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Chapter 7

BIO120H1 Chapter 7: Struggle for Existence Part VII


Department
Biology
Course Code
BIO120H1
Professor
Spencer Barret
Chapter
7

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Saturday, December 3, 2016
Struggle for Existence!
Part VII: Community ecology, primarily regarding animals
-Definition of ecological community: all of the organisms or the biotic entities in
some spatially defined locality!
Ecosystem would be including all of the abiotic components (ex. atmosphere,
weather, hydrology, mineral nutrients, etc.)!
-“Community study” is a subset of the species in a local ecosystem, separated
into two!
taxonomically-defined!
functionally-defined!
-Guild level: organisms with the same guild level have similar functional niches!
-Population ecology —> abundances and age structure !
-Community ecology —> number of species present (species richness) and the
abundance of each of them!
-Communities!
have a few species that are very common (the dominants)!
many species that are rare!
distribution of abundance and rarity determines species diversity!
-ex. community with ten equally abundant bird species is more diverse than one
with one common bird species and nine rare ones!
-Primary productivity!
functional biological property of communities!
ex. how much plants growth occurs, biochemical variables!
-Ecosystem properties!
includes abiotic components!
won’t be discussed in the course!
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Saturday, December 3, 2016
-Properties of community!
resilience in face of stress and disturbance!
stability of species composition!
changes that occur through time!
interactions that link species together (ex. predation, competition, mutualism)!
-especially interested in trophic interaction (who eats whom)!
-species exerting selective pressure on each other —> leads to coevolution!
-Two traditions of community ecology!
botany department!
-“big thing with slow dynamics”!
-starts from descriptive data!
zoology department!
-trend from extending logistic ecology from single-species models to multiple-
species models!
-difficult to track down animals —> studies are done in small contained (simplified
“communities”) under controlled condition !
-Logistic —> Lotka-Volterra competition model by adding a second braking term
that accounts for interspecific competition exerted by second species!
4 possible outcomes of the equation!
-K1 > K2/ α21 and K1/ α12 > K2: Species 1 outcompete Species 2 (species 2
goes extinct!
-K1 < K2/ α21 and K1/ α12 < K2: Species 2 outcompete Species 1!
- K1 < K2/ α21 and K1/ α21 > K2: two species in equilibrium, stably coexisting,
both are below their K values!
-K1 > K2/ α21 and K1/ α21 < K2: unstable competition, winner depends on
starting numbers (species with initial advantage will outcompete the other)!
-Condition for stable coexistence: each species inhibits its own population growth
more than it inhibits the growth of other species’ population!
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Saturday, December 3, 2016
-Experiment by Gause!
experimented with aquatic protozoan genus Paramecium!
done in labs, not real ponds!
a good description of competition (saw both stable coexistence and competitive
exclusion — often unstable)!
predator-prey tend to be unstable!
- Thomas Park’s experiment!
another example of “container” trials!
experimented with 2 congeneric species of flour beetles (stored-product pests)!
showed that competitive exclusion was the rule!
experimented under varying environmental condition (temperature, moisture, etc)!
result: one species tended to win!
showed the importance of abiotic condition in determining outcome of biotic
interactions!
showed that it’s unlikely/ difficult for two competing species to coexist —> the
Principle of Competitive Exclusion!
Principle of Competitive Exclusion: “two species that compete for the same
recourses can’t coexist for long”!
-Grinnell’s niche concept!
it’s unlikely for two species to occupy the same niche; if they’re doing the same
thing, one would be better than the other at it!
can’t really be considered as a law!
-Robert MacArthur (1957)!
conducted one of the most influential studies ever!
asked “how so many species of insectivorous warblers were able to coexist in their
summer breeding habitat of northern coniferous forests?”!
habitat doesn’t have a large diversity of tree species or food species —> no scope
of specializing into different niches!
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