LECTURE 21 CH 18 + 19: COMMUNITY STRUCTURE + DEVELOPMENT
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 can switch between alternative stable states.
10. Communities vary in whether or not they are in equilibrium. Disturbance keeps a
community from reaching equilibrium
11. Qualities of disturbances vary greatly among communities.
12. Primary succession is more slow than secondary succession because of the lack of
soil and seeds and harsh abiotic conditions.
13. Life history differences influence rank order of species entry during succession.
14. Various mechanisms govern succession: facilitation, inhibition, tolerance
Define community:
Assemblage of interacting species (populations) occurring in same area
Often named after conspicuous members
Questions: Spatially-defined communities
Ecotone: sharp boundary between communities; caused by abrupt changes in
physical feature (e.g. soil type) 18.4, 18.5
Growth on poor soils may be do tolerance or inability to compete on better soils 18.6
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 18.9
(Interactions support holistic nature of communities)
Questions: Functionally-defined in terms of interactions
How strong are connections among species?
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
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
Feeding relationships organize communities in food webs 18.11
Effect of species richness on food web structure
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
Variety of food web types
Connectedness, energy flow, function webs 18.14
Effect of food web structure on species diversity
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
What controls abundance within each trophic level? 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
Does greater food web complexity contribute to community stability? 18.15
Constancy (resist change);
Pros: Alternative resources; less dependent on fluctuations in any one resource
Energy can take many routes; disrupt one pathway shunts more energy to
another
Cons: More links creates widespread, destabilizing time lags in pop. processes
Resilience (return to prior state after disturbance) 18.17
Does species diversity help communities to bounce back from disturbance?
Communities can switch between alternative stable states. 18.18
Summary Chap 18: 1-5; 7-12
CHAPTER 19: ECOLOGICAL SUCCESSION AND COMMUNITY DEVELOPMENT
Are communities stable (in equilibrium) or transient (in non-equilibrium)?
Historically: emphasis on equilibrium view
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?
Disturbance keeps communities in non-equilibrium state.
Differ in size, frequency, intensity, level of environmental heterogeneity, and
remnant species and seed sources.
Succession
Directional replacement of species after disturbance
Sere – all stages of successional change 19.2
Succession follows as colonists alter environmental conditions
Theoretically ends in climax community that is self-replacing pg. 398-9; 19,13
Primary Succession pg. 392-393
On new bare substrate; no soil 19.4, 19.5
No seed bank; seeds dispersed from outside 19.11
Extreme abiotic conditions
Very slow
Secondary Succession 19.3
After disturbance to existing vegetation
Soil remains; seed bank may remain
Life history differences influence species rank order of entry during succession 19.10
r-selected pioneer species in early succession Table 19.2
K-selected species in late succession 19.12
Mechanisms governing succession
Facilitation – (drives ‘relay floristics’); common in primary succession 19.9
Early species modify environment in ways that later-stage species can establish
Inhibition – can lead to ‘arrested succession’
‘Jump-starting’ succession in restoration ecology
Tolerance – (explains ‘initial floristics’); passive
Animal communities follow stages of plant succession
Summary: 1-8