1. Mark J Gibbons, Room 4.102, BCB Department, UWC
Community Ecology
BDC331
Mark J Gibbons, Room 4.102, BCB Department, UWC
Tel:
Image acknowledgements –

2. Environmental Condition
Some Definitions
Environmental Condition
Physical environmental variable or factor, that varies in space and time, and to which organisms respond
Examples include:
Temperature, salinity, moisture, elevation, depth, nitrogen concentration of water, beach grain size etc etc etc
Environmental Gradient e.g. Temperature
Performance or Abundance
Species A
Species B
Species C
Species D
Species E
Species F
Species G
Species H
Species I
Species J

3. Resource
Something that is required or used by an organism, the quantities of which can be reduced by the organism
Examples include:
Dissolved oxygen, sunlight, water, carbon dioxide, mineral nutrients, organisms as food

4. Population
A group of individuals of the same species that coexist in space and/or time

5. Characteristic of intra-specific competition
Population size / density
Rate
Birth
Death K
Born
Population size / density
Numbers
Dying
Difference = NET Recruitment
N - Shaped K
S-Shaped Growth Curves
Characteristic of intra-specific competition

6. Community?
A group of interacting populations of different species that coexist in time/space

7. Inter-specific Interactions
Competition
Inter-specific Interactions
Species A
Species B +
Mutualism +
Commensalism - +
Predation -
Amensalism
Outcomes of interactions between two species

8. A group of interacting populations of different species that coexist in time/space
A community as viewed from a predator-prey perspective

9. Community subsets Guild
Communities can have very many interacting populations of different species and to study all of them requires a suite of expert taxonomists at the very least.
Community ecologists tend to get around this issue by studying subsets of the community
Community subsets
Guild
A number of other trophic based units also used

10. Taxocene

11. Dominant species – abundance or biomass
Easy to work with – common
Can work across Guilds as number of taxa lower
Quantitative errors reduced – common
Assumed to play an important role within community

12. Morpho-species

13. How big is a community? ANY SCALE
Broad patterns in terrestrial vegetation can be recognized at the global scale - BIOMES

15. How small is a community?

16. Determinants of Community Composition and Structure
Physiological Constraints
Historic Events
Evolutionary Processes
Regional Species Pool
Habitat Selection – Habitat Species Pool
Dispersal Ability – Geographic Species Pool
Inter-specific Interactions
Local Community
Rules – a species will only be present if:
It can disperse there
Conditions and resources allow it to survive
Predators and competitors etc don’t preclude it

17. Local Community

18. Regional Species Pool Evolutionary Processes
Local Community
Evolutionary Processes
Physiological constraints –
Warm water species will not survive variable, and sometimes low temperatures experienced
Historic Events –
Tectonic events have separated species that could coexist but which don’t
Niche model for Coelacanth
Regional Species Pool

19. Regional Species Pool Habitat selection – Habitat Species Pool
Dispersal Ability – Geographic Species Pool

20. Inter-Specific Interactions
Local Community

21. We assume Equilibrium Communities – spp abundance constant wrt time
Equilibrium or non-equilibrium communities?
We assume Equilibrium Communities – spp abundance constant wrt time
Remain locally stable* (in balance) within environmental range - habitat
Unstable B2
A1 within range X-Y B1
Stable communities show strong biological interactions, high levels of density dependence.
Community Attribute A2 A1 C X Y Z
Neutral
Range of Environments
* Time it takes to recover from disturbance; variability over time; persistence
Non-equilibrium
Equilibrium
Biotic coupling
Competition
Saturated
Resource limitation
Density dependence
Optimality
Few stochastic effects
Tight patterns
Biotic decoupling
Species independence
Unsaturated
Abiotic limitation
Density independence
Opportunism
Large stochastic effects
Loose patterns

22. ARTIFICIAL? Identifying or Delineating Communities
1 – physically defined communities
Assemblages of species found in a particular place or habitat
ARTIFICIAL?

23. Great Smoky Mountains Tennessee
2 – taxonomically defined communities
Identified by presence of one or more conspicuous species that dominate biomass and/or numbers, or which contribute importantly to the physical attributes of the community
Topographic distributions of the characteristic dominant tree species of the Great Smokey Mountains, Tennessee, on an idealized west-facing mountain and valley
BG, beech gap; CF, cove forest; F, Fraser fir forest; GB, grassy bald; H, hemlock forest; HB, heath bald; OCF, chestnut oak-chestnut forest; OCH, chestnut oak-chestnut heath; OH, oak-hickory; P, pine forest & heath; ROC, red-oak-chestnut forest; S, spruce forest; SF, spruce-fir forest; WOC, white oak-chestnut forest.
Great Smoky Mountains Tennessee
SUBJECTIVE?

24. 3 – statistically defined communities
Sets of species whose abundances are significantly correlated, positively or negatively, over space and/or time.
Look at numerical and specific composition of samples
Determine similarities between samples
Look for a pattern in the similarities between samples
And so identify communities OBJECTIVELY

25. 4 – interactively defined communities
Subsets of species in a particular place or habitat, whose interactions influence their abundance.
Only some, and perhaps none, of the species in a physically defined community may constitute an interactively defined community.
Hairston (1981: Ecology, 62: 65-72) noted that of the seven species of plethodontid salamander in his study (North Carolina, USA), only the two most common influenced each others abundances: the balance, while ecologically similar, remained unaffected by each others abundance.

26. THE END
Image acknowledgements –