Samenvatting van de stof voor het tweede tentamen uit het boek 'Elements of Ecology' in het Engels. Na de stof van ieder deel is een kleine opsomming gemaakt (in het Nederlands) over de belangrijke punten uit de reader.
Samenvatting Ecologie deel
2
HS 16: Community Structure (par. 1 – 5 & quantifying
ecology 16.1 in detail)
16.1 Biological structure of community defined by
species composition
The group of species that occupy a given area, interacting either directly or indirectly, is
called a community. The biological structure of a community is defined by its species
composition.
You can examine the patterns of relative abundance of species within communities by
plotting the relative abundance of each species against rank, the order of species from the
most to the least abundant. The resulting graph is a rank-abundance diagram. This graph
shows the species richness and species evenness.
16.2 Species diversity is defined by species richness and
evenness
Species diversity covers both the number and relative abundance of species within the
community, for example the Simpson’s index (D) which measures the probability that two
individuals randomly selected from a sample will belong to the same species: D=∑ p i2 in
which pi is the proportion of the total individuals in the community represented by species i.
When only one species is present, D = 1, when species richness and evenness increase, the
value approaches 0. Sometimes 1/D is used, in which 1 is the lowest value and the number
of species is the maximum value.
1
( )
You can also calculate the value of evenness: D in which S is the number of species.
E D=
S
ED can range from 0 to 1, with a value of 1 representing complete evenness.
Another index is the Shannon index: H=−∑ ( pi )¿ ¿ with species evenness:
EH = H/Hmax.
16.3 Dominance can be defined by a number of criteria
Dominance can be defined in terms of biomass, numbers or both.
16.4 Keystone species influence community structure
disproportionately to their numbers
A species that has a disproportionate impact on the community relative to its abundance is
referred to as a keystone species.
16.5 Food webs describe species interactions
The maximum number of links in a food web is S2. The actual number of observed links in a
food web (L) expressed as a proportion of the maximum possible number of links (S2)
,provides a measure of food web connectance (C): C = L / S2. If you only use unidirectional
links, the maximum number of links is S(S – 1)/2.
Linkage density (LD) is a measure of the average number of links per species in a food web:
LD = L/S. The mean chain length (ChLen) is the average of the lengths of all chains in a food
web.
Quantifying ecology 16.1 Community similarity
The Sorensen’s coefficient of community (CC) is an index that measures the similarity
between two areas or sample plots based on species composition. CC = 2c / (s1 + s2) in which
s1 is the number of species in community 1, s2 is the number of species in community 2 and c
is the number of species common to both communities. The outcome ranges from 0 to 1 in
which 0 is when the two communities share no species in common and 1 is when the species
composition of the two communities is identical.
Another index is the percent similarity (PS) which is calculated by calculating the species
abundance as a percentage and then adding the lowest percentage for each species that the
communities have in common. The outcome ranges from 0 to 100.
HS 17: Factors Influencing the Structure of
Communities
17.1 Community structure is an expression of the
species’ ecological niche
The null model assumes that the presence and abundance of the individual species found in
a given community are solely a result of the independent responses of each individual
species to the prevailing abiotic environment. Interactions among species have no significant
influence on community structure. However, evidence indicates that species interactions do
influence both the presence and abundance of species within communities.
17.2 Zonation is a result of differences in species’
tolerance and interactions along environmental
gradients
Differences in environmental tolerances among species and changes in the nature of species
interaction result in shifts in the species present and their relative abundance. These spatial
changes in community structure are referred to as zonation.
Spatial changes in species distribution along the intertidal gradient are a result of the trade-
off between tolerance to environmental stress and competitive ability.
Patterns of zonation reflect an interaction between the relative competitive abilities of
species in terms of acquiring nutrients and the ability of plant species to tolerate increasing
physical stress.
17.3 Species interactions are often diffuse
Studies sometimes underestimate the importance of species interaction on the structure
and dynamics of communities because interactions are often diffuse and involve a number of
, species. Diffuse competition describes the total competitive effects of a number of
interspecific competitors.
Experiments that remove only one or a small number of species may show little effect on the
abundance of the focal species. However, removing all competing species may result in a
significant positive impact on the focal species.
17.4 Food webs illustrate indirect interactions
Indirect interactions occur when one species does not interact with a second species directly
but instead influences a third species that does directly interact with the second.
In keystone predation, the predator enhances one or more less competitive species by
reducing the abundance of the more competitive species.
Apparent competition occurs when two species that do not compete with each other for
limited resources affect each other indirectly by being prey for the same predator. For
example when one prey species increases in population size, the predator is attracted and
the other prey species can decline in population size. But there can also be a positive
interaction between the prey species like indirect commensalism and indirect mutualism.
17.5 Food webs suggest controls of community structure
To understand the influence of species diversity on the structure and dynamics of
communities, you can group species together. By aggregating species into a smaller number
of functional groups, researchers can explore the processes controlling community structure
in more general terms.
Two types of control:
Bottom-up control: the productivity and abundance of populations at any given
trophic level are controlled by the productivity and abundance of populations in the
trophic level below them.
Top-down control: predator populations control the abundance of prey species.
o Trophic cascade: when a predator in a food web suppresses the abundance
for their prey such that it increases the abundance of the next lower trophic
level on which the intermediate species feeds.
17.6 Environmental heterogeneity influences community
diversity
Environmental conditions are not homogeneous within a given community.
Increased vertical structure means more resources and living space and a greater diversity of
habitats.
17.7 Resource availability can influence plant diversity
within a community
Tropical forest communities on soils with lower nutrient availability supports a greater
number of tree species than do communities on more fertile soils. Species that might
dominate under higher nutrient availability cannot realize their potential growth rates and
biomass and thus are unable to displace slower-growing, less competitive species.
Under low nutrient availability, plant growth rate, size and density are low for all species.
Competition primarily occurs belowground and therefore is symmetric (larger roots linearly
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