BIOS 3010: Ecology Lecture 20: Community Structure & Predationhomepages.wmich.edu/~malcolm/BIOS3010-ecology/Lectures/L20-Bios... · Lecture 20: Community Structure & Predation: ...

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Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 1

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


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.”


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.

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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.).


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.


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).

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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.



•  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.



•  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.”

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Figure 21.1 (3rd ed.): Effect of rabbit grazing on sand-dune plant species richness.


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


Figure 21.3 (2nd ed.): Paine’s rocky shore community.

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Figure 21.3 (3rd ed.): Switching between planktonic and sediment waterfleas by roach.


Figure 21.4 (3rd ed.): Effect of powdery mildew on inter-specific competition between barley & wheat


Figure 21.5 (3rd ed.): Changes in community structure in a German pasture.

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Figure 21.6 (3rd ed.):

Theoretical effect of density independent population reductions on competitive exclusion.


Figure 21.7 (3rd ed.): Theoretical effect of frequency of population reduction on community diversity.


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.

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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 20 (3rd ed.)


Figure 16.17: Rates of disturbance and effects on species richness

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Figure 16.18:

Insect species richness and disturbance in a New Zealand stream.


Figure 16.21: Hypothetical competitive lottery among species A-E for community gaps

BIOS 3010: Ecology Lecture 20: Community Structure & Predationhomepages.wmich.edu/~malcolm/BIOS3010-ecology/Lectures/L20-Bios... · Lecture 20: Community Structure & Predation: ...

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