Island Evolution Readings Integrated Science 4 Name:...
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Island Evolution Readings 4/08 Integrated Science 4 Name: Per. The course of evolution is more than just natural selection. In nature random events and other factors play a significant role in determining the course of changes over time. The following readings explain some of these other factors. Both readings are from: Quamman, D. (1996). Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner, N.Y., N.Y. Reading 1: Why is Rarity Perilous? Why is rarity perilous? Why do small populations go extinct? The answer is simple in outline and complicated in its scientific details. For now, let's stick with the outline version. Small populations go extinct because (1) all populations fluctuate in size from time to time under the influence of two kinds of factors, which ecologists refer to as deterministic and stochastic; and (2) small populations, unlike big ones, stand a good chance of fluctuating to zero, since zero is not far away. What are the deterministic factors? Those involving straightforward cause-and- effect relations that to some extent can be predicted and controlled. Essentially, that means human activities: hunting, trapping, destroying habitat, introducing exotic animals that compete with or prey on native species, applying pesticides, swiping the eggs of birds or sea turtles, blockading the migrational routes of salmon or eels, and any other willful deed that either directly or indirectly exerts a negative influence. Because they are predictable, rational (well, arguably) and subject to control, we can eliminate such factors if we really care to, and in most cases, we can rectify the damage that has been done. What are the stochastic factors? Those that operate in a realm beyond human prediction and control, either because they are truly random or because they are linked to geophysical or biological causes so obscurely complex that they seem random. For practical purposes, we can consider them accidental. Weather patterns, for instance. A hard winter is an accidental factor. So is a drought. So is an ice storm in spring. So is a forest fire set by lightning. Each might cause a downward fluctuation in the population size of some species. If a hard winter were followed immediately by a drought, then by a fire, then by another hard winter, the downward fluctuation might be large. Other accidental factors? A hurricane or a typhoon might strike, battering habitat, interrupting courtship, wrecking nests. A population boom among some predator species might lead to a population bust among its prey. An epidemic disease, too, could reduce the size of a population. An infestation of parasites. A statistical aberration in the gender ratio of newborns, that is, too many males and not enough females. On the longer scale of time, an ice age would be an accidental factor. The population size of any animal or plant species fluctuates naturally, routinely, in response to this sort of accident. Often the fluctuations are small. Sometimes they are big. And occasionally they are numerically small but big in significance. Think of two species that live on the same tiny island. One is a mouse. Total population, ten thousand. The other is an owl. Total population, eighty. The owl is a fierce and proficient mouse eater. The mouse is timorous, fragile, easily victimized. But the mouse population as a collective entity enjoys the security of numbers. Say that a three-year drought hits the island of owls and mice, followed by a lightning- set fire, accidental events that are hurtful to both species. The mouse population drops to five thousand, the owl population to forty. At the height of the next breeding season a typhoon strikes, raking the treetops and killing an entire generation of unfledged owls. Then a year passes peacefully, during which the owl and the mouse populations both remain steady, with attrition from old age and individual mishaps roughly offset by new births. Next, the mouse suffers an epidemic disease, cutting its population to a thousand, fewer than at any other time within decades. This extreme slump even affects the owl, which begins starving for lack of prey. Weakened by hunger, the owl suffers its own epidemic from a murderous virus. Only fourteen birds survive. Just six of those fourteen owls are female, and three of the six are too old to breed. Then a young female owl chokes to death on a mouse. That leaves two fertile females. One of them loses her next clutch of eggs to a snake. The other nests successfully and manages to fledge four young, all four of which happen to be male. The owl population is now depressed to a point of acute vulnerability. Two breeding females, a few older females, a dozen males. Collectively they possess insufficient genetic diversity for adjusting to further troubles, and there is a high chance of inbreeding between mothers and sons. The inbreeding, when it occurs, tends to yield some genetic defects. Meanwhile, the mouse population is also depressed far below its original number. Ten years pass, with the owl population becoming progressively less healthy because of inbreeding. A few further females are hatched, precious additions to the gender balance, though some of them turn out to be congenitally infertile. During the same stretch of time, the mouse population rebounds vigorously. Good weather, plenty of food, no epidemics, genetically it's fine, and so the mouse quickly returns to its former abundance. Then another wildfire scorches the island, killing four adult owls and, oh, six thousand mice. The four dead owls were all breeding-age females, crucial to the beleaguered population. The six thousand mice were demographically less crucial. Among the owls there now remains only one female who is young and fertile. She develops ovarian cancer, a problem to which she's susceptible because of the history of inbreeding among her ancestors. She dies without issue. Very bad news for the owl species. Let's give the mouse another plague of woe, just to be fair: A respiratory infection, contagious and lethal, causes eight hundred fatalities. None of this is implausible. These things happen. The owl population, reduced to a dozen mopey males, several dowagers, no fertile females, is doomed to extinction. When the males and the dowagers die off, one by one, leaving no offspring, that's that. The mouse population fluctuates upward in response to the extinction of owls, a rude signal that life is easier in the absence of predation. Twelve thousand mice. Fifteen thousand. Twenty-thousand. But while its numbers are so high, it will probably overexploit its own resources and eventually decline again as a consequence of famine. Then rise again. Then decline again. Then ... The mouse population is a yo- yo on a long string. Despite all the accidental disasters, despite all the ups and downs, the mouse doesn't go extinct because the mouse is not rare. The owl goes extinct. Why? Because life is a gauntlet of uncertainties and the owl's population size, in the best of times, was too small to buffer it against the worst of times. • Read the above article and answer the questions below on a separate sheet. Use complete sentences. 1. Define deterministic factor . List 3 examples of deterministic factors that can lead to variation within populations. 2. Define stochastic factor . List 3 examples of stochastic factors that can lead to variation within populations. Examples of factors that can lead to evolution. 3. List 2 reasons from the article for why populations go extinct 4. In sequential order, 6 events that lead to the extinction of the owl population. 5. Mice were living in the same habitat as the owl. Explain why these species survived.
Transcript of Island Evolution Readings Integrated Science 4 Name:...
Island Evolution Readings 4/08 Integrated Science 4 Name: Per.
The course of evolution is more than just natural selection. In nature random events and other factors play a significant role in determining the course of changes over time. The following readings explain some of these other factors. Both readings are from: Quamman, D. (1996). Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner, N.Y., N.Y. Reading 1: Why is Rarity Perilous?
Why is rarity perilous? Why do small
populations go extinct? The answer is simple in outline and complicated in its scientific details. For now, let's stick with the outline version. Small populations go extinct because (1) all populations fluctuate in size from time to time under the influence of two kinds of factors, which ecologists refer to as deterministic and stochastic; and (2) small populations, unlike big ones, stand a good chance of fluctuating to zero, since zero is not far away.
What are the deterministic factors? Those involving straightforward cause-and-effect relations that to some extent can be predicted and controlled. Essentially, that means human activities: hunting, trapping, destroying habitat, introducing exotic animals that compete with or prey on native species, applying pesticides, swiping the eggs of birds or sea turtles, blockading the migrational routes of salmon or eels, and any other willful deed that either directly or indirectly exerts a negative influence. Because they are predictable, rational (well, arguably) and subject to control, we can eliminate such factors if we really care to, and in most cases, we can rectify the damage that has been done.
What are the stochastic factors? Those that operate in a realm beyond human prediction and control, either because they are truly random or because they are linked to geophysical or biological causes so obscurely complex that they seem random. For practical purposes, we can consider them accidental. Weather patterns, for instance. A hard winter is an accidental factor. So is a drought. So is an ice storm in spring. So is a forest fire set by lightning. Each might cause a downward fluctuation in the population size of some species. If a hard winter were followed immediately by a drought, then by a fire, then by another hard winter, the downward fluctuation might be large. Other accidental factors? A hurricane or a typhoon might strike, battering habitat, interrupting courtship, wrecking nests. A population boom among some predator species might lead to a population bust among its prey. An epidemic disease, too, could reduce the size of a population. An infestation of parasites. A statistical aberration in the gender ratio of
newborns, that is, too many males and not enough females. On the longer scale of time, an ice age would be an accidental factor. The population size of any animal or plant species fluctuates naturally, routinely, in response to this sort of accident.
Often the fluctuations are small. Sometimes they are big. And occasionally they are numerically small but big in significance. Think of two species that live on the same tiny island. One is a mouse. Total population, ten thousand. The other is an owl. Total population, eighty. The owl is a fierce and proficient mouse eater. The mouse is timorous, fragile, easily victimized. But the mouse population as a collective entity enjoys the security of numbers.
Say that a three-year drought hits the island of owls and mice, followed by a lightning-set fire, accidental events that are hurtful to both species. The mouse population drops to five thousand, the owl population to forty. At the height of the next breeding season a typhoon strikes, raking the treetops and killing an entire generation of unfledged owls. Then a year passes peacefully, during which the owl and the mouse populations both remain steady, with attrition from old age and individual mishaps roughly offset by new births. Next, the mouse suffers an epidemic disease, cutting its population to a thousand, fewer than at any other time within decades. This extreme slump even affects the owl, which begins starving for lack of prey.
Weakened by hunger, the owl suffers its own epidemic from a murderous virus. Only fourteen birds survive. Just six of those fourteen owls are female, and three of the six are too old to breed. Then a young female owl chokes to death on a mouse. That leaves two fertile females. One of them loses her next clutch of eggs to a snake. The other nests successfully and manages to fledge four young, all four of which happen to be male. The owl population is now depressed to a point of acute vulnerability. Two breeding females, a few older females, a dozen males. Collectively they possess insufficient genetic diversity for adjusting to further troubles, and there is a high chance of inbreeding between mothers and sons. The inbreeding, when it occurs, tends to yield some genetic defects. Meanwhile, the mouse population is also depressed far below its original number.
Ten years pass, with the owl population becoming progressively less healthy because of
inbreeding. A few further females are hatched, precious additions to the gender balance, though some of them turn out to be congenitally infertile. During the same stretch of time, the mouse population rebounds vigorously. Good weather, plenty of food, no epidemics, genetically it's fine, and so the mouse quickly returns to its former abundance.
Then another wildfire scorches the island, killing four adult owls and, oh, six thousand mice. The four dead owls were all breeding-age females, crucial to the beleaguered population. The six thousand mice were demographically less crucial. Among the owls there now remains only one female who is young and fertile. She develops ovarian cancer, a problem to which she's susceptible because of the history of inbreeding among her ancestors. She dies without issue. Very bad news for the owl species. Let's give the mouse another plague of woe, just to be fair: A respiratory infection, contagious and lethal, causes eight hundred fatalities. None of this is implausible. These things happen. The owl population, reduced to a dozen mopey males, several dowagers, no fertile females, is doomed to extinction. When the males and the dowagers die off, one by one, leaving no offspring, that's that. The mouse population fluctuates upward in response to the extinction of owls, a rude signal that life is easier in the absence of predation. Twelve thousand mice. Fifteen thousand. Twenty-thousand. But while its numbers are so high, it will probably overexploit its own resources and eventually decline again as a consequence of famine. Then rise again. Then decline again. Then ...
The mouse population is a yo-yo on a long string. Despite all the accidental disasters, despite all the ups and downs, the mouse doesn't go extinct because the mouse is not rare. The owl goes extinct. Why? Because life is a gauntlet of uncertainties and the owl's population size, in the best of times, was too small to buffer it against the worst of times.
• Read the above article and answer the questions below on a separate sheet. Use complete sentences. 1. Define deterministic factor. List 3 examples of deterministic factors that can lead to variation within
populations. 2. Define stochastic factor. List 3 examples of stochastic factors that can lead to variation within
populations. Examples of factors that can lead to evolution. 3. List 2 reasons from the article for why populations go extinct 4. In sequential order, 6 events that lead to the extinction of the owl population. 5. Mice were living in the same habitat as the owl. Explain why these species survived.
Reading 2: Theory of Island Biogeography • Read the above article and answer the questions below on your separate sheet. Use complete sentences. 1. How are species lost from an island? 2. How are species gained on an island? 3. What is the equilibrium theory of island biogeography and why is it useful? 4. What is the area effect? 5. What is the distance effect?
