LECTURE 21 CH 18 COMMUNITY STRUCTURE
MAJOR CONCEPTS
1. Ecologists hold diverse concepts of communities. Some have a holistic view; others
an individualistic view of the nature of the community.
2. Gradient analysis usually shows support for the individualistic view; interactions
support holistic view.
3. Communities change gradually along a continuum unless an ecotone sharply
divides communities.
4. Communities may have tight connection among species – or redundant species
with less strong connection.
5. Feeding relationships organize communities in food webs that can be quantified
with multiple measures.
6. The activities of a keystone species may control the structure of communities.
7. Debate continues about whether greater food web complexity leads to stability.
8. Abundance within trophic levels is influenced from above by predation (top-down
control) and from below by production (bottom-up control).
9. Communities vary in whether or not they are in equilibrium. Disturbance keeps a
community from reaching equilibrium
Define community:
Assemblage of interacting species (populations) occurring in same area
I. Questions: Spatially-defined communities 371-373
A. Ecotone: sharp boundary between communities; caused by abrupt changes in
physical feature (e.g. soil type) 373-375 18.4, 18.5
Growth on poor soils may be due to tolerance or inability to compete on better soils 18.6
B. What is the ‘nature’ of the community? Is it a natural unit of organization? pg. 369-370
Pro: Holistic view 18.3a
Closed with discrete boundaries
If true, then can focus on classifying communities
Con: Individualistic view 18.3b, 18.7
Haphazard assemblages of species
Open with no natural boundaries; each species has unique range
If true, then focus on community dynamics and functional organizatio
Continuum concept: species replace one another continuously along physical gradients 18.7
Gradient analysis: most studies support continuum, hence open communities 375-6 18.9
(Interactions support holistic nature of communities)
II. Questions: Functionally-defined in terms of interactions
A. How strong are connections among species?
1.Rivet model: Tight linkage among species
Obligate associations or exclusions
Conservation implications: requires community dynamics approach
Expect removal of one species to have cascading effects
2. Redundancy model: Loose linkage among species
Most species have little impact on other species
Multiple species can play same ecological role
If remove one species, another takes over
B. Feeding relationships organize communities in food webs 377-378 18.11
1. Effect of species richness on food web structure 378
2. What changes with increased food web complexity? 18.12
Number of trophic levels
Number of guilds per trophic level
Number of interactions/species (doesn’t change)
Connectance = # interactions/total possible = # interactions/[S(S-1)]
Linkage density = # interactions/ # species
3. Variety of food web types 380-381
Connectedness, energy flow, function webs 18.14
4. Effect of food web structure on species diversity 378-380
What is the role of keystone species in communities? 18.13
Non-redundant species that maintains community stability and diversity
System-specific: Can be plant, herbivore, or predator
C. What controls abundance within each trophic level? 385-389 18.19
Top-down control (by predators)
Bottom-up control (by nutrients, plant productivity, plant defense)
Trophic cascade: ripple effects from top level through multiple trophic levels 18.23, 24, 25
Indirect effects on lower levels; abundance alternates at each trophic level
D. Does greater food web complexity contribute to community stability? 380-382 18.15
1. Constancy (resist change);
a. Pros: Alternative resources; less dependent on fluctuations in any one resource
Energy can take many routes; disrupt one pathway shunts more energy to
another
b. Cons: More links creates widespread, destabilizing time lags in pop. processes
2. Resilience (return to prior state after disturbance) 18.17
Does species diversity help communities to bounce back from disturbance? 382-3
III. Communities through time
A. Communities can switch between alternative stable states. 383-384 18.18
B. Are communities stable (in equilibrium) or transient (in non-equilibrium)?
Historically: emphasis on equilibrium view; ignored disturbance
Disturbance keeps communities in non-equilibrium state.
Current: raises importance of disturbance and non-equilibrium view
Not ‘either-or’; rather under what circumstances does one or other prevail?
Importance for conservation: easier to conserve a predictable equilibrium community?
C. Communities recover from disturbance via primary and secondary succession 392-399
Summary Chap 18: 1-5; 7-12