Lecture 31

Section IV Outline

Ecological Community Structure

Community Change - Succession

Ecological Biogeography

Ecosystem Structure, Energy Flux

Energy Flux Between Trophic Levels

Energy Flux Efficiency

Biological Magnification and Pollution

Nutrient Cycles

Nutrient Regeneration and Limitation

Global Ecology and Pollution

Lecture 31: Community Structure

Reading: Economy of Nature, pp. 499-517.

Biological Communities
Any assemblage of species in a given area or habitat a given time.

Possible Definitions

1. Minimum property
presence of several species in a given area

2. Constant composition
same species occur together in time and space for a given community type

3. Dynamic stability
group of species make a self-regulating, homeostatic, super-organism

Community level variables

Species richness: number of different species
Relative abundance: population sizes of each species compared
Diversity indices: combines species richness and relative abundance

Growth forms, structure (plants), and seasonality:
forms: trees, shrubs, herbs
structure: stratification, vertical layering

Dominance: based on size, numbers, or activity

Trophic structure: who eats whom, energy flow pattern
food webs and subwebs
food guilds

Diversity Indices

Simpson’s Index

The proportion of total species abundance represented by species i in a community = p_i

Given i species, D may vary from 1 to i depending on evenness of abundances.

Shannon-Weaver Index

H is proportional to ln(i), e^H is proportional to ln(eⁱ), so e^H is proportional to i.

Comparisons of the diversity indices D, H, and e^H are shown for five hypothetical communities each containing five species (A - E) (from Ricklefs, 1996, p 517, Table 22.1).

Proportion of Each Species in Community					Diversity Index
A	B	C	D	E	D	H	e^H
0.20	0.20	0.20	0.20	0.20	5.00	1.609	5.00
0.25	0.25	0.25	0.25	0.00	4.00	1.386	4.00
0.24	0.24	0.24	0.24	0.04	4.31	1.499	4.48
0.25	0.25	0.25	0.25	0.001	4.02	1.393	4.03
0.50	0.30	0.10	0.07	0.03	2.81	1.229	3.42

What is a community?

Hypotheses

(A) Communities are fundamental units in nature

(B) Communities are classification for observer convenience but not fundamental functional units

Predictions

Stands and associations are (A) or are not (B) discontinuously distributed.

Species are (A) or are not (B) organized in discrete groups corresponding to associations and stands.

Distributions are not discontinuous except when there are environmental discontinuities. Most distributions are continuous, overlapping, and change gradually with local conditions and resources. This is seen in the distributions of four species of Maple (Acer) in eastern North America (Ricklefs, 1996, pp 105 and 106, Fig. 5.1 and 5.2).

Similar continuous, overlapping distributions are seen in three species of pine (Pinus) whose abundances change with elevation (after Krebs, 1994, Fig. 20.2).

Species are not in discrete groups. Individual species distributions are independent of each other in most cases.

Comparison between freshwater lake communities

Lake Erie and Lake Ontario are adjacent North American Great Lakes which contain communities of crustacea zooplankton, rotifers, and whitefish (Coregonidae). If communities were fundamental units in nature, we would expect the same species to occur in these two lakes which are of the same community type.

The extent of similarity in species composition in any two locations can be evaluated as the Index of Similarity (IOS):

IOS values have a range from 0 - 1.0.

a and b are the number of species in the two locations being compared.

c is the number of species found in both locations.

If communities are fundamental units of nature, then the IOS of Lake Erie and Lake Ontario should be 1.0, indicating that they have the same species composition.

IOS	Organisms	Locations
0.90	Zooplankton Crustacea	Lakes Erie and Ontario
0.55	Rotifers
0.40	Whitefish

Both lakes are the same community type, but they have very different species compositions for rotifers and whitefish, suggesting that communities are not fundamental units of nature.