COMMUNITIES IV
ENERGY FLOW AS A FACTOR STRUCTURING COMMUNITIES
I. Stability of communities
A. Definitions - kinds of stability
1. Relative resistance - how hard it is to move the community from its
equilibrium state
2. Relative resilience - how quickly the altered system returns to its original state
B. Food chain complexity and stability
1. Hypothesis #1 the more possible ways that energy can flow through a food web,
the more stable the system will be (MacArthur)
2. Hypothesis #2 Too many paths and links in food webs lead to instability (May)
3. Hypothesis #3 Real food webs are shorter than random and also have less
omnivores (Pimm)
. Experimental tests of community stability
II. Effects of single species
A. Keystone species
1. Predation
2. Action on potentially dominant competitor
B. Key industry species
1. Herbivores
2. So abundant many predators depend on them for food
C. Herbivores with the potential to alter the primary producer level
1. Structurally
2. Biomass
3. Species composition
4. Productivity
III. Differences among natural communities
A. Terrestrial Communities
1. Forests
a. Primary producers are large, long lived, physically complex, slow growing
strong competitors
b. Herbivores are: much smaller than their "prey" (usually insects) shorter
lived than the plants have higher potential population growth rates
can decimate "prey"
c. Carnivores are: equal or slightly larger than prey have approximately equal
generation times can track prey numbers in many cases
2. Grasslands
a. Primary producers are small relative to herbivores, fast growing simple in
structure, able to recover from over grazing
b. Herbivores are large relative to their "prey" long lived and slow growing
relative to plants can over eat food under most conditions migrate
through feeding areas
c. Carnivores are the same size or smaller than prey have life history patterns
similar to their prey
B. Planktonic communities
a. Primary producers are small relative to prey, fast growing with
simple physical structure and life history patterns, have
little or no energy put into defense from predation
b. Herbivores are larger than prey, have life cycles somewhat longer
than prey but can decimate prey under suitable conditions tend
to be relatively unselective in prey choice
c. Carnivores tend to be
i. similar in size to prey
ii. very much larger than prey
IV Differences in food chain structures
A. Three level systems
autotroph standing crop is high
autotrophs expend energy on efficiency & defense
first order biophage density is low
small % of primary production is used by first order biophages
large % of primary production is used by first order saprophages
B. Four level systems
autotroph standing crop is low
autotrophs expend energy on reproduction
autotroph turnover is high
first order biophage energy is relatively high
large % of primary production is used by first order biophages
small % of primary production gets to first order saprophages
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