to the prevailing environmental conditions.
Plants involved in succession are those that arrive first on the site
and are able to establish themselves under prevailing
environmental conditions.
o As time passes, plants modify the environment, and
competition and other interactions among species determine the
final outcome.

62. o His view became known as the individualistic continuum
concept.
The continuum concept states that the relationship between
coexisting species is a result of
similarities in their requirements and tolerances,
not a result of strong interactions or common evolutionary
history, as viewed by Clements.
o Boundaries between communities are gradual and difficult to
identify. What is referred to as a community is merely the
group of species found to coexist under any particular set of
environmental conditions.

63. Connell and Slatyer (1977) proposed a theoretical framework for
understanding succession that included three different models:
1. Facilitation model - the organisms themselves bring about
changes within the community, modifying the environment
in such a way that they prepare the site for later
successional species, thus facilitating their success.
This is a holistic and Clementsian model.

64. 2. Inhibition model - species interactions are purely competitive and
no species is competitively superior to another.
The site belongs to those species that become established first
and are able to hold their positions against all invaders.
They make the site less suitable for both early and late
successional species through consumption of resources and
modification of the environment.
Although early successional species may suppress their growth
for a long time, ultimately, species that are long-lived come to
dominate.
Such succession is not orderly and is less predictable than that
observed under the facilitation model. This is a reductionist
(Gleason) approach with competition driving vegetational
change.

65. 3. Tolerance model - involves the interaction of competition and
life history traits. It suggests that later successional species
are neither inhibited nor aided by species of earlier stages.
• Later-stage species can invade a site, become established,
and grow to maturity in the presence of those preceding
them, because they have a greater tolerance for the lower
level of resources created by earlier species.
• As time progresses, the early successional species decline
in abundance and the community is dominated by the
tolerant species.
• This model suggests that early stages of succession are
driven by competition, whereas later stages are dominated
by species that can invade and tolerate lower resource
regimes than previously existing species on the site.

66. • Noble and Slatyer (1981) proposed a framework for understanding
succession that focused on species life history characteristics that
determine the place of a species in a succession rather than on
species interactions.
These characteristics are called vital attributes and they fall into
three categories:
1. Ability and method by which a species recovers following
disturbance.
2. The ability of a species to grow and reproduce under
competition.
3. Species longevity.
o Species within an area are classified based on their vital
attributes, and predictions about successional sequences are
possible.
o This model explains succession in terms of species
characteristics and is grounded in the Gleasonian view of
communities.

67. Another frame work… the Fundamental Niche
A model of community dynamics based on a species’ fundamental
niche is based on four premises:
1. The fundamental niche of a species acts as a primary
constraint on its distribution and abundance.
2. Species vary in their fundamental niches (environmental
tolerances). Characteristics that allow an organism
to prosper under one set of environmental conditions
often limit its ability to do equally well under differing
environmental conditions.
3. Environmental conditions change in time and space.
4. The fundamental niche is modified by species interactions
(realized niches). Organisms interact through direct
contact (i.e., competition and predation) or indirectly
through modification of the physical environment.

68. Fundamental niches of seven hypothetical species along an
environmental gradient in the absence of competition from other
species. The species all have bell-shaped responses to the
gradient, but each has different tolerance limits defined by a
minimum and maximum value along the gradient. As conditions
change, for example from e1 to e2, the set of species that can
potentially occur in the community changes. These changes can
occur in either time or space.

69. Plants, the environment & community dynamics
There are two components of plant response to the environment
that are critical for understanding community dynamics.
One is the response of the individual to the prevailing environment,
such as light, nutrients, and moisture.
The other is how the individuals modify the environment
= autogenic environmental change.
It is the combination of these two plant responses to the
environment that give rise to the dynamics of communities across
the landscape.

70. There are 3 current models of plant community dynamics:
1. Plant strategies and vegetative processes (Grime 1977, 1979)-
the concept of the r and K life history classification was
expanded to include three primary plant strategies:
(a) R or ruderal strategy species - Species exhibiting the R, or
ruderal, strategy rapidly colonize disturbed sites but are small
in stature and short-lived.
Allocation of resources is primarily to reproduction, with
characteristics allowing for a wide dispersal of propagules to
newly disturbed sites.
(b) C or competitor strategy species - Predictable habitats with
abundant resources favor species that allocate resources to
growth, favoring resource acquisition and competitive ability (C
species).

71. (c) S or stress tolerators strategy species- Habitats where
resources are limited favor stress-tolerant species (S species) that
allocate resources to maintenance.
 This model considers plant succession to be a sequence of life
history strategies beginning with ruderals invading the site,
followed by competitors, and eventually stress tolerators.
Changes in species dominance result from autogenic changes
in resource availability as a direct result of resource
consumption by the plants with resource availability decreasing
as succession progresses.
R C S

72. 2. Resource-ratio model (Tilman 1985, 1988)- based on the trade-off
in characteristics that enable plants to compete for the essential
resources of nitrogen and light.
-The ability to effectively compete for light is associated with
allocation of carbon to the production of above-ground tissues—
leaves and stems.
-Conversely, the ability to effectively compete for nitrogen is
associated with the production of root tissues.
According to the resource-ratio model, succession comes about as
the relative availability of nitrogen and light change through time.
• As biogeochemical processes make more soil nitrogen
available, plant growth increases, reducing the availability of
light at the soil surface - leads plant species replacement
(plants adapted to high nutrient and low light availability)
Low soil nitrogen High soil nitrogen
High light Low light

73. 3. Individual-based model (Huston and Smith 1987)- based on the
cost-benefit concept that plant adaptations for the simultaneous use
of two or more resources are limited by physiological and life history
constraints.
Their model focuses on the resources of light and water.
The plants themselves largely influence variations in available light
within the community (i.e. autogenic),
while the availability of water is largely a function of climate and soils
(i.e. allogenic).

74. The following three premises summarize the consequences of
constraints on the simultaneous use of light and water by individual
plants:
i) There is an inverse relationship between the ability to
survive and grow under low-light conditions and the
ability to photosynthesize and grow at high rates
when the availability of light is high.
ii) There is an inverse relationship between the ability to
survive and grow under low-water conditions and the
ability to photosynthesize and grow at high rates
when water is freely available.
iii) Tolerances to conditions of low light and low water are
interdependent.
i.e. The set of physiological and morphological characteristics that
enable a plant to survive and grow under shaded conditions (e.g.,
allocation of carbon to the production of leaves and stem) is in
direct conflict with its ability to tolerate low water availability (e.g.,
high allocation to the production of roots).

75. Allogenic & Autogenic changes
Most of the plants succession discussion so far was about autogenic
(self-made) changes in environment.
Allogenic (abiotic environmental) changes can produce patterns of
succession over time scales ranging from days to millennia or longer.
Fluctuations in the environment that occur repeatedly over the lifetime
of an individual are unlikely to influence patterns of succession among
species with that general life span.
In contrast, shifts in environmental conditions that occur at periods as
long or longer than the organism’s life span are likely to result in shifts
in species dominance—succession.

76. Temporal changes in the abundance of dominant phytoplankton
species over the period of May through October in Lawrence Lake,
Michigan (1979) as a result of allogenic changes (seasonal
temperature and photoperiod changes)
The mean generation time of the phytoplankton species is in the
range of 1-10 days.

77. Allogenic environmental change is the dominant influence on
spatial variations in community structure — i.e. zonation.
Patterns of temperature and moisture resulting from regional variations in
climate are the major determinant of regional and global patterns of
vegetation distribution, and they form the basis of most vegetation
classification systems.
On a more local scale, climate interacts with soil and topography to
influence patterns of temperature and soil moisture.
The underlying geology of an area interacts with climate to influence soil
characteristics such as texture.
In turn, texture has a direct effect on soil moisture-holding capacity,
cation exchange capacity, and base saturation,
influencing the moisture and nutrient environment of plants.

78. In aquatic environments, water depth and salinity are
examples of allogenic environmental gradients that
directly influence the distribution and dynamics of
communities.
The distinction between autogenic and allogenic environmental
change is often blurred when abiotic (allogenic) environmental
factors influence the manner in which organisms modify the
environment.
By affecting rates of survival, growth and reproduction, allogenic
variations influence competition for resources.

79. SPECIES RICHNESS & SUCCESSION
During succession, two opposing forces affect species richness.
Colonization increases species richness
and species replacement decreases species richness.
This results in a pattern of increasing and decreasing species diversity
from early to late successional stages.
The particular pattern can be influenced by factors such as the growth
rate of a population and disturbance.
At both high and low disturbance frequencies, species diversity
remains low.
However, at an intermediate frequency of disturbance, species
diversity remains high.
As a final note… herbivory can influence plant community dynamics
both directly and indirectly (causing disturbance etc.)

80. And finally…
you’re climbing up a mountain…

81. •A transect up a mountain in an area with four plant species
- the distribution of the four species is presented in 2 ways.
1 - In one view, the species distribution is plotted as a
function of altitude or elevation.
NB the four plants exhibit a continuum of species regularly
replacing each other in a sequence of A, B, C, and D with
increasing altitude.

82. 2- Species distribution described as you walk up the
mountainside - the distributions of the four species are not
continuous.
As a result, you would see a number of species
associations you walk along the transect.
These communities composed of coexisting species are a
consequence of the spatial pattern of the landscape.

83. • The two views are quite different yet consistent.
Each species has a continuous response along an
environmental gradient, elevation.
• Yet it is the spatial distribution of that environmental
variable across the landscape that determines the
overlapping patterns of distribution—the composition
of the community.
• The simple example presented here examines only one
feature of the environment—elevation.
• Yet the structure of communities is the product of a
complex interaction of pattern and process. Species
respond to a wide array of environmental factors that
vary spatially and temporally across the landscape, and
the interactions among organisms influence the nature
of those responses.