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Figure 18-11 represents the amount of energy stored as organic material in each trophic level in an ecosystem. The pyramid shape of the diagram indicates the low percentage of energy transfer from one level to the next. On average, 10 percent of the total energy consumed in one trophic level is incor- porated into the organisms in the next. Why is the percentage of energy transfer so low? One reason is that some of the organisms in a trophic level escape being eaten. They eventually die and become food for decomposers, but the energy contained in their bodies does not pass to a higher trophic level. Even when an organism is eaten, some of the molecules in its body will be in a form that the consumer cannot break down and use. For example, a cougar cannot extract energy from the antlers, hooves, and hair of a deer. Also, the energy used by prey for cellu- lar respiration cannot be used by predators to synthesize new bio- mass. Finally, no transformation or transfer of energy is 100 percent efficient. Every time energy is transformed, such as during the reactions of metabolism, some energy is lost as heat. Limitations of Trophic Levels The low rate of energy transfer between trophic levels explains why ecosystems rarely contain more than a few trophic levels. Because only about 10 percent of the energy available at one trophic level is transferred to the next trophic level, there is not enough energy in the top trophic level to support more levels. Organisms at the lowest trophic level are usually much more abundant than organisms at the highest level. In Africa, for exam- ple, you will see about 1,000 zebras, gazelles, and other herbivores for every lion or leopard you see, and there are far more grasses and shrubs than there are herbivores. Higher trophic levels con- tain less energy, so, they can support fewer individuals.A population is a group of organisms that belong to the same species and live in a particular place at the same time. All of the bass living in a pond during a certain period of time make up a pop- ulation because they are isolated in the pond and do not interact with bass living in other ponds. The boundaries of a population may be imposed by a feature of the environment, such as a lake shore, or they can be arbitrarily chosen to simplify a study of the population. The humans shown in Figure 19-1 are part of the pop- ulation of a city. The properties of populations differ from those of individuals. An individual may be born, it may reproduce, or it may die. A population study focuses on a population as a whole—how many individuals are born, how many die, and so on. Population Size A population’s size is the number of individuals that the population contains. Size is a fundamental and important population property but can be difficult to measure directly. If a population is small and composed of immobile organisms, such as plants, its size can be determined simply by counting individuals. Often, though, individ- uals are too abundant, too widespread, or too mobile to be counted easily, and scientists must estimate the number of individuals in the population. Suppose that a scientist wants to know how many oak trees live in a 10 km2 patch of forest. Instead of searching the entire patch of forest and counting all the oak trees, the scientist could count the trees in a smaller section of the forest, such as a 1 km2 area. The scientist could then use this value to estimate the population of the larger area. SECTION 1 OBJECTIVES ● Describe the main properties that scientists measure when they study populations. ● Compare the three general patterns of population dispersion. ● Identify the measurements used to describe changing populations. ● Compare the three general types of survivorship curves. VOCABULARY population population density dispersion birth rate death rate life expectancy age structure survivorship curve FIGURE 19-1 A population can be widely distributed, as Earth’s human population is, or confined to a small area, as species of fish in a lake are. Copyright © by Holt, Rinehart and Winston. All rights reserved. 382 CHAPTER 19 If the small patch contains 25 oaks, an area 10 times larger would likely contain 10 times as many oak trees. A similar kind of sampling technique might be used to estimate the size of the pop- ulation shown in Figure 19-2. To use this kind of estimate, the sci- entist must assume that the distribution of individuals in the entire population is the same as that in the sampled group. Estimates of population size are based on many such assumptions, so all esti- mates have the potential for error. Population Density Population density measures how crowded a population is. This measurement is always expressed as the number of individuals per unit of area or volume. For example, the population density of humans in the United States is about 30 people per square kilome- ter. Table 19-1 shows the population sizes and densities of humans in several countries in 2003. These estimates are calculated for the total land area. Some areas of a country may be sparsely popu- lated, while other areas are very densely populated. Dispersion A third population property is dispersion (di-SPUHR-zhuhn). Dispersion is the spatial distribution of individuals within the popu- lation. In a clumped distribution, individuals are clustered together. In a uniform distribution, individuals are separated by a fairly con- sistent distance. In a random distribution, each individual’s location is independent of the locations of other individuals in the popula- tion. Figure 19-3 illustrates the three possible patterns of dispersion. Clumped distributions often occur when resources such as food or living space are clumped. Clumped distributions may also occur because of a species’ social behavior, such as when animals gather into herds or flocks. Uniform distributions may result from social behavior in which individuals within the same habitat stay as far away from each other as possible. For example, a bird may locate its nest so as to maximize the distance from the nests of other birds. These migrating wildebeests in East Africa are too numerous and mobile to be counted. Scientists must use sampling methods at several locations to monitor changes in the population size of the animals. FIGURE 19-2 TABLE 19-1 Population Size and Density of Some Countries Population size Population density Country (in millions) (in individuals/km2) China 1,289 135 India 1,069 325 United States 292 30 Russia 146 8 Japan 128 337 Mexico 105 54 Kenya 32 54 Australia 20 3 dispersion from the Latin dis-, meaning “out,” and spargere, meaning “to scatter” Word Roots and Origins Copyright © by Holt, Rinehart and Winston. All rights reserved. POPULATIONS 383 The social interactions of birds called gannets, which are shown in Figure 19-3b, result in a uniform distribution. Each gannet chooses a small nesting area on the coast and defends it from other gannets. In this way, each gannet tries to maximize its distance from all of its neighbors, which causes a uniform distribution of individuals. Few populations are truly randomly dispersed. Rather, they show degrees of clumping or uniformity. The dispersion pattern of a population sometimes depends on the scale at which the popu- lation is observed. The gannets shown in Figure 19-3b are uni- formly distributed on a scale of a few meters. However, if the entire island on which the gannets live is observed, the distribution appears clumped because the birds live only near the shore. POPULATION DYNAMICS All populations are dynamic—they change in size and composition over time. To understand these changes, scientists must know more than the population’s size, density, and dispersion. One important measure is the birth rate, the number of births occur- ring in a period of time. In the United States, for example, there are about 4 million births per year. A second important measure is the death rate, or mortality rate, which is the number of deaths in a

According to 〜によれば add to 増やす add up 合計する after a while しばらくして against the idea その考えに反対して all of a sudden 突然に all the time いつも all through the night 一晩中 along with 〜と一緒に apply for 〜に申し込む apply to 〜に適用する as a rule 原則として aside from 〜のほかに / 〜を除いて at any cost どんな犠牲を払っても at heart 心の底では at last ついに at least 少なくとも at length 詳細に at most 多くても at once すぐに / 一度に at the sight of 〜を見て at times 時々 attach to 〜に付ける / 〜に結びつける back up 支援する、バックアップする based on 〜に基づいて be absent from 〜を欠席している be against 反対する be based on 〜に基づいている be confident of 〜に自信がある be curious about 〜に好奇心を持つ be derived from 〜に由来する be filled with 〜で満たされている be full of 〜でいっぱいである be made up of 〜で構成されている be pleased with 〜に満足している be short of 不足している be similar to 〜に似ている because of 〜のせいで / 〜のおかげで before long まもなく break out 突発する break out in （急に）〜になる break up 解散する / 別れる bring out 引き出す / 公表する bring up 育てる / 持ち出す burst into 急に〜し始める by heart 暗記して by mistake 間違えて by now 今頃までには by the way ところで by way of 〜を通じて call for 要求する / 呼びかける call out 大声で呼ぶ carry on 続ける carry out 実行する、行う catch up with 〜に追いつく close to 〜に近い come across 偶然出会う / 見つける come into 〜に入る / 〜になる come out 出てくる / 公表される come up with 〜を思いつく compared with 〜と比べて depend on 〜に依存する do him good 彼に利益をもたらす drive at 意図する、狙う drop by 立ち寄る drop down 落ちる feel at home くつろぐ feel like doing 〜したい気分 feel sorry 気の毒に思う figure out 理解する find fault with 〜に文句をつける find out 知る、解明する for fear of 〜を恐れて for free 無料で for good 永遠に、完全に for once 一度だけ / 今回だけは for sale 売り物の for the best 最善のために get over 乗り越える get ready 準備する get rid of 〜を取り除く give away 与える、寄付する give in to 〜に屈する give off 放つ give out 配る / 発表する go ahead 続けて行う go down 下がる / 沈む hand in 提出する hand over 手渡す hang on 待つ / 頑張る hang up 受話器を置く hear from 〜から連絡をもらう help yourself 自由に取る / 自由にどうぞ hold back 控える hold on 待つ / 持ちこたえる hold up 停止させる、遅らせる in a jacket ジャケットを着て in a word 一言で言えば in advance 前もって in case 〜の場合に in common 共通して in detail 詳細に in hand 手元に in part 部分的に in place of 〜の代わりに in return 見返りに in terms of 〜の観点から in the distance 遠くに in the habit of 〜する習慣がある in the way 妨げになって instead of 〜の代わりに keep a secret 秘密を守る keep an eye on 見守る keep away from 近づかないようにする keep on Ving 〜し続ける keep pace with 〜に遅れずについていく keep the change お釣りはいりません keep track of 記録をつける / 追跡する keep up with 〜に遅れずについていく lay it down それを置く / 規定する less than 〜未満 / 〜より少ない look after 世話をする look back on 〜を振り返る look down on 〜を見下す look like 〜のように見える look up 調べる / 見上げる made up of 〜で構成されている major in 〜を専攻する make efforts 努力する make it out 理解する / 成し遂げる make out 見分ける、うまくいく make progress 進歩する make sense 理解できる、意味を成す make up your mind 決心する mind your own business 自分のことに集中しろ move on 次に進む no longer もはや〜ない not always 必ずしも〜でない nothing but ただ〜だけ on air 放送中 on behalf of 〜を代表して on business 仕事で on fire 火がついている on purpose わざと on the point 〜の点で on time 時間通りに one another お互いに out of the question 問題外で pass by 通り過ぎる pay attention 注意を払う play a part in 〜で役割を果たす pour out 注ぎ出す、溢れ出る prefer A to B BよりAを好む put away 片付ける put off 延期する put on 着る / 演じる　(weightで太る） put out 消す / 発表する reach for 手を伸ばす rely on 〜に頼る result in 【自動詞】結果として〜になる run it over それをひく / 読み返す run out of 〜を使い果たす run over ひいてしまう、走り回る see about 手配する / 調べる see off 見送る see through 見抜く / 見通す set out 出発する、始める set up 設置する / 設定する show off 自慢する、見せびらかす show up 現れる / 到着する sit up 座る、起き上がる speak up はっきり話す stand out 目立つ stand out 目立つ stand up for 〜を支持する suffer from 〜に苦しむ take away 持ち去る / 奪う take in 理解する、取り入れる take notice of 注意を払う take on 引き受ける / 挑む take out 取り出す / 持ち帰る take over 引き継ぐ、乗っ取る take part 参加する take place 起こる / 開催される take risks 危険を冒す take turns 交代で行う talk over 相談する tear off 引き裂く the second largest 二番目に大きい think better of 考え直す throw away 捨てる try on 試着する turn in 提出する turn off 消す / 切る turn on （スイッチを）入れる turn out 結果的に〜になる turn over ひっくり返す under control 制御下にある up to date 最新の with ease 容易に with regard to 〜に関して

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