Ecology

Geology/Geography/Oceanography/Atmospheric Sciences

Quiz 22 :

Geographic Ecology

Quiz 22 :

Geographic Ecology

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Now, suppose you are going to study the bird communities on the islands shown on the below, which lie equal distances from the mainland but differ in area. According to the equilibrium model of island biogeography, what should be the relative rates of immigration to the two islands? On which islands should rates of extinction be lowest? Explain. img
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The equilibrium model of the island biogeography states that the number of species on islands or the species richness is determined by a dynamic balance between immigration and extinction of species.
The rate of immigration and extinction has been found to be influenced by number of factors such as the number of species on the island, extent of isolation of the island from the source land, and area of the island.
The ecologists have shown that rate of immigration increases with decrease in isolation.
The rate of extinction has been shown to increase with decrease in area of the island.
Hence, large islands near to the source land have been found to have the highest species richness. The small far islands have been found to have the lowest species richness.
The islands those are far and large along with islands that are near and small showed intermediate number of species.
The islands to be studied are illustrated below:
Island 1 and island 2 are stated to lie at equal distances from the mainland. The islands differ in the area with island 1 larger than island 2. As per the equilibrium model of island biogeography, island 1 and 2 should experience approximately equal rate of immigration of birds due to similar extent of isolation. However, other ecologists have suggested that area of island can influence rate of immigration.
As per the equilibrium model of island biogeography, the rate of extinction is more in smaller islands compared to larger islands. Hence, island 2 should show high rate of extinction and island 1 should show lowest rate of extinction.
However, other ecologists have suggested that high rate of immigration reduces the rate of extinction. Hence, the distance of the islands from the mainland may also govern the rate of extinction on both the islands.
Hence, the equilibrium model of island biogeography is being extended by ecologists to account for the other environmental factors affecting species richness.

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Diamond's estimates (1969) of numbers of species immigrating and numbers that became extinct (six versus five) were virtually identical. Is this near equality in numbers of extinction and immigration consistent with the equilibrium model of island biogeography? Explain. img
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The equilibrium model of island biogeography states that the species richness on islands is governed by the dynamic interplay of immigration and extinction.
As per the model, the rate of immigration and the rate of extinction are dependent on many factors such as the number of species on the island, the area of island, and the isolation of island.
As the number of species goes on increasing on the island, the rate of immigration from the source land to the island has been found to decrease.
As the number of species goes on increasing on the island, the rate of extinction of species has been found to increase.
The rate of immigration and the rate of extinction can become equal leading to equilibrium. This leads to a constant number of species on the island. However, this equilibrium is found to be dynamic as there is a constant change in the composition of species which is called species turnover.
The model also explains the variation in number of species on different islands depending on the area of island and the isolation.
A larger island closely situated to the source island has been shown to harbor high number of species at equilibrium. A smaller island situated far from the source island harbors a comparatively lower number of species at equilibrium.
The islands assessed by Diamond might be near equilibrium. Hence, the rate of extinction and immigration observed might have been approximately equal. Hence, his results are consistent with the equilibrium model of island biogeography.

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The following data (Preston 1962a) give the area and number of bird species on islands in the West Indies: img The numbers are expressed in two ways: as simple measurements and counts and as the logarithms of area and numbers of species. Use these data to plot your own species-area relationship. Plot area on the horizontal axis and number of species on the vertical axis. First plot the simple measurements of area and species number on one graph, and then plot the logarithms of area and species number on another graph. Which gives you the tightest relationship between area and species richness?
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The plot of the logarithms of area and species number should yield the tightest relationship between area and species richness.

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Explain how speciation and extinction rates might be affected by the area of continents. What evidence is there to support your explanation? What does the influence of area on rates of extinction and speciation have to do with higher species richness in tropical regions compared to temperate and highlatitude regions?
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Ricklefs (1987) pointed out that many large-scale contrasts in species richness and composition cannot be explained by local processes such as competition and predation. Ricklefs proposed that differences in history and geography can leave a unique stamp on regional biotas. The mammals of Australia, including kangaroos, koalas, and duck-billed platypuses, are one of the best-known examples of a unique biota. How have history and geography, as opposed to local processes, combined to produce this unique assemblage of mammals?
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We discussed how Diamond (1969) documented immigrations and extinctions on the California Channel Islands by comparing his censuses of the birds of the islands with the birds recorded over 50 years earlier. Disregarding the numbers for San Miguel and Santa Rosa Islands, which were not well censused in 1917, Diamond showed that an average of approximately six bird species became extinct on California Channel Islands between 1917 and 1968. During the same period, an average of approximately five new bird species immigrated to the islands. Diamond suggested that his estimates of immigration and extinction were likely underestimates of the actual rates. Explain why his comparative study produced underestimates of rates of immigration and extinction.
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Most examples of regional and latitudinal variation in species richness cited in chapter 22 have been terrestrial. Consider regional variation in marine biotas. Like birds on land, fish are one of the best-studied groups of marine organisms. Moyle and Cech (1982) cite the following patterns of fish species richness: img As you can see, fish species richness decreases northward on both coasts. However, the Pacific coast generally supports a larger number of species. This contrast may be another situation requiring historical- and geographic-level explanations. Explore and explain this contrast in species richness using information from the fields of marine biology, oceanography, and ichthyology. Moyle and Cech (1982) and Briggs (1974) are good starting points.
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Refer to figure 22.5, which MacArthur and Wilson (1963) used to show how isolation affects species richness on islands. Find a detailed map of the Pacific Ocean and locate New Guinea. Next locate as many of the "near," "intermediate," and "far" islands on the map as you can. This will give you a better sense of the distances represented by the islands. How do the numbers of species on near, intermediate, and far islands support the hypothesis that island isolation tends to reduce species richness?
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How might whole-earth scale, global ecology be affected by sample size considerations?
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virtually identical. Is this near equality in numbers of extinction and immigration consistent with the equilibrium model of island biogeography? Explain.
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Review the major hypotheses proposed to explain the higher species richness of tropical regions compared to temperate and high-latitude regions. How are each of these hypotheses related to relative rates of speciation and extinction in tropical regions and temperate and high-latitude regions?
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