Wednesday, January 7, 2009

Chapter 53: Population Ecology

Population ecologists describe two general patterns of population growth as follows:

1. Exponential growth occurs whenever the reproductive rate is greater than zero. On a graph where population size is plotted against time, a plot of exponential growth rises quickly, forming a J- shaped curve.

2. Logistic growth occurs when limiting factors resrtic the size of the population to the carrying capacity of the habitat. In this case, the equation for reproductive rate is modified as follows:

Delta(N)/ Delta(t) = r N (K-N/K)

- K represents the carrying capacity. In logisitic growth, when the size of the population increases, its reproductive rate decreases until, at carrying capacity (that is, when N=K), the reproductive rate is zero and the population size stabilizes. A plot of logisitc growth forms an S- shaped, or sigmoid curve.

Populations cycles are fluctuations in population size in response to varying effects of limiting factors. For Example, since many li miting factors are density- dependent, they will have a great effect when the population size is large as compared to when the populatoin is small.
Exponential and logistic growth patterns are assocaited with two kinds of life- history strategies as follows:

1. An r-selected species exhibits growth (J-shaped curve). This type of reproductive strategy is characterized by opportunistic species, such as grasses and many insects, that quickly invade a habit, quickly reproduce, and then are small, mature quickly, and require little if any parental care.

2. A K- selected species is one whose population size remains relativly constant ( at the carrying capacity, K). Species of this type, such as humans, produce a small number of relatively large pffspring that require extensive parental care until they mature. Reproduction occurs repeadly during their lifetime.

Chapter 54: Community Ecology
Community ecology is connected with the interaction of populations. One form of interaction is interspecific competition (competition between different species). The following concepts describe the various ways in which competition is resolved:

1. The Competitive Exclusion Principle ( Gause's principle). When two species compete for exactly the same resources ( or occupy the same niche), one is likely to be more successful. As a result, one species outcomes the other, and eventually, the second species is eliminated.

2. Resource partitioning: some species coexist in spite of apparent competition for the same resource. Close stugy, however, reveals that they occupy slightly differnt niches. by pursuing slightly different resources or securing their resources in slightly different ways, individuals minimize competition and maximize sucess. Dividing up the resources in this manner is called resource partitioning.

Insolitus usually perches on shady branches.
Distichus perches on fence posts and other sunny surfaces.

3. Character displacement (niche shift): As a result of resource partitioning, certain characteristics may enable individuals to obtain resources in their partitions more successfully. Selection of these characteristics (or character) reduces competition with individuals in other partitions and leads to a divergence of features, or character displacement.

4. Realized niche: the niche that an organism occupies in the absence of competing species is its fundamental niche. When competitions are present, however, one or both species mat be able to coexist by occupying their realized niches, that part of their existence where niche overlap is absent, that is, where they do not compete for the same resources.

Predation: is another form of community integration. In a general sense, a predator is any animal that toally or partly consumes a plant or another animal. More specifically, predstors can be categorized as follows:

1. A true predator kills and eats another animal
2. A parasite: spends m0st (of all) of itd life living on another organism (the host), obtaining nourishment from the host by feeding on its tissues. Although the host may be weakened by the parasite, the host does not usually die unitl the parasite has completed at least one life cycle, through usually many moves.

3. A parasitoid: is an insect that lays its eggs on a host (usually aninsect or spider). After the eggs hatch, the larvae obtain nourishment by consuming the tissues of the host. Th ehost eventually dies, but not unitl the larvae compete their development and begin pupation.

4. A herbivore: is an animal that eats plants. Some herbavor, especially seed eaters (granivores), act like predators in that they totally consume the organism. Other animals, such as those that eat grasses (grazers) or leaves of other plants (browsers), may eat only part of the plant but many weaken it in the process.


Symbiosis: is a term applied to two species that live together in close contact during a portion (or all) of their lives. A description of three forms of symbiosis follows:
1. Mutualism: is a relationship in which both species benefit.
Review Questions:
1. A population's carrying capacity
a. can be accurately calculated using the logistic growth model.
b. generally remians constant over time.
c. increases as the per capita growth rate (r) decreases.
d. may change as environmetal conditions change.
e. can never be exceeded.
2. The competitive exclusion principle states that
a. two species cannot exists in the same habitat.
b. competition between two species awlways causes extiniction or emigration of one species.
c. competition in a population promotes survival of the bestadapted individuals.
d. two species with the exact same niche cannot coexist in a community.
e. species that compete usually coevolve.
3. A community's species diversity is
a. increased by frequent massive disturbance.
b. increased by stable conditions with no disturbance.
c. increased by moderate levels of disturbance.
d. increased when humans interene to eliminate disturbance.
e. increased by intensive disturbance by humans.
Answers: 1. d, 2. d, 3. c
Created by Alyssa Hamilton

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