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Ecology, 9th edition by Sher and Molles Answers to End-of-Chapter Review Questions Chapter 1 1. The advantage of dividing the field of ecology into subdisciplines, each focusing on a single level of organization is that such concentration and simplification tends to make the hypotheses addressed more likely to be testable. 2. The pitfalls of subdividing nature is that important ecological relationships may go unrecognized. Figure 1.1 misrepresents nature by converting the complexity of nature into an abstract simplification and by picturing the natural world as a vertically nested hierarchy, when in fact natural relationships and influences are more like a tangled web. 3. In the case of MacArthur’s warblers, you might compare the use of foraging zones by warbler species in areas with the full complement of warbler species with the use of foraging zones where one or more species is missing from the community. One way to test the role of competition in excluding some American redstarts from productive feeding areas would be to capture and remove the, presumably, dominant individuals from those areas to see if other redstarts, especially females and young males would move in to replace the individuals removed. 4. New technology allows us to see things we were not able to observe before, and ask questions that we were not previously able to address. For example, drones can carry cameras and other sensors to locations that are not easily accessible. Stable isotope analyses allow us to trace water and nutrients through ecosystems. Genetic analyses allow us to identify gut contents. Sound equipment allows us to hear things out of the range of human hearing. Citizen science initiatives increase the number of eyes and ears collecting evidence. Improved computing allows us to analyze larger data sets and run complex models to more accurately predict future events. 5. The species composition of these forests has changed substantially and their composition is likely to change in the future in response to climatic change. 6. There are trade-offs when comparing simple and complex models. While complex models include more data/variables and potentially provide greater accuracy, they may also incorporate more assumptions and therefore have a greater potential for error if any of those assumptions are invalid. Chapter 2 1. Use biome discussions as a guide. 2. Use figure 2.8 and the associated discussion as a guide. 3. Use figure 2.4 and associated discussion as a guide. High precipitation in the tropics is produced by high rates of evaporation and subsequent condensation of water vapor in ascending air masses. The clouds formed in this way produce the heavy precipitation associated with the tropics. High precipitation at temperate latitudes is produced when warm moisture bearing subtropical air meets cold polar air, which forces condensation of the water vapor in the subtropical air mass. Many tropical environments experience drought during periods of El Niño (see chapter 23). 4. The seasonal rainfall of these biomes is produced by shifts in the latitude at which the sun is directly overhead. Figure 2.4a, which shows the sun directly overhead at the equator, which Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

generates storms through the processes of evaporation, condensation and precipitation of water, is really a snapshot of a dynamic system. The latitude at which the sun is directly overhead, which is sometimes called the "solar equator", and acting as a generator of tropical storms shifts between 23.5° N and 23.5° S. The wet season comes during the warmer time of the year when the sun is more nearly overhead. The dry season comes when the solar input is less direct (sun's angle is more oblique). 5. Anywhere that the presence of mountains which can force precipitation in rising air masses and form a "rain shadow" in the leeward (downwind) side. The height of mountains will generally make a difference in the rain shadow effect, since higher mountains will induce more precipitation in rising air masses. 6. One of the greatest differences is the presence of permanent snow and ice on mountains at high latitudes. 7. One of the main differences between midlatitude and tropical alpine zones is that on tropical mountains daily variation in temperature is nearly as great as seasonal variation. Tropical alpine ecosystems lack fht every strong seasonality in input of solar radiation and temperature that occurs in midlatitude alpine zones. 8. In tropical climates the dominant seasonal change is from wet season to dry season. At very high latitudes it is cold much of the year with a brief cool growing season. In some tropical countries, such as Costa Rica, people have divided the seasonal changes significantly than has been done customarily in temperate climates. In the middle of the wet season, for instance, there is a period of reduced rainfall which Costa Ricans call veranillo, which means "little summer". The point here is that the seasonal terms associated with midlatitudes do not fit the annual variations that occur in other parts of the earth. It would be interesting to catalog the traditional terms for seasonal changes used by native peoples living in different climatic zones. 9. There has been much greater connection and exchange across the tundra and boreal forests which form nearly continuous bands in the northern hemisphere. In contrast, the temperate woodland tropical rain forest biomes are highly fragmented with few opportunities for biotic exchange over long periods of evolutionary history. 10. The most heavily impacted biomes have been temperate forest, temperate grassland, desert, and temperate woodland. Areas of tropical dry forest have also been fairly heavily impacted. Tropical rain forest, boreal forest, and tundra have received less impact. Satellite imagery could be used to assess the density of human-induced change on landscapes. The very high rates of human population growth in tropical countries and increasing resource extraction in the northern biomes will likely increase human impact in these biomes.

Chapter 3 1. Use Figure 3.2 as a guide for your review. The major sources of freshwater are groundwater, lakes, and rivers. A large amount of freshwater is also present as ice. Water shortages already affect large regions of the world and groundwater levels are dropping in many regions. Even where water remains plentiful, pollution is reducing water quality. 2. The proportion of the ecosystem receiving sufficient light to support photosynthesis is 200 m divided by 4000 m, about 0.05 or 5% of the total. 3. Major challenges faced by organisms moving from an aquatic to a terrestrial environment included lower support in the less dense aerial medium, greater temperature variation on

land, and desiccating water loss to air. Some advantages were less resistance to movement through air, lower rate of heat loss to air, and higher oxygen concentrations. 4. Refer to figure 3.13 as you develop your answer. Darwin's developmental sequence was tested by drilling on atolls, which revealed deep accumulations of ancient coral reefs deposited on sunken volcanic mountains. 5. The main factor here is that, though urchins feed on young corals, they also suppress the growth of algae, which reduces algal competition with the young corals. Thus the net effect of urchins on coral establishment is positive. 6. The wide environmental fluctuations of the intertidal environment has probably selected for greater physiological tolerance among intertidal species. Because of differences in the intensity of selection, tolerance of variation in salinity should be higher in the upper intertidal zone compared to the lower intertidal and subtidal zones. 7. Because small grain size reduces rates of water flow, oxygen concentration is generally lower in the interstitial waters of fine sediments compared to coarse sediments. Oxygen concentrations should be lower in tide pools in sheltered locations because of wave-related circulation. 8. The most direct way to test these predictions would be to study the relative contributions of litterfall from streamside forests and photosynthesis within the stream to the total energy budget of the stream. 9. Two approaches have been used to study the influence of nutrient availability on lake primary production. The first has involved measuring nutrient availability and rates of primary production in a large number of lakes and looking for a correlation between these two variables. The second approach has been to increase the nutrient availability in lakes and measure primary production responses (see Chapter 18). 10. It is clear that that the introduction of exotic species has greatly altered the structure and functioning of the Great Lakes ecosystem. However, the Great Lakes of Africa have also been impacted by species’ introductions. See Chapter 17 online, which includes a discussion of the effects of introducing Nile perch on the rich fish fauna of Lake Victoria. Chapter 4 1. Darwin mainly conducted comparative studies of natural populations, while Mendel conducted numerous experimental studies in his garden. In addition, Mendel used mathematics extensively in his work, while Darwin did not. Darwin proposed potential mechanisms for evolutionary change in species. Mendel discovered mechanisms of inheritance. The studies of Darwin and Mendel prepared the way for the later studies reviewed in this chapter by providing a mechanistic theory to explain evolution in organisms and the genetic transmission of characteristics between generations. 2. Genetic variation within and between populations is required for evolutionary change. On the surface, it is difficult to know if the phenotypic variation observed within and between populations is due to genetic differences and local adaptation, or phenotypic plasticity (identical genotypes producing different phenotypes when exposed to different environmental conditions). Common garden experiments can be used to help distinguish between these two sources of phenotypic variation. The observation of distinct differences between individuals from the same source population grown in different environments suggests phenotypic plasticity; while differences between individuals from different source populations grown in the same environment supports true genetic variation and local

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adaptation. An experiment showing consistent expression of a trait regardless of environment would support genetic variation and local adaptation rather than plasticity. The Hardy-Weinberg principle states that in populations mating at random in the absence of evolutionary forces, allele frequencies will remain constant. Hardy-Weinberg equilibrium is the condition under which allele frequencies do not change as a result of the conditions assumed by the Hardy-Weinberg principle. The conditions required for Hardy-Weinberg equilibrium are random mating, no mutations, large population size, no immigration, and equal fitness for all genotypes within the population. In the Hardy-Weinberg equation, p2, q2, and 2pq represent genotype frequencies. How these genotype frequencies translate into phenotype frequencies depends on the inheritance system. Phenotype and genotype frequencies are not always the same. Genetic drift is change in gene or allele frequencies in a population as a result of random processes. Genetic drift is most common in small populations and least likely in large populations. Genetic drift decreases genetic variation in populations, since it can eliminate alleles from populations. In your breeding program, you should create a breeding population that incorporates as much of the genetic variation within the remaining population as possible. Random mate selection with respect beak size in the population of G. fortis at El Garrapatero would tend to mask the influence of disruptive selection and even out the distribution of individuals across the range of beak size in figure 4.13. Marlene Zuk, Robin Tinghitella, and their colleagues demonstrated rapid evolutionary adaptation to introduced parasitic flies when they discovered that non-singing males, which do not attract the parasitic flies, increased in frequency in the population following the introduction of the parasitic flies. Genetic drift reduced the genetic diversity of field crickets on the islands compared to mainland populaitons and, coupled with founder effect, may have increased the frequency of non-singing males in island populations. This set the stage for rapid increase of non-singing males following introduction of the parasitic flies. Classical approaches to genetic studies, such as common garden experiments have the potential to reveal the significance or function of genetic differences among populations. Meanwhile modern molecular techniques, such as DNA sequencing, reveal genetic differences or variation directly. The classical approaches require little in the way of technology. However, they are labor-intensive and require a great deal of time. Modern methods often require costly equipment but they are less labor-intensive and can yield results in a shorter time.

Chapter 5 1. On Southern mountains, boreal plants and animals should be most prevalent at high elevations and on north-facing slopes, which are cooler than south-facing slopes. 2. During the warmest parts of the year, the beetle would likely avoid spending long periods of time on bare ground (see figure 5.3) in full sun. It could keep its temperature at 35°C by moving between sun and shade. During the hottest time of the day it might seek the shelter of a burrow. During the cooler times of the year the beetle would likely seek out full sun and be active during the warmest time of the day. 3. Although sharks and tuna are both fish, they are only distantly related. Their lineages diverged approximately 450 mya, and they did not inherit RM endothermy from a common ancestor. Endothermy is especially costly in aquatic environments due to the heat absorption

and conductive capacity of water (significantly greater specific heat, and latent heat of vaporization in comparison to air). In order for this trait to be the result of natural selection (multiple times), the energetic benefits of endothermy in these animals must outweigh the costs. Endothermic fish are able to decrease the energetic costs through circulatory countercurrent exchange mechanisms. Additionally, they have been observed to swim faster and have generally larger ranges than comparably sized exothermic fish. This gives these large predatory fish significantly improved hunting efficiency and enough of a fitness benefit to have resulted in the convergent evolution of RM endothermy in sharks and tuna. 4. Because of declining enzyme function, swimming speed should decrease above 20°C. 5. These results leave open the possibility that reduced egg hatching at higher temperatures has interacted with other factors, such as competition with other snail species, rates of predation, or mortality due to disease, to produce local extinctions. We need to consider birth rates, death rates, and the balance between them. 6. Basking should be most prevalent among butterflies in cooler climates at higher latitudes. Butterflies should also be more likely to be found basking in early morning than in the middle of the day. 7. A major disadvantage is greater vulnerability to predators when in torpor and slower behavioral response to environmental fluctuations generally. 8. Plants use orientation of their surfaces to increase or decrease heat gain. In addition, plants avoid temperature extremes by going into dormancy. The most obvious examples of this are the plants of temperature deciduous forests. 9. This example suggests that predation exerts strong selection for background matching, or camouflage, in these beetles. The broader implication here is that natural selection for one characteristic may be opposed to natural selection for other characteristics, which could place constraints on the ability of natural selection to optimize. 10. One of the reasons behind the mismatch between organisms and environment was the subject of Question 9. In addition, during selection, the present environment acts upon existing genetic variation within populations to produce differential survival and reproduction among genotypes. However, future environments may be quite different from past environments and if environmental change is too rapid or the genetic variation within populations too low, the population may become extinct. Chapter 6 1. The higher humidity under stones reduces the gradient of water concentration from the isopod to the air and so reduces the rate of evaporation. 2. The vapor pressure deficit is the difference between the actual water vapor pressure in air and the saturation water vapor pressure of air at a particular temperature. Osmotic pressure is the force necessary to prevent the movement of water down a gradient of water concentration. Water potential is the capacity of water to do work. Since vapor pressure deficit, osmotic pressure, and water potential can all be measured as pressures, they can be expressed in the same units. 3. Lower leaf water potential at midday should increase the rate of water movement from soil to plant, since water moves down gradients of water potential. 4. The kangaroo rat obtains most of its water from metabolic water, while the beetle obtains most of its moisture from condensed fog. Compared to the beetle, the kangaroo rat loses a greater proportion of its water through urine.

5. Water loss rates should be higher among tenebrionid beetle species from moist temperate environments. This prediction is based on the assumption that there is some energetic cost associated with water proofing and that natural selection favors investment in water proofing where it increases the evolutionary fitness of individuals. One way to test this prediction would be to go out and measure water loss rates in tenebrionid species from different environments. 6. The need for water is a strong selective force for all organisms in arid environments, so individuals with variations in traits and behaviors that enabled them to collect more water would have improved fitness, and be more likely to pass those traits on to future generations. The similarity of these water collecting strategies in such different species (convergent evolution), makes sense due to the physical properties of water. These properties dictate how water is collected via condensation, or moved from one location to another via channeling and gravity. Successful strategies to collect water used by plants and animals living in arid environments could potentially be “mimicked” in order to collect water for human use. 7. Plants and animals must exchange carbon dioxide and oxygen with the surrounding air in order to engage in respiration and photosynthesis. Complete water proofing would eliminate these vital gas exchanges. 8. Use figures 6.26, 6.27 and 6.28 as guides. Marine bony fish are hypoosmotic (lower concentration of salts, higher concentration of water) relative to their environment. Freshwater bony fish are hyperosmotic. The kidneys of marine and freshwater sharks would both function to pump excess water out of the environment but the kidneys of freshwater sharks would need to pump water at a higher rate. Chapter 7 1. Ultraviolet light, while energy rich, is highly damaging to organic molecules including those involved in photosynthesis. The existence of vision systems which detect UV light suggests that a photosynthetic system capable of using UV light may be theoretically possible. Infrared light has been generally considered to be too low in energy to drive photosynthesis. The physiology of photosynthetic bacteria seems to indicate otherwise. 2. C3 plants are best adapted to cool and moist climates, where they should predominate, while C4 and CAM plants should predominate in arid and semiarid environments of high light intensity. Within a macroclimate, some microclimates would tend to favor one or another of these groups of plants. In addition, one must consider seasonal variation and longer-term variation (e.g., drought cycles) in climate. 3. At low CO2 concentrations, C4 plants are better at conserving water, since PEP carboxylase, the main enzyme involved in concentrating CO2 in C4 plants, has a high affinity for CO2. 4. Herbivores generally consume food that is abundant but well defended. Detritivores consume food that may be even more abundant than the green tissues eaten by herbivores but detritus is often low in some essential nutrients, especially nitrogen. Carnivores consume foods high in nitrogen but that is generally very well defended. Omnivores were the main organisms left out of the discussion. 5. The main advantage of advertising is that it reduces the probability of attack by predators. Mullerian mimicry increases the abundance of similar-looking noxious individuals in the environment, which reduces the probability of attack on any particular individual by a "naive" predator. In Batesian mimicry, the mimic reduces its probability of being eaten (a

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benefit) but the risk to the model is increased because of the existence of palatable individuals that display warning coloration to potential predators. Begin by choosing one or more chemosynthetic autotrophs and building the rest of the biota on this foundation. Type 1: filter feeding aquatic organisms that feed on small prey; Type 2: consumers limited by prey availability at low prey densities and by prey handling time at high densities; Type 3: predators that require some learning to be effective on a prey species and/or that feed on prey that is well protected at low densities. Selection for prey defense should lower the height of functional response curves, while selection for more effective predators should raise the height. Specializing on crayfish (which has apparently occurred) would increase the rate of energy intake E/T. Allocation to roots versus shoots influences the rates of nutrient uptake by roots versus energy fixation by photosynthesis by shoots. Balancing these two processes may require different levels of investment in roots versus shoots in different environments.

Chapter 8 1. The idea here is to use the descriptions of mating behavior and physical characteristics of species described in the introduction to make predictions about sexual selection. For instance, the chapter opens with schooling fish that are identical in morphology. The absence of obvious features that distinguish males and females suggests that predation on the species overrides potential sexual selection for dimorphism. In the second example, damselfish males compete with other males for territories, which suggests intrasexual competition for spawning territories. This competition should act as a source of intrasexual selection. Though the example doesn't mention how females choose males, they may distinguish between males on the basis of territory quality and/or male characteristics, such as coloration, size, and so forth. Female choice of male damselfish should create the conditions for intersexual sexual selection. Use a similar approach to predict the forms of sexual selection occurring in each of the remaining species described in the introduction. 2. Within a population of a species that reproduces sexually, there are often complex social behaviors associated with either attracting or choosing mates. In a species having separate sexes, there are often male-specific and female-specific mating behaviors. However in a hermaphroditic species all individuals should exhibit male and female forms of social behavior. As the number of forms of males or females increase in a population, the diversity of behavioral interactions within the population will likely increase. 3. Physical factors that might influence the color of male guppies include the turbidity of the water, the color of the materials forming the bottom or banks of the stream, and the quantity and quality of light shining onto the stream. 4. The greenhouse experiments provided the opportunity to control several factors within the experimental environment, while varying the factor of interest, intensity of predation. The shortcomings of the greenhouse experiments were that many other factors of potential importance to guppy coloration, for instance a wide variety of invertebrate predators or a diverse diet that includes a wide variety of invertebrate prey, were absent in the greenhouse environment. Field experiments give the scientist less control over environmental conditions. However, if carefully designed, field experiments reduce the chances of experimental

artifact. Ideally, the ecologist uses several sources of evidence when exploring a natural phenomenon. There is evidence for intrasexual and intersexual selection in scorpionflies. For example, competition among male scorpionflies can result in injuries during fights, a strong indicator of intrasexual selection among males in competition for nuptial offerings. Intersexual selection is suggested by mate selection by female scorpionflies, which appears to be mostly mediated by the quality of nuptial offerings presented by males from which females get nutritional benefit. The maternal plant may influence the rate of growth by pollen tubes of different genotypes, creating conditions that increase the probability of successful mating by some pollen types over others. The maternal plant may also invest more energy resources into some seeds pollinated by particular pollen genotypes, while reducing energy allocated to other seeds. Maternal plants may even selectively abort seeds based on their paternity. Competition among pollen may take the form of a race, determined by rate of pollen tube growth, or involve some type of interference among pollen on the stigma. Jennifer Jarvis waited one year after establishing her laboratory colony of naked mole rats before attempting to quantify the behavior of the laboratory population so that she could be sure that the colony had adjusted to the laboratory environment If Jarvis had begun her studies soon after the colony was established, her results may have been strongly influenced by atypical behaviors associated with adjustment to the new environment. Eusociality is generally thought to include three major characteristics: 1) Individuals of more than one generation living together, 2) cooperative care of young, and 3) division of individuals into sterile, or nonreproductive, and reproductive castes. Naked mole rats show all three of these characteristics. Use the many case histories of laboratory and field studies discussed in this chapter as models for your proposed study.

Chapter 9 1. The highly reflective leaves of Encelia farinosa reduce heating of leaves and lower the rate of water loss. The deeper penetrating roots of E. frutescens provides access to deeper groundwater, which sustains transpiration sufficient to evaporatively cool the plant’s leaves. 2. Spruce trees in the desert mountains of North America are concentrated at higher elevations and on north facing slopes, because temperatures are lower and precipitation greater at high elevations and temperatures and evaporative water losses are lower on north facing slopes. 3. Figure 9.10 summarizes the answer to this question. 4. If plants are distributed mainly in response to the distribution of soil characteristics, there should be a strong correspondence between the distribution of soil characteristics and the distribution of individuals within a plant population. 5. The plant reproducing from seeds should have a less clumped distribution pattern compared to the plant species reproducing mainly by budding. 6. Assuming that the habitat could be restored, the refuges should be established in areas that once supported high densities of fish crows (historic hotspots). Several refuges should be stocked simultaneously in case one or more fails. 7. Mammals tend to have higher densities than birds of similar size. Terrestrial invertebrates tend to live at lower densities than aquatic invertebrates.

8. Consult figure 9.22. Species that have an extensive geographic range, broad habitat tolerances, and large local populations are the least vulnerable to extinction, while those with restricted geographic range, narrow habitat tolerance, and small local populations are the most vulnerable. 9. Yes, at least somewhat since large organisms tend to live at the lowest population densities, they automatically show at least one aspect of rarity. However, size of geographic range and habitat tolerances are not included in the analyses by Damuth and Peters and Wassenberg. Chapter 10 1. Use the discussion in this chapter to construct your outline of Muller's colonization cycle. One of the best ways to follow the colonization cycle of an organism like N. latissima would be to mark the shells of a large number of individuals in the colonizing waves and use other marks to identify individuals in local stationary populations. 2. A cohort life table is made by identifying a large number of individuals born at about the same time and following them from birth to death and recording the age at which each individual dies. One of the main assumptions of a cohort life table is that your cohort is representative of the population as a whole. A cohort life table is most practically used when it is possible to observe and keep track of a large number of individuals born nearly simultaneously. A static life table is constructed by recording the age at death for a large number of individuals in a population. The individuals in a static life table are not all part of a cohort. A static life table assumes that... 3. Type III survivorship is difficult to study because of the large number of individuals that must be studied to adequately record mortality during the very early portion of the life cycle. 4. It is reasonable to assume that most species with very high reproductive rates have type III survivorship because otherwise their populations would be growing at extremely rapid rates. 5. A population with highly episodic reproduction will often show an age structure with age classes, often early age classes, missing. These reproductive failures may suggest that the individuals in a population are not replacing themselves and that the population is dying out. In some populations, of desert shrubs for example, significant reproduction occurs every few decades. The best way to avoid such misinterpretations is to conduct long-term population studies and where possible to do historical reconstructions of population structure and reproduction. 6. The main point here is that while Leverich and Levin accurately measured the reproductive and mortality rates of the 996 individuals in their study, these 996 individuals were a sample of a much larger population. Therefore, the population statistics for this sample of 996 individuals were estimates of those parameters for the larger population. 7. R0> 1.0 is growing, R0 = 1.0 is stable, R0< 1.0 is declining, r > 0 is growing, r = 0 is stable, r < 0 is declining. 8. The minimum information needed to construct a life table and fecundity schedule are lx, the proportion of the population surviving to each age x, and mx, the average number of offspring produced by each individual at each age x. 9. In a predator population increasing in density in response to increased prey density r will be greater than 0 and R0 will be greater than 1. Because prey availability can affect survivorship, lx, and reproductive rates, mx, as well as R0, a predator population undergoing a strong numerical response (i.e., growing rapidly) may also show an altered generation time, T.

Chapter 11 1. The geometric model of population growth is appropriate for organisms with nonoverlapping generations. The exponential model of population growth is most appropriate for organisms that have overlapping generations. 2. You would need to know more about relative reproductive rates. In fact, the much longer interval between births among north Atlantic right whale populations, every 4 to 7 years compared to every 2 years for gray and blue whales, indicate a very low potential rate of population growth. Consequently, these whales can be expected to increase very slowly even with full protection from whaling. 3. To build the logistic model for population growth from the exponential model, first substitute rm, the intrinsic rate of increase which is a constant, for r, the per capita rate of increase which is variable. Next, take the resulting expression for exponential grow, rmaxN, times 1N/K. This last factor, 1-N/K, produces sigmoidal growth. See figure 11.13 for further explanation. 4. The factors that limit population size are those aspects of the environment that affect birth and death rates in populations. These include environmental factors such as limited habitat, food availability, predation, disease, parasites, and so forth. Make this listing more concrete by listing the limiting factors for a specific organism that you know something about or about which you can readily obtain information. 5. The per capita rate of increase, r, is an actual rate that varies with environmental conditions. The intrinsic rate of increase, rmax, is a theoretical rate of increase for a population of an organism increasing at a maximum rate under "ideal" conditions for the species. You can estimate the rm of a species experimentally by providing ideal conditions for population growth and measuring rates of increase. 6. Biotic factors limiting population growth include competition, predation, disease, and parasitism. Abiotic factors that limit population growth include fire, drought, excessively high or low temperatures, floods, toxins, and so forth. 7. Because they are capable of killing all vulnerable individuals, an abiotic factor such as a severe freeze can influence populations independently of local population density. In contrast, the proportion of a population attacked by a predator or infected by a disease organism usually changes with the density of the attacked or infected population. Typically density independent factors may be at least partially influenced by population density if, for instance, there are a limited number of safe sites that can be filled, leaving the remainder of the population more vulnerable. For instance, at high population densities individuals in a population may move into floodplain habitats where they are more vulnerable to floods. 8. See figure 11.23. Human population density tends to be highest in coastal regions and lowest in the interior of continents and in the Arctic and Antarctic. Very few regions are completely devoid of human inhabitants. Population growth is fastest in developing countries such as Rwanda and approximately stable in many developed countries such as Japan and many countries of Western Europe. Many developed countries have declining populations. 9. The projected global population in 2050 is over 9 billion. This population size may not be sustainable over the long term because the resource requirements of such a large population may damage the environment (e.g., change atmospheric composition, pollute freshwaters, destroy forests, deplete fish stocks) to such an extent its capacity to support humans will decline. Such a population might persist over the long term if average per capita resource consumption is greatly reduced.