plant communities is the most important what you should know. from samara university ethiopia

1. 4. Plant Communities
CHAPTER FOUR

2. 3.1. Definition &Concepts of Plant Community
• A plant community is a group of recurring spp that: share a
common habitat; collectively create a unique physiognomy;
attain a typical spp richness & annual productivity.
• Populations of many different organisms in a particular place,
interacting to one another by feeding relationships and other.
• These interacting popns are called communities and their study
is called community ecology.
• The plant community is simply all of the plants spp occupying
an area of a particular niche in a wider ecosystem.
• Community study focuses on how biotic interactions such as
predation, herbivory & competition influence the number and
distribution of organisms.
• Popn ecology focuses on the influences of environment on the
density, abundance and distribution a particular popn.

3. 4.2. The Nature of Plant Community
• A single popn do not usually monopolize a habitat alone.
Normally, there is a mixture of coexisting plant, animal and
microbial populations
• These clustered popns are associated with each other and to be
members of a biotic community
• The nature of the plant community remains today as one of the
biggest controversies.
• Europeans oversee communities as distinct and discrete entities.
North Americans see communities as entities that blend
together continuously
• Our current view of plant communities "evolved" over 100
years and can be discretely categorized in to three historical
eras or paradigms:

4. 4.2.1. Clementsian Paradigm
 In 1889, W. Morris put forth the idea of a Geographical Cycle;
i.e., landforms were created through a very orderly set of
processes.
 This cycle was likened to the orderly development of a human
being (i.e., birth, childhood, adolescence, etc.)
 This concept spread throughout the scientific community in
the post-Darwinian era and became known as the Organismal
metaphor.
 H. Cowles in1899, arose a new concept, he actually extended
the concept of geographical cycle to a vegetation cycle
(succession)
 He produced the pioneering work on vegetation succession on
the Lake.

5.  Clements (1904) continued on the work of Cowles and used it
to formulate the theory of vegetation dynamics.
 Clements used the Organismal Metaphor to demonstrate that
communities changed over time in very discrete ways,
ultimately ending at a specific point or "climax".
 In this view, the climax community was a static developmental
endpoint of stability.
 Clement's view of the community was intransigent, stated:
 Communities were distinct spatial entities developed with one
superorganism complex giving way to another (in space or
time).
 Clements did admit the role of competition, mutualism, and
predation in influencing community structure

6.  He did recognize the role of environment, soils, and history.
 But, his focus was on the idealized nature of communities.
 Clements perception of the community dominated for many
years and still be found in many ecology textbooks.
4.2.2. Gleasonian Challenge
 HA. Gleason, in 1939, argued that communities were the
result of interactions between individual spp and the
environment (biotic and abiotic) in combination with
historical events.
 Each species has its own environmental tolerance and
responds individualistically to the environment.

7.  Gleason's view of the plant community became known as the
“Individualistic Concept” and opposed Clement's Organismic
View in all aspects.
 The implication of Gleason's view was that spp distributed
along environmental gradients, with their boundaries
determined by their tolerances to the environment.
 Communities were not tightly linked superorganisms, for eg.
Fig 1. Each community (A, B, C) is an arbitrary section

8. 4.2.3. Modern Synthesis
 The primary issues surrounding the nature of plant
communities divide roughly into pattern and process.
 The issues of pattern focus on how spp and communities are
distributed over the landscape
 Are boundaries abrupt or gradual? How predictable are the
patterns?
 The issues of process focus on what processes (e.g.,
competition, herbivory, etc.) function in natural communities
and which of these are most important to determine the
observed patterns.
 Do some process predominate? do processes vary among
communities? Are communities static or dynamic?

9. 4.3. Plant Community Attributes
 Each plant community has unique attributes that have to do
with the architecture, spp richness and the spatial patterns in
which individuals are arranged
 The community also differ in the efficiency which they trap
light and cycle energy or nutrients, and the stability of the spp
in face of environmental stress
 Common Elements
1. Communities structure is a population process
2. Communities are sections of continuous gradients
3. Communities show some directionality & predictability
4. Communities are strongly influenced by historical effects
5. Communities do not develop to a stable climax
6. Communities are dynamic & influenced by disturbance

10.  Types of community attributes:
a. Physiognomic characteristics
b. Species richness
c. Analytic and Synthetic characteristics
4.3.1. Physiognomy
 Physiognomy is the general architecture of the community
 It is the external appearance or the stand area coverage.
 It is measured by canopy cover & leaf area index, and growth
forms of dominant spp.
 Communities in different agroecology have different
physiognomy.

11.  A desert tree in north rift valley dominated by acacia bush, w/c
has a single open canopy layer
 A tropical rain forest in Africa has several tree canopy layers.
 Leaf area index (LAI) is a measure of canopy thickness: the
no. of layers of overlapping leaves through which light passes.
 The desert community has LAI of 1, and the tropical rain
forest has LAI of 10.

12. 4.3.2. Species richness
 It is also known as species diversity, w/c is the total no of
spp that occur in a given community
 Communities differ in the number of spp they contain. For eg:
 Tropical rain forests can have the greatest diversity of plant
spp, up to 365 per 10,000 m2.
 Temperate forest and woodland communities have moderate
diversities of 50 to 100 spp in 10,000 m2.
 The desert communities have less than 50 spp in the same area
 Every spp in a community are not equally important, their
importance is laid on their abundance.
 It is quantified by counting the density, canopy cover, biomass,
and frequency of individuals.

13. 4.3.3. Analytical and Synthetic Characters
 Frequency
 Frequency is the number of occurrence of a particular species in
a sampling units (as %).
 Thus, frequency of each species is calculated as:
Frequency(%) = No. of sampling units in which the spp occurred/
Total no. of sampling units studied x 100
 Density:
 The number of individuals of the spp in unit area is its density;
it gives an idea of degree of competition.
 It is calculated as: Density = Total no. of individuals of the spp
in all sampling units / Total no. of sampling units identified.

14.  Cover and Basal area:
 The above ground parts (leaves, stems and flowers) cover a
certain area
 If this area is demarcated by vertical projections, the area of the
ground covered by the plant canopy is called canopy cover
 It is one of the chief characteristics to determine dominance.
 Abundance
 Abundance is the number of individuals of any spp per sampling
unit of occurrence.
 Plants are not found uniformly distributed in an area. They are
found in smaller groups, differing in number at each place.
 Abundance is divided in five arbitrary groups depending upon
the number of plants.
 These are: very rare, rare, common, frequent and very much
frequent.

15.  Phenology:
 It is the scientific study of seasonal change i.e., the periodic
phenomenon of plants in relation to the climate.
 Different spp have different periods of seed germination,
vegetative growth, flowering, fruiting, leaf fall, seed dispersal,
etc.
 A study of such date and time of events is phenology. It is the
calendar of events in the life history of the plant
 Environmental factors tend to influence the phenological
behaviour of a species popn.

16. 4.4. Distribution of Species along Environmental Gradients
 Plant spp are not randomly distributed on land surface along
environmental gradients but display spp specific tolerances to
have non-random assemblages
 Plant spp show different characteristics (traits) that enable them
to live and reproduce in extreme environments
 The distribution of plants along environmental gradients is
constrained by abiotic and biotic factors.
 Among abiotic drivers, climate and soil properties are
recognized to strongly constrain the distribution of spp.
 In the same way, biotic interactions constrain individual spp
ranges and, thus, the spatial variation in spp assemblages
 For eg., fungi display mutualistic and antagonistic interactions
with plants and influence plant growth and distribution.

17.  Among the biotic factors, decomposers (fungi & bacteria) and
disturbance (human factor) are the main constraints affecting
spp distribution
 Fungal distribution and community composition are also
patterned along abiotic gradients
 The majority of plants establish mycorrhizal symbioses with
fungi.
 Some of these symbiotic fungi are obligate biotrophs that
require the host plant to complete their life cycle.
 In these associations, the fungus supplies the plant with
inorganic nutrients and the plant provides the fungi with
photosynthates.
 Plant-fungal symbioses influence different key aspects of
plants life.

18.  For eg. symbioses with AMF significantly increase the
tolerance of the host plant to abiotic and biotic stresses, and
hence favor the composition of plant communities.
4.5. Habitat and Niches
 Habitat refers to places where the organism live with all biotic
and abiotic factors
 For plants it could be aquatic, terrestrial or epiphytic.
 The concept of habitat associated with descriptive analyses of
the environments of organisms
 Niche refers to the position or function a species plays within its
community
 An ecological niche includes all of the factors that a species
needs to survive, stay healthy, and reproduce

19. 4.6 Interactions Between Species in a Community
• Communities are defined as a group of interacting species
occupying the same location
• In community we examine how spp interact with each other
and the resulting effects that occur in each spp.
• Interaction can be described as positive, neutral, or negative
effects
• Symbiosis is a biological or ecological interactions b/n two
spp that live in close physical association
• There are three forms of symbiosis; Mutualism, Parasitism &
Commensalism
• Mutualism is a symbiotic interaction b/n two species where
both spp benefit; eg., Bees and flowering plants.

20.  Commensalism is an association between two spp in which
one benefits, but the other neither benefited nor harmed; eg.,
milkweed & monarch butterfly
 Parasitism is an association b/n two spp in which one benefits
and the other harmed; eg., Aphid and plants.
 Competition is a relationship b/n two spp in which both
parties are harmed; Plant spp compete for temperature, light,
water, space
 Predation is a relationship b/n two spp in which the predator
eats the prey
 A plant community is a complex assemblage of plant spp
which interact with each other and with other elements of the
environment.
 It is not a static, rather it may vary in structure and species
composition from place to place and also over time

21. 4.6. Community Change: Succession
• The change in communities over time is termed as ecological
succession
• It is caused as a result of different disturbance factors such as
fires, floods, grazing, and human factors
• As the vegetation returns to a disturbed area, it may be the same
spp that were before, or replaced by other spp, this change is
known as succession.
• Natural ecological succession proceeds in a gradual process, it
may takes 100 years.
• During this time, one community evolve to another through a
series of stages
• Eventually, with absence of further disturbance, a climax
community develops, w/c is at equilibrium with the environment

22. Characteristics of ecological succession
a) A continuous change in the kinds of plants and animals
b) A tendency towards increase in the species-diversity.
c) An increase in the organic matter and biomass
d) Decrease in net community production or annual yield.
4.7 Climax Community
• The occurrence of a definite sequence of communities over a
long time in one area results in stable community or climax
• The first community to inhabit an area are called pioneer
community
• while the last and stable community in an area is called climax
community.
• The intermediate community between the pioneer and climax
communities are called transitional or seral community