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Flashcards in Exam 7 Deck (38)
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Group of interacting species
All species interact in some way with other species


Examples of community interactions (4)

Interspecific competition


Interspecific competition

Competition between individuals of different species



An organism's role in an ecosystem (profession)
What it eats, where it lives, who eats it, what it needs to survive, who competes with it


Competition coefficient

Indicates strength of effect on the other species

If a=1, the species have no net effect on each other

When a12 is < 1 the effect of species 2 on species 1 is less than the effect of species 1 on its own members.

Conversely, when a12is > 1 the effect of species 2 on species 1 is greater than the effect of species 1 on its own members.


Ex: alpha1,2

Effect of species 2 on species 1


Stable equilibrium

K1 and K2 are lower than K1/a12 and K2/a21
When INTRAspecific competition is stronger than interspecific competition
2 species can coexist because there is little overlap between needs of the 2 species


Unstable equilibrium

INTERspecific competition greater than intraspecific competition
One of the 2 species is likely to drive the other out, but it is random who wins


Which population fluctuates in size as a result of changes in carrying capacity?

Predator, because prey is a resources for the predators


Coevolution of predator and prey leads to

Adaptations to capture prey
Adaptations to avoid being detected/eaten


3 types of adaptations to capture prey

Mechanical adaptations
Chemical adaptations (venom)
Camouflage of predator


5 types of adaptations to avoid being eaten/detected

Mechanical defenses
Chemical defenses (toxins)
Synchronized reproduction



Harmless animal resembles danger


Synchronized reproduction

Satiates predators before all offspring are consumed
Ex: cicada



2 species live in close association with each other


3 forms of symbiosis




One species benefits to detriment of the other
Host species evolve mechanisms to minimize effects of parasites (cysts/galls, immune response)
Parasites have greatest impact on host population when newly introduced (no prior coevolution)
Parasites evolve to manipulate host for improved transmission



One species benefits and other is unaffected
Ex: barnacles on whales



Both species benefit
Cooperation or reciprocal exploitation?
Ex: microrrhizae: fungus-plant mutualism
Ex: Plant-insect mutualisms (defense-ants and acacia trees; pollination by bees)


Why aren't all species engaged in mutualistic relationships?

Costs may be prohibitive (small benefits don't outweigh costs)
Conflict of interest between participants generates selective advantage to "cheaters"


Community structure

Interactions within community are often diffuse, involving many species (competition/predation between any 2 species may be weak, even if it has a strong effect overall on the community)
Interactions are often indirect, propagating throughout the community (keystone species)


Keystone species

Have significant influence on community structure
Ex: removing a predator species can reduce diversity by increasing competition among prey


2 types of community structure control

Top-down control
Bottom-up control
Hard to test if populations are more under top-down or bottom-up control


Top-down control

Through the effects of predators


Bottom-up control

Through availability of resources to higher levels


‘No difference in rates of multiplication can be so slight as to negate the exclusion principle.’ What does this mean?

No 2 processes in the real world are exactly equal. There is always going to be some difference, even if small, in reproduction rates between species. But even if the difference is very small, eventually one species will overtake the other given enough time, according to the exclusion principle. There is no small enough difference to make the principle false.

No matter how small the difference between the two rates (so long as there is a difference) such will be the outcome.


According to the paper, why is it difficult to test the competitive exclusion principle? In what way do the interpretations become circular?

Suppose you believe the principle is true and set out to prove it empirically. First you find two noninterbreeding species that seem to have the same ecological characteristics. You bring them together in the same geographic location and await developments. What happens? -Either one species extinguishes the other or they coexist. If the formerS you say, "The principle is proved.v But if the species continue to coexist indefinitely, do you conclude the principle is false? Not at all. You decide there must have been some subtle difference in the ecology of the species that escaped you at firstS so you look at the species again to try to see how they differ ecologically, all the while retaining your belief in the exclusion principle. There is a danger of a circular process here


David Lack wrote a ‘little classic’ called 'Darwin’s Finches.' How did ‘Gause’s contention’ change the way Lack interpreted the differences in beak shape among these birds?

Two species with similar ecology cannot live in the same region (Gause, 1934). This is a simple consequence of natural selection. If two species of birds occur together in the same habitat in the same region, eat the same types of food and have the same other ecological requirements, then they should compete with each other, and since the chance of their being equally well adapted is negligible, one of them should eliminate the other completely. Nevertheless, three species of ground finch live together in the same habitat on the same Galapagos islands, and this also applies to two species of insectivorous tree-finch. There must be some factor which prevents these species from effectively competing.


The paragraph that begins ‘Economics’ on page 1296 explains the application of the competitive exclusion principle to aspects of human economic behavior. How is the example described in this paragraph analogous to that of Darwin’s finches?



Under what circumstances do the equations predict that two species can co-exist?

When both species' carrying capacities are lower than the other's carrying capacity divided by the competition coefficient