This lecture discusses mutualism, species abundance, and diversity

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Biology 205
Ecology and Adaptation
Mutualism,
Species Abundance, and Diversity
Dr. Erik D. Davenport

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Outline (chapter 15-16)
 Introduction of mutualism.
– Plant Mutualisms
– Coral Mutualisms
 What is community.
 Species Abundance ----Lognormal Distribution
 Species Diversity
 Environmental Complexity --- Niches
 Disturbance and Diversity

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Introduction
 Mutualism: Interactions between individuals of
different species that benefit both partners.
– Facultative Mutualism occurs when a species can live
without its mutualistic partner.
– Obligate Mutualism occurs when a species is
dependent on a mutualistic relationship.

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Concept1: plant mutualisms
Plants benefit from mutualistic partnerships
with a wide variety of bacteria, fungi, and
animals.
Mycorrhizae: the symbiotic association of the a
fungus with the roots of a seed plant

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Mycorrhizae and Plant Water
Balance
 Hardie suggested mycorrhizal fungi improve
water relations by providing more extensive
contact with moisture in rooting zone and
providing extra area for water absorption.

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Mycorrhizae and Plant Water
Balance
 Allen and Allen studied water relations of
grass Agropyron smithii.
– Plants with mycorrhizae maintained higher leaf
water potentials.
– Plants with greater access to phosphorus may
develop roots that are more efficient at extracting
and conducting water.

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15_03.jpg

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Nutrient Availability
 Fungal partner received an equal or greater
quantity of photosynthetic product in trade for
low quantity of nutrients.
 Results suggested mycorrhizal fungi from
unfertilized soils supplied plants with more
nutrients.
 Plants able to invest more energy in above-
ground photosynthetic material.

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15_06c.jpg

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plant mutualisms -- Mycorrhizae
 Benefits for plants: Plants receive more water
and nutrients.
 Benefits for Fungus: Fungus receive
photosynthetic products (sugars) from plants

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Coral Mutualisms
 Zooxanthallae and Corals
– Zooxzanthallae is one type of phytoplankton
(dinoflagellates) live within coral tissues.
– It receives nutrient from coral.
– In return, coral receives organic compounds
synthesized by zooxanthallae during
photosynthesis.

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Coral Mutualisms
 Corals also control rate of zooxanthallae
population growth and density by influencing
organic matter secretion.
 Main zooxanthallae benefit appears to be
access to higher nutrient levels, especially N
and P.
 Uptakes ammonium excreted by coral.

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Coral Protection Mutualism
 Glynn found 13 coral
species protected by
crustacean mutualists.
 Crustacean mutualists
substantially improved
chances coral will avoid
attack by sea stars.

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Community
(what is population???)

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Introduction
 Community: Association of interacting
species (population) inhabiting some defined
area.
 Community Structure includes attributes such
as number of species, relative species
abundance, and species diversity.

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Pattern of Species Abundance
 There are regularities in the
relative abundance of species
in communities that hold
irregardless of the ecosystem.
 Most species are moderately
abundant; few are very
abundant, or extremely rare.

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Lognormal Distribution
 Preston graphed abundance of species in
collections as frequency distributions.
– Lognormal Distributions
– Bell-shaped curves.
– In most lognormal distributions, only portion of
bell-shaped curve is apparent.
– Sample size has large effect.
– Significant effort to capture rare species.

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Lognormal Distribution

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16_04.jpg

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Species Diversity
Which one do you think that has the higher diversity? Why?

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Species Diversity!!!!
 Two factors define species diversity:
– Species Richness
 Number of species in the community.
– Species Evenness
 Relative abundance of species.

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Quantitative Index of Species
Diversity (you don’t need to remember the
equation)
 Shannon Wiener Index:
 H’ = Value of SW diversity index.
 Pi = Proportion of the ith species.
 Loge = Natural logarithm of pi.
 s = Number of species in community.
ie
s
i
i ppH log'
1



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Rank Abundance Curves
 Can also portray relative abundance and
species diversity within a community by
plotting relative abundance of species
against their rank in abundance.
 Greater evenness indicated by lower slope.

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Rank Abundance Curves

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Environmental Complexity
 In general, species diversity increases with
environmental complexity or heterogeneity. why???
– Higher environmental complexity will introduce a more
diversified environments -- more niches
– More niches  higher species diversity
 Many studies have shown positive relationship
between environmental complexity and species
diversity.

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Environmental Complexity

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Niches and Diversity of Algae and
Plants
 Hutchinson:
– Phytoplankton communities present a paradox
because they live in relatively simple
environments and compete for the same
nutrients, yet many species coexist without
competitive exclusion.
– Environmental complexity may account for
significant portion of the diversity.

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Niches and Diversity of Algae and
Plants
 Algal niches are defined by their nutrient
requirements.
 Difference algae species require different type of
nutrients
 Tilman found coexistence of freshwater diatoms
depended upon ratio of silicate and phosphate.
– Found conditions allowing coexistence.
– Diatoms held different trophic niches.
– Thus different diatoms would dominate different areas.

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16_12.jpg

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Algal and Plant Species Diversity and
Increased Nutrient Availability
 Repeatedly observed negative relationship
between nutrient availability and algal and plant
species diversity.
 What is eutrophication?
 Adding nutrients to water or soils generally
reduces diversity of plants and algae. Why???
 Reduces number of limiting nutrients.

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Disturbance and Diversity
 Disturbance difficult to define as it involves
departure from “average conditions.”
– Average conditions may involve substantial
variation.
 Sousa defined disturbance:
– Discrete, punctuated, killing, displacement, or
damaging of one or more individuals that directly
or indirectly creates an opportunity for new
individuals to be established.

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Disturbance and Diversity
 White and Pickett defined disturbance:
– Any relatively discrete event in time that disrupts
ecosystem, community, or population structure
and changes resources, substrate availability, or
the physical environment.
– Two major characteristics:
 Frequency
 Intensity

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Intermediate Disturbance Hypothesis
 Connell proposed disturbance is a prevalent
feature that significantly influences community
diversity.
– Proposed both high and low levels of disturbance
would reduce diversity.
– Intermediate levels promote higher diversity.
– Sufficient time between disturbances allows wide
variety of species to colonize, but not long enough to
allow competitive exclusion.

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Disturbance and Diversity in the Intertidal
Zone