BIOS 3010: Ecology
Lecture 20: Community Structure & Predation:
•  Lecture summary:
–  Effects of grazing
herbivores.
–  Effects of predators.
–  Effects of parasites
& disease.
–  Variation in time.
–  Disturbance &
community patch
dynamics:
•  Dominance control.
•  Founder control.
Fig 7-10, Miller (2002) Essentials of Ecology
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 1
2. The effect of grazing herbivores:
•  Herbivory, predation, disease and parasitism,
competition, earthquakes, fire, rain, wind,
temperature etc. can all disturb communities.
•  Disturbance is:
–  any relatively discrete event in time that removes
organisms or otherwise disrupts the community by
influencing the availability of space or food resources,
or by changing the physical environment. A general
consequence is likely to be the opening up of space,
or freeing up of resources, that can be taken over by
new individuals.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 2
3. The effect of grazing herbivores:
•  Disturbance (continued):
–  For example, a predator, or herbivore, or lawn
mower, or a strong wave, or a strong wind can
open gaps in communities.
–  Thus grazing by rabbits can strongly influence the
structure of plant communities (Fig. 21.1).
–  Intermediate grazing promotes most diversity
through its influence on competition.
•  exploiter-mediated coexistence - Fig. 21.2.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 3
1
4. The effects of predators:
•  Removal of predatory starfish by Paine (1966):
–  Led to reduction in community species number from
15 to 8 (Fig. 21.3, 2nd ed.):
•  Because space made available by the predator helped
competitively subordinate species and increased species
diversity.
•  Exploiter-mediated coexistence.
•  Frequency-dependent effects can also influence
community structure.
–  e.g. switching according to prey density in fish
(Fig. 21.3, 3rd ed.).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 4
5. The effects of parasites and disease:
•  Exploiters can affect other interactions and facilitate
coexistence of species (Fig. 21.4).
–  Strongly negative effects on communities can also
occur for highly pathogenic invading diseases:
•  e.g. malaria and bird pox in Hawaii:
–  may have exterminated 50% of the endemic bird species.
•  Also destruction of chestnut and elm forests in North
America by introduced pathogens.
•  Effects are likely to be frequency dependent
–  Influenced by the frequency of encounter in high density
populations.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 5
6. Community structure and temporal
variation in conditions:
•  Conditions constantly change in space and time:
–  They have a marked impact on ecological processes and
hence on species composition of communities
(Fig. 21.5).
•  Thus periodic, density independent reductions in
population size may be expected to promote
coexistence of competing species (Fig. 21.6).
•  Frequency of disturbance also impacts diversity, as
does growth rate (rate of increase) of species
(Figs. 21.7 and 21.8).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 6
2
7. Disturbances and patch dynamics:
•  Includes the effects of movement among
patches of resource available for
colonization:
– Within patches are like Lotka-Volterra
interactions.
– But extinctions can be reversed or created
by immigration and emigration
•  Like metapopulations.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 7
8. Disturbances and patch dynamics:
•  Dominance-controlled communities:
–  Some species are competitively superior
•  Gaps lead to succession-like changes in species
composition.
•  Disturbance knocks the community back to an earlier
stage (Fig. 16.6 & Table 21.1):
–  From pioneer species (p) to mid-succession species (m) to
climax competitor species (c).
–  Diversity is also greatest at intermediate levels of
disturbance (Figs. 16.17 & 16.18).
•  The intermediate disturbance hypothesis.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 8
9. Disturbances and patch dynamics:
•  Founder-controlled communities:
–  All species are competitively similar.
•  Thus succession is not expected.
•  There is no competitive exclusion.
•  Presence in the community becomes a lottery
(Fig. 16.21).
–  Some tropical reef fish communities may be like this and
diversity may remain high.
–  Species may be competing for living space in which larvae
are the tickets, and the first arrival at the vacant space
wins the site, matures quickly and holds the space for
its lifetime.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 9
3
Figure 21.1 (3rd ed.): Effect of rabbit grazing
on sand-dune plant species richness.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 10
BIOS 3010: Ecology
Lecture 20: slide 11
Figure 21.2
(3rd ed.):
Effect of Littorina
littorea density on
species richness
(S) and species
diversity (Shannon
H) in (a) tide pools,
(b) emergent rocks
Dr. S. Malcolm
Figure 21.3 (2nd ed.): Paine s rocky
shore community.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 12
4
Figure 21.3 (3rd ed.): Switching between planktonic
and sediment waterfleas by roach.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 13
Figure 21.4 (3rd ed.): Effect of powdery mildew on
inter-specific competition between barley & wheat
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 14
Figure 21.5 (3rd ed.): Changes in community
structure in a German pasture.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 15
5
Figure 21.6
(3rd ed.):
Theoretical effect
of density
independent
population
reductions on
competitive
exclusion.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 16
Figure 21.7 (3rd ed.): Theoretical effect of frequency
of population reduction on community diversity.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 17
Figure 21.8 (3rd ed.): Huston s model communities: Predicted
relationships between diversity and (a) frequency of population
reduction, (b) population growth rates, (c) contour of both.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 18
6
Figure 16.6: Hypothetical mini-succession in a
gap by pioneer (pi), mid-successional (mi) and
climax (ci) species.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 19
BIOS 3010: Ecology
Lecture 20: slide 20
(3rd ed.)
Dr. S. Malcolm
Figure 16.17: Rates of disturbance
and effects on species richness
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 21
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Figure 16.18:
Insect species
richness and
disturbance in a
New Zealand
stream.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 22
Figure 16.21: Hypothetical competitive lottery
among species A-E for community gaps
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 20: slide 23
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