Day 19 chapter 14 november 18th
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Population Ecology
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Day 19 chapter 14 november 18th
- 1. Day 19 Chapter 14 November 18th Dr. Amy B Hollingsworth The University of Akron Fall 2014
- 2. • Exam scores were released. Overall, they were WAY better than exam two, so YAY! • 65 questions, I threw out 9, total out of 56 questions. • Final Exam – Chapters 14, 15, 16 in CBT during finals day -
- 3. Chapter 14: Population Ecology Planet at capacity: patterns of population growth Lectures by Mark Manteuffel, St. Louis Community College
- 4. 14.1–14.6 Population ecology is the study of how populations interact with their environments.
- 5. 14.1 What is ecology?
- 7. 14.2 A population perspective is necessary in ecology.
- 9. 14.3 Populations can grow quickly for a while, but not forever. There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the Earth would soon be covered by the progeny of a single pair. —Charles Darwin, The Origin of Species
- 10. In stable populations, How many of the five million eggs that a female cod might lay over the course of her life will, on average, survive and grow to adulthood? Who leaves more surviving offspring, a pair of elephants or a pair of rabbits?
- 13. 14.4 A population’s growth is limited by its environment.
- 15. Density-dependent Factors The limitations on a population’s growth that are a consequence of population density This ceiling on growth is the carrying capacity, K, of the environment.
- 17. Density-independent Forces Factors that strike populations without regard for the size of the population Mostly weather-based
- 19. How many people can earth support? Why does the answer keep increasing?
- 21. 14.5 Some populations cycle between large and small.
- 24. Do lemmings jump off cliffs committing suicide when their populations get too big?
- 25. 14.6 “Maximum sustainable yield” is a useful but impossible-to-implement concept.
- 27. Almost all natural resource managers working for the U.S. government fail to do their job exactly as mandated. Why?
- 29. What We Often Do Not Know… Population carrying capacity Number of individuals alive Stability of carrying capacity from year to year Which individuals to harvest
- 31. 14.7–14.9 A life history is like a species summary.
- 32. 14.7 Life histories are shaped by natural selection.
Editor's Notes
- Lobster tastes delicious. Many people consider it one of the finest delicacies. It’s not surprising, then, that catching and selling lobsters is big business. Now imagine for a minute that you were placed in charge of the lobster industry. The 6000 lobster fisherman in Maine depend on your managing the lobster fishery so that not only can they catch and sell enough lobsters to survive this year, but there must also be enough lobsters left behind to ensure that there are lobsters available to catch in all of the years to come. How do you do it? You would be faced with some tough questions: How many lobsters should you allow each fisherman to take each day? Each year? Should you require that certain lobsters be thrown back? If so, should they be the biggest or the smallest? Does it matter whether they are males or females? Is it best to increase the population size or should it be maintained at current levels? But wait a minute, forget about these obviously complex questions and start with a seemingly simpler question: How many lobsters currently live in the waters off the coast of Maine? Is it even possible to have a clue as to what this number is? (Figure 14-1 Managing valuable resources) Welcome to ecology.
- These difficult questions are all part of ecology, a sub-discipline of biology defined as the study of the interactions between organisms and their environments. But don’t be fooled by the simple definition. Ecology encompasses a very large range of interactions and units of observation and is studied at different levels (Figure 14-2 From individuals to ecosystems). These include: Individuals: How do individual organisms respond biochemically, physiologically, and behaviorally to their environment? Populations: How do groups of interbreeding individuals change over time in terms of their growth rates, distributions, and genetic makeup? Communities: How do the populations of different species within a locale interact with each other? Ecosystems: At the highest level of organization within ecology, how do the living and non-living elements interact in a particular area, such as a forest, desert, or wetland?
- As we begin our study of population ecology, a subtle but critical shift in perspective is required. That is the shift from viewing the individual as the primary focus of our attention to the population—a group of organisms of the same species living in a particular geographic region. Most ecological processes cannot be observed or studied within an individual. Rather, they emerge when considering the entire group of individuals that regularly exchange genes in a particular locale.
- As an example of how a population rather than individual perspective is needed, consider the case of adaptations produced by natural selection. Genetic changes don’t actually occur within an individual. That is, no one giraffe ever evolved a larger neck. Instead, the genetic changes occurred within a population over time. As a consequence of differential reproductive success among the individuals of a population with different neck lengths, over time there come to be more individuals with longer necks. Birth rates, death rates, immigration and emigration rates, too, are features possessed not by individuals but by populations (Figure 14-3 A change in perspective).
- Think about those two questions, how many cod eggs and baby elephants will make it to adulthood? Just two. If a population is not growing, each individual is ultimately replacing itself with a new one. This is true for all species. Two of those cod eggs will make it. And two of those baby elephants will make it. (Two survive rather than one because both a male and female contributed to each. If each pair only produced one offspring, the population would get smaller and smaller.)
- What would happen if more than two eggs survived? If the number were more than one offspring per individual, the population would grow and grow and grow until the earth was covered with cod and elephants and every other species. But that hasn’t happened and it can’t. Let’s investigate why. Figure 14-4 part 1 Unchecked growth.
- With the same calculations, we could determine the population size for the next 10 or even 50 years—these numbers are plotted in Figure 14-4 part 2 (Unchecked growth). When a population grows at a rate that is proportional to its current size—in other words, the bigger the population, the faster it grows—the growth is called exponential growth. The graph reveals that very quickly the size of a population growing exponentially becomes astronomical. In 10 years, our population would have grown from 500 individuals to 2580. In 50 years, it would reach almost 4 million. In fact, after 80 years, the population would pass one billion individuals. It’s clear that exponential population growth ends badly.
- Life gets harder for individuals when it gets crowded. Whether you are an insect, a plant, a small mammal, or a human, difficulties arise in conjunction with increasing competition for limited resources.
- In particular, as population size increases, organisms experience: reduced food supplies due to competition diminished accessibility to places to live and breed due to competition increased incidence of parasites and disease which can spread more easily when their hosts live at higher density increased predation risk as predator populations grow in response to the increased availability of their prey and also as the more densely packed prey become more visible. Figure 14-5 Fighting over scarce resources.
- The limitations on a population’s growth that are a consequence of population density—the number of individuals within a given area—are called density-dependent factors, and they cause more than discomfort. With increased density, a population’s growth is reduced as limited resources slow it down. This ceiling on growth is the carrying capacity, K, of the environment. And as a population size approaches the carrying capacity of the environment, death rate increases; migration rates increase (as individuals seek more hospitable places to live); and a reduction in birth rates usually occurs, too, as low food supplies give rise to poor nutrition which, in turn, reduces fertility.
- When a population grows exponentially at first but its growth slows as the population size approaches the environment’s carrying capacity—an “S”-shaped growth curve—the population growth is called logistic growth. Logistic growth is a much better approximation of how populations grow in the real world than is exponential growth (Figure 14-6 Lack of resources limits growth).
- Density-independent forces can also knock a population down. These are factors that strike populations without regard for the size of the population, increasing the death rate or decreasing the number and rate of offspring that are produced. Density-independent forces are like “bad luck” limits to population growth. They are mostly weather-based, including calamities such as floods, earthquakes, fires, and lightning. They also include habitat destruction by other species, such as humans. The population hit may be at its carrying capacity or it may be in the initial stages of exponential growth. In either case, the density-independent force simply knocks down the population size. The population then resumes logistic growth.
- In an environment in which these “bad luck” events repeatedly occur, a population might never have time to grow as high as the carrying capacity. Instead, it might perpetually be in a state of exponential growth, with periodic massive mortality events. The population’s growth would appear as a series of jagged curves as seen in Figure 14-7 (Events can limit population growth).
- The growth of populations doesn’t always appear as a smooth “S”-shaped logistic growth curve. For some populations, particularly humans, the carrying capacity of an environment is not set in stone.
- Consider that in 1883, an acre of farmland produced an average of 11.5 bushels of corn. By 1933 this was up to 19.5 bushels per acre. And by 1992 it had increased to 95 bushels per acre! How did this happen? The development of several agricultural technologies—including the use of vigorous hybrid varieties of corn, rich fertilizers, crop rotation, and effective pest management—have all made it possible to produce more and more food from the same amount of land. This is just one example of how the carrying capacity of an environment can be increased (Figure 14-8 Efficient crop production). Of course, at this same time, the carrying capacity has likely been decreased for many other species trying to live in the same environment as humans and their crops.
- Nature is not as tidy as biologists might have you believe. Exponential and logistic growth equations, for instance, help us understand the concept of population growth. But real populations don’t always show such “textbook” growth patterns. Instead, they sometimes vary greatly in their rates and patterns of growth.
- Locust swarms of biblical proportions certainly attest to the unpredictability of population growth (Figure 14-9 Population explosion!). In northwest Africa, the desert locusts normally live as solitary individuals in relatively small, scattered populations. In 2004, though, the population size increased rapidly, probably due to unusually good rains and mild temperature. As the rainy season ended and green areas gradually shrunk, however, the locusts became progressively concentrated in smaller and smaller areas. At this point, for reasons that are related to their overcrowding but are not completely understood, the locusts began behaving like a mob rather than like solitary individuals. Giant swarms—some including tens of millions of insects—began flying across huge expanses of land in search of food. They completely consumed huge swaths of farmland, causing more than $100 million in damage.
- There is more than one way for populations to deviate from the standard population growth pattern. The explosive locust population growth just described occurs at unpredictable intervals. Another unusual pattern is the population oscillations of the lynx and snowshoe hare populations of Canada. As seen in Figure 14-10 (Predator and prey), rather than smooth logistic growth, the populations of both the snowshoe hare and their predators, the lynx, have regular cycles between very large numbers and crashes to much smaller numbers. Thanks to the Hudson Bay Company, which kept detailed records on the number of pelts it purchased from fur trappers, this population cycling is very well-documented. Although its cause is not fully certain, to some extent this predator and prey cause their own cycling: The hare population size grows, providing more food for the lynx, which then reproduce at a higher rate, causing them to eat too many of the hares, thereby reducing their food source, and causing the lynx population to crash, enabling the hare population to grow and the cycle to begin anew.
- One population-growth myth that demands debunking concerns lemmings and their supposed suicidal response to overcrowding. Do lemmings jump off cliffs, committing suicide when their population size becomes too large? In a word: No. This is an urban myth. Lemming populations do occasionally experience large increases in size. Then, just as locust behavior changes when population density gets too great, their behavior changes a bit and many individuals migrate away in search of less-crowded habitat and food. As they enter unfamiliar territory, they may suffer increased rates of death. But these deaths are not suicidal and do not occur in large groups (Figure 14-11 An urban myth debunked).
- Suppose that you are in charge of an eastern hardwood forest, working for a logging company, responsible for selecting which trees to cut down. Or, suppose you are in charge of the lobster fishery discussed at the beginning of this chapter, responsible for deciding how many lobsters should be harvested. In either case, your responsibility would be very similar. In the case of the hardwood forest, how many trees would you cut down in the forest for maximum wood yield? This is a trick question. The maximum yield on any given day is obtained by cutting everything down (also known as clear-cutting). No other strategy yields more right now. But such a harvest can be done only once, so it isn’t really a management strategy. A more sensible strategy would include harvesting some individuals and leaving others for the future. With that type of strategy, which trees or lobsters would you harvest so as to minimally affect the population’s growth? Ideally, you would harvest those that are post-reproductive. After all, they are no longer contributing to the population growth. But there may not be many post-reproductive individuals, or it may be too hard to identify them.
- What’s a better solution? For long-term management, it is best to harvest some but leave others still growing and reproducing for harvest at a later time. With such a strategy, the population can persist indefinitely. The special case, in which as many individuals as possible are removed from the population without impairing its growth rate, is called the Maximum Sustainable Yield (Figure 14-12 Tree harvest). The value in such a harvest comes from the fact that it can be carried out forever, clearly yielding more than the short-sighted strategy of a complete harvest. Your first step as the manager of a hardwood forest or lobster fishery is to determine the maximum sustainable yield for the resource. This is, in fact, straightforward. Maximum Sustainable Yield is calculated as that point at which the population is growing at its fastest rate. If we examine the logistic growth curve, we can see that the population is getting larger at the fastest rate when it is equal to half of the carrying capacity. At this midpoint, scarcity of mates is not a problem as it can be at low population levels, and competition is not a factor as it can be near the carrying capacity—one of the reasons why the population doesn’t grow at all when it is at its carrying capacity.
- Maximum sustainable yield is a useful concept, applicable not just to wood harvesting but also to livestock, agriculture, and nearly every other useful natural resource. And, in fact, there are 31 official U.S. agencies that, in their charters, are mandated to utilize the maximum sustainable yield concept in determining their harvest levels. But here’s the rub: This is nearly always an impossible task. Why?
- There are numerous reasons, most of which become apparent only when you leave the simplicity of the theoretician’s desk and head into the messy real world. For starters, if maximum sustainable yield occurs when a population is half its carrying capacity, do we first have to wait until the population stabilizes at its carrying capacity to determine what half of the carrying capacity was? But isn’t it inefficient to sit around waiting for the population to reach its carrying capacity when you want to maintain it at half that size? Or can you just estimate carrying capacity in order to calculate half of it? But that can be difficult since lobsters live underwater (Figure 14-13 How many lobsters can be harvested sustainably)? Put simply, we rarely know K.
- And the problem gets worse. For one thing, with many species, not only do we not know carrying capacity, we don’t even know the number of individuals alive. It is even difficult to accurately count humans, so imagine how hard it is for a species in which the individuals are underwater or fly or are microscopic. And if we were to solve the mysteries of counting individuals and knowing carrying capacity, we would still have to figure out whether carrying capacity is stable from year to year. If carrying capacity depends on levels of resources, it may be cyclic, for instance. And even with knowledge of carrying capacity and population size, we would not be certain which individuals ought to be harvested. Often the individuals in a population are not contributing equally to population growth. The post-reproductive individuals mentioned above, for example, do not contribute to population growth. As a natural resource manager you do your best, knowing that the theory behind the concept of maximum sustainable yield can almost never be put into practice perfectly.
- Harvesting natural resources for maximum efficiency turns out to generate insights into fighting biological pests such as cockroaches or termites. The problem is just turned on its head: Which pest animals would you concentrate on killing so as to most effectively slow population growth? The pests to kill are those at the age of maturity, with the highest reproductive value because they contribute most to the growth of the population. Similarly, because a population can still grow very rapidly with only a few males—because any one male can fertilize a large number of females—it is most effective to focus pest control on females.
- Think of life histories as strategies for (population) survival in particular environments.