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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

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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

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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

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