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1
Species Abundance and Diversity
Chapter 16
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Outline
• Introduction• Species Abundance
Lognormal Distribution• Species Diversity• Environmental Complexity
Niches• Disturbance and Diversity
Intermediate Disturbance Hypothesis
3
Introduction
• Community: Association of interacting species inhabiting some defined area. Community Structure includes attributes
such as number of species, relative species abundance, and species diversity.
• Guild: Group of organisms that all make their living in the same fashion. Seed eating animals in the desert.
• Life Form: Combination of structure and growth dynamics.
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Species Abundance
• There are regularities in the relative abundance of species in communities that hold regardless of the ecosystem.
• Preston developed concept of distribution of commonness and rarity.
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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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Lognormal Distribution
• May proposed lognormal distribution is a statistical expectation.
• Sugihara suggested lognormal distribution is a consequence of the species within a community subdividing niche space.
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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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Species Diversity
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Quantitative Index of Species Diversity
• Shannon Wiener Index: s
H’ = -Σpi logepi
i=l
• H’ = Value of SW diversity index.• Pi = proportion of the ith species.
• Loge = natural logarithm of pi.
• S = Number of species in community.
11Molles: Ecology 2nd Ed.
H’ example
• Community: S = 7AAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABCDEFG
• Sum values across all species• pi = 44/50 = 0.88, pi = 1/50 = 0.02• Calculate ln for each (ln 0.88 = - 0.128)
12Molles: Ecology 2nd Ed.
H’
• Sum pi’s
• = 0.88 (-0.128) + 0.02 (- 3.91) +0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) = - 0.58
• Change sign; H’ = 0.58
13Molles: Ecology 2nd Ed.
Another example:
• Community: S = 5ABCDEABCDE
ABCDEABCDEABCDEABCDE
ABCDEABCDE
ABCDEABCDE
• H’ = 1.61
14Molles: Ecology 2nd Ed.
What’s the purpose of H’?
• Includes both species richness and evenness
15
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
17Molles: Ecology 2nd Ed.
Environmental Complexity
In general, species diversity increases with environmental complexity or heterogeneity
MacArthur found warbler diversity increased as vegetation stature increased
Measured env. complexity as foliage height
Many studies find positive relationship between env. complexity and species diversity
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Environmental Complexity
19Molles: Ecology 2nd Ed.
Fig 16.9
20Molles: Ecology 2nd Ed.
Diversity of Algae and Plants
Hutchinson:
Phytoplankton communities present paradox – they live in relatively simple environments and compete for the same nutrients, yet many species coexist without competitive exclusion
Env. complexity may account for diversityEnv. complexity may account for diversity
21Molles: Ecology 2nd Ed.
�Diversity of Algae and Plants
� Algal niches appear to be defined by nutrient requirements Tilman (1977)
found coexistence of freshwater diatoms depended upon ratio of silicate and phosphate
22Molles: Ecology 2nd Ed.
• Tilman (1977) found conditions allowing coexistence
• Diatoms held different trophic niches• Thus different diatoms would dominate
different areas• Trophic niches =
nutrient requirements
23Molles: Ecology 2nd Ed.
Heterogeneity and Diversity of Tropical Forests• Jordan – Amazon tropical forest diversity
organized in two ways: 1. Large number of species live within
most tropical forest communities
Fig 16.14
24Molles: Ecology 2nd Ed.
2. Large number of plant communities in a given area, each with distinctive species composition
Fig 16.14
25Molles: Ecology 2nd Ed.
Algal and Plant Species Diversity And Increased Nutrient Availability
• Repeatedly observed negative relationship between nutrient availability and algal and plant species diversity
• Adding nutrients to water or soils reduces diversity of plants and algae Reduces number of limiting nutrients
26Molles: Ecology 2nd Ed.
Disturbance and Diversity
• Disturbance difficult to define because it involves departure from “average conditions.” Average conditions may involve substantial
variation
27Molles: Ecology 2nd Ed.
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
28Molles: Ecology 2nd Ed.
Disturbance and Diversity
• Another definition of disturbance: Any relatively discrete event in time that
disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment
29Molles: Ecology 2nd Ed.
Two major characteristics of disturbance:
Frequency
Intensity
30Molles: Ecology 2nd Ed.
�Intermediate Disturbance Hypothesis
� Connell � disturbance is a prevalent feature that
significantly influences community diversity Proposed that both high and low levels of
disturbance reduce diversityIntermediate levels promote higher
diversity
31Molles: Ecology 2nd Ed.
• Sufficient time between disturbances allows wide variety of species to colonize, but not long enough to allow competitive exclusion
32Molles: Ecology 2nd Ed.
Disturbance and Diversity in The Intertidal Zone
• Sousa studied effects of disturbance on diversity of algae and invertebrates growing on boulders in the intertidal zone Predicted level of disturbance depends on
boulder size
33Molles: Ecology 2nd Ed.
Large boulders move less = heavier
Boulders with highest diversity of species had intermediate levels of disturbance
Fig 16.18
34Molles: Ecology 2nd Ed.
Disturbance and Diversity inTemperate Grasslands - burrowing as a
disturbance• Whicker and Detling • prairie dogs (Cynomys spp.)
source of disturbance on N. A. prairies Build extensive burrow
systems Move 200-225 kg soil from
underground to entrance
35Molles: Ecology 2nd Ed.
• They removed vegetation around burrows Area opens to colonization Pest control programs reduced prairie
dog populations 98% Eliminated dynamic
influences on plant communities
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Review
• Introduction• Species Abundance
Lognormal Distribution• Species Diversity• Environmental Complexity
Niches• Disturbance and Diversity
Intermediate Disturbance Hypothesis
37