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Summary ZOL3702 learning unit 9
- Course
- ZOL3702 - Ecology (ZOL3702)
- Institution
- University Of South Africa (Unisa)
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ZOL 3702 LEARNING UNIT 9THE NATURE OF COMMUNITIES:
Textbook reference: pg 469 - 498
Learning outcomes:
Describe, evaluate and apply the concept of community and the characteristics of a community.
Explain, evaluate and apply the methods for measuring and describing aspects of communities using, for
example, diversity indices and rank-abundance diagrams.
Describe, evaluate and apply patterns in the community structure in space using gradient analysis,
ordination and classification.
Describe, evaluate and apply community patterns in time, such as allogenic and autogenic succession as
well as the climax concept.
APPROACHES REGARDING COMMUNITY STUDIES IN ECOLOGY pg 469
Two kinds of characteristics can be distinguished for communities, due to their hierarchical organisation:
- Collective characteristics (community characteristics of the components) such as species diversity,
community biomass and community productivity.
- Emergent characteristics (characteristics that cannot be deduced from the separate components and
that emerge as a result of interactions between the species) such as the food web structure, energy
and nutrient flow and the similarities/differences between competing species.
Community ecology is the study of patterns in the structure and behaviour of multispecies groupings.
Patterns from the cornerstone of all sciences, that is, repetitive patterns such as the grouping of the same
species at different places, growth forms and productivity.
In the case of community ecology repetitive patterns in collective and emerging characteristics are studied
Since studies of communities can be very complex, the search for patterns involves a search for simple
ways to describe complex systems.
These patterns lead to hypotheses about the causes of the patterns. Such hypotheses can then be tested
by observation or conducting experiments.
DESCRIPTIONS OF COMMUNITY COMPOSITION: pg 470 - 473
1 way to characterise a community is to count or list species present. In practice it is difficult due to
taxonomic problems, but also because only a subsample of organisms in an area can usually be counted.
The number of species recorded then depends on the number of samples that have been taken or on the
volume of the habitat that has been explored.
The most common species are likely to be represented in the first few samples and as more samples are
taken, rarer species will be added to the list.
When does one stop taking more samples? Ideally it should continue until no. of species reaches a plateau
Species richness of different communities should be compared on the basis of same sample sizes (in terms
of area of habitat explored, time devoted to sampling, or no. of individuals/modules included in samples).
Communities can only be compared if the sampling input was the same.
DIVERSITY INDICES:
By only looking at the number of species, an important factor is ignored – the distribution of the
abundance of individuals among the species. The rareness and abundance of species must be considered.
Communities can have the same species richness, but some can be more diverse.
Richness and equitability combine to determine community diversity.
If the community is closely defined, counts of the number of individuals in each species may suffice for
many purposes.
However, if we are interested in all animals in an area, then their enormous disparity in size means simple
counts could be misleading.
, There are also problems if we try to count plants and other modular organisms. Do we count the number
of shoots, leaves or stems.
One way around this problem is to describe the community in terms of biomass per species per unit area.
The simplest measure of the character of a community that considers the
abundance (or biomass) patterns, and the species richness is Simpson’s
diversity index.
This is calculated by determining, for each species, the proportion of
individuals or biomass that it contributes to the total in the sample.
Pi is the proportion for the ith species
S is the total number of species in the community (richness)
D increases with equitability, and decreases with richness
Another index used is Shannon diversity index, H.
One of the limitations of
diversity indices is that it can be difficult to count the organisms. The comparison of indices of groups of
organisms that differ widely in terms of size, could be misleading.
RANK-ABUNDANCE DIAGRAMS:
Attempts to describe a complex community structure by one single attribute, such as richness, diversity or
equitability can be criticized because so much valuable information is lost.
A more complete picture of the distribution of species abundances in a community makes use of the full
array of Pi values by plotting Pi against rank.
Therefore P for the most abundant species is plotted first, then the next most common and so on.
A rank-abundance diagram can be drawn for the number of individuals, or for the area of ground covered
by different sessile species, or for the biomass contributed to a community by various species.
Rank abundance diagrams offer a visual picture/pattern of equity and species richness.
If this index is drawn as log abundant against species, we get certain descriptive patterns.
A flatter graph indicates a more equitable distribution of species and is associated with a higher diversity
index for a given species richness.
Different patterns are obtained for a straight line (geometric series model) for communities with low
species diversity and high dominance or poor distribution, against the broken stick model, which
represents high diversity and a high degree of equitable distribution.
For many species, rank abundance diagrams can be difficult to compile.
All 3 methods (species richness, diversity indices and rank abundance diagrams) have limitations and uses.
COMMUNITY PATTERNS IN SPACE pg 474 - 478
The boundaries between communities are rarely sharply defined. Ecologists prefer to describe gradients
between communities than define finite boundaries.
On the basis of their species composition and other characteristics such as relevant abundance of species,
communities can be compared to establish the differences and similarities between them.
Therefore, based on specific species composition, 2 communities that have same biodiversity index could
be completely different.
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