Community Ecology

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Community Ecology BCB331
Mark J Gibbons, Room Z108, BCB Department,
UWC Tel [phone]. Email mgibbons_at_uwc.ac.za
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EQUILIBRIUM theories
An equilibrium theory (e.g competitive exclusion)
is a theory used to explain patterns in the
absence of temporal and spatial variability it
focuses attention on the properties of a system
at equilibrium
A non-equilibrium theory is concerned with
behaviour of system away from an equilibrium
point and focuses attention on temporal and
spatial variability e.g. intermediate
disturbance hypothesis.
Historically most theories of community
organisation have focused on equilibrium theories
but this has changed in recent time must be
realised, however, that BOTH are important
When we think of disturbance need not confine
thoughts to physical disturbance by the physical
environment but also biological disturbance
predation or parasitism
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Effects of Grazers
Observations of Charles Darwin
Grazer-mediated co-existence
Similar observations in other environments
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Response of community to grazers dependent upon
environment
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Effects of Carnivores
Paine (1966) American Naturalist 100 65-75
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4
2
2
1
3
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Total Number of Species 15
KEYSTONE SPECIES Carnivore mediated co-existence
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Primary space occupiers
KEYSTONE SPECIES Carnivore mediated co-existence
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KEYSTONE SPECIES Carnivore mediated co-existence
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A terrestrial Example
Kullberg Ekman (2000) Oikos 89 41-45
Small Exploitation
Larger Interference
Glaucidium passerinum
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BUT.
Spiller Schoener (1998) Ecology 79 503-516
Predation reduced diversity!
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Predators can increase diversity if they prey on
competitive dominants, but can decrease diversity
if they prey on fugitives
Response of community will vary with predator
behaviour prey selection
If predators display switching to most common
prey items the diversity can be maintained some
evidence BUT not universal. Some predators
can be maintained by most common prey item whilst
exterminating rarer species!
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Boiga irregularis
Savidge (1987) Ecology 68 660-668
Most of the 18 species of endemic birds reduced
in density following snakes introduction 7 now
extinct
By including lizards in its diet, snake
maintained high densities, whilst exterminating
more vulnerable birds
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Effects of predators an appraisal
Selective predation can increase diversity IF
preferred prey are competitively dominant AND if
environment is productive. However, can reduce
diversity if environment unstable or if preferred
prey are rare
An intermediate intensity of predation is most
likely to be associated with higher diversity
Role of predators may be least when physical
environment is severe or unpredictable as the
predators themselves may be absent e.g. exposed
vs sheltered rocky shores
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Multiple factors
Disequilibrium AND Equilibrium theories
Competition, Predation, Disturbance and
Recruitment (etc) can ALL influence community
organisation but to different degrees in
different communities and environments
Competition and predation may be more important
when recruitment levels are high or in more
stable environments
Wilbur (1987) Ecology 68 1437-1452
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Predators
Stable environment
Predator eats dominant species
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Unstable environment
Unstable environment causes increase in
competitive stress, slowing growth and delaying
time of metamorphosis
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Interactions are NOT just negative!
Facilitation also important..
..though again varies with environment
At upper distribution reduces rock temperature
and evaporative losses positive impact on
recruitment
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Determinants of Community Composition and
Structure
Physiological Constraints
Historic Events
Evolutionary Processes
Dont forget communities are not static and vary
in time and with chance!
Regional Species Pool

• Rules a species will only be present if
• It can disperse there
• Conditions and resources allow it to survive
• Predators and competitors etc dont preclude it

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Overview
Introduction Definitions
Defining a community Summarizing
characteristics Examining links
Inter-specific Interactions I Competition
Inter-specific Interactions II Predation
Community changes in space and time Succession Di
sturbance
Effect of Competition in structuring communities
Effect of Predation in structuring communities
Theory, Modeling and Field
Field Analytical
Theory Modeling
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Important aspects NOT covered!
Theoretical
Parasitism Mutualisms.. Co-evol
ution... Optimal foraging Supply
side ecology. Island biogeography. Food
webs and energy flow....
Analytical
Ordination techniques.
Field
Illustrative examples
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• Readings
• Although there are no prescribed books for this
  course, the following texts are recommended
  (especially those in bold-typeface) all are
  currently placed on short-loan at the UWC
  library.
• Begon, M., Harper, J.L. and Townsend, C.R.
  (1990). Ecology Individuals, Populations and
  Communities. Blackwell Scientific Publications,
  945pp.
• Begon, M. and Mortimer, M. (1986). Population
  Ecology A Unified Study of Animals and Plants.
  Blackwell Scientific Publications, 220pp.
• Krebs, C.J. (1999). Ecological Methodology.
  Benjamin Cummings, 620pp.
• Morin. P.J. (1999). Community Ecology. Blackwell
  Science, 424pp
• Zar, J.H. (1984) Biostatistical Analysis.
  Prentice-Hall

PDF DOCUMENTS
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THE END
Image acknowledgements http//www.google.com
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Need to bring in work of Menge et al in next
years classes