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Chapter 19

Biology 2483A Chapter 19: Species Diversity in Communities


Department
Biology
Course Code
BIOL 2483A
Professor
Hugh Henry
Chapter
19

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Lecture 18 Species Diversity in Communities
Introduction
What is the importance of species diversity?
o Functional reasons:
More redundancy, so if one species goes extinct than it won’t suffer the community as much if the community
wasn’t as diverse. Ie. If only one species holds a single function, then it could alter how the community works
entirely.
If more variability, then can overcome disease easier.
Higher productivity = ecosystem services.
o Ethical reasons ie. Not our right to put species extinct.
Species diversity at the local scale two important questions:
1. What are the factors that control species diversity within communities?
2. What is the effect of species diversity on community function
Distribution and abundance of species in communities depends on:
1. Regional species pools and dispersal ability.
2. Abiotic conditions.
3. Species interactions.
These factors act as filters, which excluding or
including species in particular communities.
- Shows regional species pool w/ factors acting as
filters ending up with final Local community pool.
1. Regional species pools and dispersal ability:
o The regional species pool provides an upper limit (filter) on number & types of species in a community.
o The importance of dispersal can be seen in cases of non-native species invasions.
Humans have greatly expanded regional species pools by serving as vectors of dispersal.
Example: Aquatic species travel around the world in ballast water carried by ships
Ie. Recall in the jelly fish that were moved from one area of the sea to another.
Ie. Zebra mussels made it into the fresh water systems in Ontario and they spread very quickly. Became
filters in the system.
Ships are now larger and faster, so trans-ocean trips take less timespecies are more likely to survive.
2. Abiotic conditions:
o A species must next be able to tolerate the abiotic conditions typically a large filter.
For example, a lake might not support organisms that require fast-flowing water.
Many species that are dispersed in ballast water cant survive in a new habitat because of temperature,
salinity, etc.
But we cant rely on physiological constraints to exclude invaders, as in the case of
Caulerpa
in the
Mediterranean Sea.
Might have a few individuals that overcome the abiotic conditions and over populate the area.
3. Species interactions:
o Coexistence with other species is also required for community membership.
o Other species may be required for growth, reproduction, or survival.
o Species may be excluded by competition, predation, parasitism, or disease.
Some non-native species do not become part of the new community:
o Biotic resistance occurs (when communities/native species resist invasion/new species) when interactions with the
native species exclude the invader.
Example: Native herbivores can reduce the spread of non-native plants by eating them ie. Non-native plants
haven’t built a form of protection against the herbivores.
Not a lot is known about biotic resistance, partly because failed introductions of non-native species tend to go
undetected.
Easier to study successful invasions.
- How can we explain high species diversity vs. low species diversity?
Resource partitioning: Competing species coexist by using resources in different ways.
o It reduces competition and increases species richness.
o In a simple model, each species resource use lies on a spectrum of available resources.
o Any kind of gradient in the resources (ie. seed size)
- Each distribution represents the use by a single
species
- How can you compact more species into
a community?
-- Narrower species: narrow the range
in the spectrum to get more specialized
individuals along the same gradient.
-- Broader resources: broader resources
for species and extend the resource
spectrum
- More overlap of resource use = more
competition between species.
- Less overlap = more specialized
species and less competition.
MacArthur (1958) studied resource partitioning in a community of warblers in New England forests.
o He recorded feeding habits, nesting locations, and breeding territories.
o When he mapped the locations of warbler activity he found that the birds were using different parts of the habitat in
different ways.

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- Were partitioning space in the trees
in terms of where they specialized in
were they got their food.
Broader resources
- looking at this more broadly across the
land scape, came up with index of
measuring the foliage height of diversity
(measures the diversity of different
spots within a tree)
- Tall tree with many different areas
(broader = high diversity), and shorter
tree (less broad = less diversity).
- Complexity of trees can fit more
species.
To explain how diatom (algea) species coexist in nature, Tilman proposed the resource ratio hypothesis: Species coexist by
using resources in different proportions.
Two diatom (algae) species were grown in media with different (Silica: Phosphorus) SiO2:PO4 ratios.
o Tilman found that
Cyclotella
dominated only when the ratio was low,
Asterionella
dominated when the ratio was high.
Coexistence occurred only when SiO2 and PO4 were limiting to both species.
- Push ratio to 2 extremes, get
dominance of one species or the
other, but if you move ratio btw 100-
10, then you get stable coexistence
btw Asterionella and Cyclotella based
on fact that they’re able to use
resources differently.
In a field study, Robertson et al. (1988) mapped soil moisture & nitrogen concentration and found variation over small spatial
scales.
o If two maps are combined, patches corresponding to different proportions of two resources emerge.
o This suggests that resource partitioning could occur in plants.
- Little coorrespondance in moisture & nitrogen (no relationship, lots of ratio variety).
- Can have different ratios of different resources for organisms, explains high species diversity due to the
heterogeneity/diversity in environment.
Processes That Promote Coexistence
Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus
promoting species coexistence and species diversity.
Fundamentally coexistence comes down to how dominant competitor responds to conditions. When the dominant competitor is
unable to reach its own carrying capacity (ie. affected by disturbance or predation) competitive exclusion cant occur
(releasing other non-dominant species, allowing them to do better), and coexistence will be maintained.
G. E. Hutchinson considered the idea in his paper The Paradox of the Plankton (1961). Lake Phytoplankton communities
have very high diversity (3040 species), all using the same limited resources, in a homogeneous environment.

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o His explanation was that conditions in the lake changed seasonally, which kept any one species from outcompeting the
others.
o As long as conditions changed before competitively superior species reached carrying capacity,
coexistence would be possible.
- might have dominant predator in June,
but conditions are constantly changing
throughout year so before species can
exert its dominance and exclude all the
others, the conditions change and allow
other species to survive.
- Keeps changing throughout season, so
you never get dominance in one species.
- Key thing is the speed in which one
species can dominant the others,
relative to how quick the conditions
change.
- Fast growth rate, and conditions
change slowly = lower species diversity,
than if the conditions were vice versa (ie.
slower growth rate & rapid change).
- When we think about how disturbance
can affect species. This is a classic view
of how 2 species of very similar resource
requirements. So in variable conditions,
(arrows representing disturbance) every
time we have disturbance come along we
knock down both species allowing for
coexistence because we’re never allowing
Species 1 to reach carrying capacity (K) /
completely dominating the other.
- Promoting disturbance can
promote diversity up to a certain
point. But too much disturbance can
knock out a species.
The intermediate disturbance hypothesis: Species diversity should be greatest at intermediate disturbance.
- At low (infrequent/mild)
disturbance, competition
determines diversity. Due to
competitive dominance.
- At high (frequent/intense)
disturbance, many species cannot
survive.
- At intermediate levels, allows for
coexistence and highest level of
diversity based on disturbance.
There have been many tests of this hypothesis.
o Sousa studied communities on intertidal boulders in southern California that were overturned by waves.
o Small boulders were overturned frequently (disturbance), large boulders were overturned less often.
- Study consistent w/ intermediate
disturbance hypothesis.
Potential role of positive interactions:
- Talked also about positive
interactions (species facilitating one
another). And when you superimpose
the positive interactions w/ the
imtermediate disturbance hypothesis.
- Positive interactions are going to
allow species to persist at both
intermediate and high
stress/disturbance, where they
wouldn’t otherwise be able to persist.
- Superimposing the two extend the
range (see that in middle the red is
higher than blue) in which you’ll have
higher species diversity.
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