1. Species Abundance and Diversity
Chapter 16

2. 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.

4. 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.

5. 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.

6. Lognormal Distribution

7. 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.

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

9. Species Diversity

10. 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.

11. Sum values across all species pi = 44/50 = 0.88, pi = 1/50 = 0.02
H’ example
Community: S = 7 AAAAAAAAAA AAAAAAAAAA AAAAAAAAAA AAAAAAAAAA AAAABCDEFG
Sum values across all species
pi = 44/50 = 0.88, pi = 1/50 = 0.02
Calculate ln for each (ln 0.88 = )
Molles: Ecology 2nd Ed.

12. H’
Sum pi’s
= 0.88 (-0.128) (- 3.91) (- 3.91) (- 3.91) (- 3.91) (- 3.91) (- 3.91) =
Change sign; H’ = 0.58
Molles: Ecology 2nd Ed.

13. Another example:
Community: S = 5 ABCDEABCDE ABCDEABCDE ABCDEABCDE ABCDEABCDE ABCDEABCDE
H’ = 1.61
Molles: Ecology 2nd Ed.

14. What’s the purpose of H’?
Includes both species richness and evenness
Molles: Ecology 2nd Ed.

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.

16. Rank Abundance Curves

17. 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
Molles: Ecology 2nd Ed.

18. Environmental Complexity

19. Fig 16.9
Molles: Ecology 2nd Ed.

20. 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 diversity
Molles: Ecology 2nd Ed.

21. 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
Molles: Ecology 2nd Ed.

22. Tilman (1977) found conditions allowing coexistence
Diatoms held different trophic niches
Thus different diatoms would dominate different areas
Trophic niches = nutrient requirements
Molles: Ecology 2nd Ed.

23. 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
Molles: Ecology 2nd Ed.

24. 2. Large number of plant communities in a given area, each with distinctive species composition
Fig 16.14
Molles: Ecology 2nd Ed.

25. 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
Molles: Ecology 2nd Ed.

26. Disturbance and Diversity
Disturbance difficult to define because it involves departure from “average conditions.”
Average conditions may involve substantial variation
Molles: Ecology 2nd Ed.

27. 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
Molles: Ecology 2nd Ed.

28. 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
Molles: Ecology 2nd Ed.

29. Two major characteristics of disturbance:
Frequency
Intensity
Molles: Ecology 2nd Ed.

30. Intermediate Disturbance Hypothesis
Connell
disturbance is a prevalent feature that significantly influences community diversity
Proposed that both high and low levels of disturbance reduce diversity
Intermediate levels promote higher diversity
Molles: Ecology 2nd Ed.

31. Sufficient time between disturbances allows wide variety of species to colonize, but not long enough to allow competitive exclusion
Molles: Ecology 2nd Ed.

32. 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
Molles: Ecology 2nd Ed.

33. Large boulders move less = heavier
Boulders with highest diversity of species had intermediate levels of disturbance
Fig 16.18
Molles: Ecology 2nd Ed.

34. prairie dogs (Cynomys spp.) source of disturbance on N. A. prairies
Disturbance and Diversity in Temperate Grasslands - burrowing as a disturbance
Whicker and Detling
prairie dogs (Cynomys spp.) source of disturbance on N. A. prairies
Build extensive burrow systems
Move kg soil from underground to entrance
Molles: Ecology 2nd Ed.

35. They removed vegetation around burrows Area opens to colonization
Pest control programs reduced prairie dog populations 98%
Eliminated dynamic influences on plant communities
Molles: Ecology 2nd Ed.

36. Review
Introduction
Species Abundance
Lognormal Distribution
Species Diversity
Environmental Complexity
Niches
Disturbance and Diversity
Intermediate Disturbance Hypothesis