【正文】 是前者滅亡，并且認(rèn)為這兩個(gè)現(xiàn)象之間，有共同的機(jī)制在起作用。 人工優(yōu)勢(shì)種 （三 )伴生種 ? 伴生種 (panion species)為群落的常見(jiàn)種類(lèi)，它與優(yōu)勢(shì)種相伴存在，但不起主要作用。群落的不同層次可有各自的優(yōu)勢(shì)種，比如森林群落中，喬木層、灌木層、草本層和地被層分別存在各自的優(yōu)勢(shì)種， 其中優(yōu)勢(shì)層的優(yōu)勢(shì)種 (此處為喬木層 )常稱為建群種 (edificator或 constructive species)。例如，地勢(shì)變化較陡的山地的垂直帶，陸地環(huán)境和水生環(huán)境的邊界處 (池塘、湖泊、島嶼等 )。誠(chéng)然，生物群落是生物種群的集合體，但不是說(shuō)一些種的任意組合便是一個(gè)群落。一個(gè)群落中種類(lèi)成分的多少及每種個(gè)體的數(shù)量，是度量群落多樣性的基礎(chǔ)。環(huán)境概論 Introduction to Environment 董 方 土木與水利工程學(xué)院 第 2講 Ecosystem, Living Organisms and Physical Environment 生態(tài)系統(tǒng) , 生命有機(jī)物與自然環(huán)境 Outline 提要 Evolution: How Populations Change Over Time 進(jìn)化 :種群隨時(shí)間如何變化 . Natural Selection 自然選擇 。因此，種類(lèi)組成是區(qū)別不同群落的首要特征。 (五 )不同物種之間的相互影響 ? 群落中的物種有規(guī)律的共處，即在有序狀態(tài)下共存。前者見(jiàn)于環(huán)境梯度變化較陡，或者環(huán)境梯度突然中斷的情形。它們通常是那些個(gè)體數(shù)量多、投影蓋度大、生物量高、體積較大、生活能力較強(qiáng)，即優(yōu)勢(shì)皮較大的種。在復(fù)層群落中，它通常居于下層，如大針茅草原中的小半灌木冷蒿就是亞優(yōu)勢(shì)種。 soil is already present. ? Climax munity頂級(jí)群落 : Ecologists initially thought that succession inevitably led to a stable and persistent munity, such as a forest, known as a climax munity. ? It is now recognized that mature ‘climax’ munities are not in a state of permanent equilibrium but rather in a state of continual disturbance. ? The time involved in ecological succession is on the scale of tens, hundreds, or thousands of years, not the millions of years involved in the evolutionary time scale. ? Primary succession on bare rock from a pioneer munity to a forest munity often occurs in this sequence: lichens地衣 → mosses臺(tái)蘚 → grasses草叢 → shrubs → trees ? Primary Succession on Krakatoa. ? Primary Succession on Sand Dunes ? Secondary Succession on abandoned field Keystone Species 關(guān)鍵物種 ? Certain species are more crucial to the maintenance of their ecosystem than others. Such species, called keystone species, are vital in determining the nature and structure of the entire ecosystemthat is, its species position and its ecosystem functioning. Other species of a munity depend on or are greatly affected by the keystone species. Keystone species are usually not the most abundant species in the ecosystem. Although present in relatively small numbers, keystone species exert a profound influence on the entire ecosystem because they often affect the available amount food, water, or some other resources. ? One example of a keystone species is a top predator such as the gray wolf. Where wolves were hunted to extinction, the populations of deer and other herbivores increased explosively. As these herbivores overgrazed the vegetation, many plant species that could not tolerate such grazing pressure disappeared. Many smaller animals such as insects also were lost from the ecosystem because the plants that they depended on for food were now less abundant. Thus, the disappearance of the wolf resulted in an ecosystem with considerably less biological diversity. Interactions Among Organisms 有機(jī)體之間相互關(guān)系 ? No anism exists independently of other anisms. The producers, consumers, and deposers of an ecosystem interact with one another in a variety of plex ways, and each forms associations with other anisms. Three main types of interactions occur among species in an ecosystem: symbiosis, predation, and petition. Symbiosis 共生現(xiàn)象 ? Symbiosis is any intimate relationship or association between members of two or more species. Usually, one species lives in or on another species. The partners of a symbiotic relationship, called symbionts共生體 , may benefit from, be unaffected by, or be harmed by the relationship. ? Mutualism互利共生 : A symbiotic relationship in which both partners benefit. ? Commensalism偏利共生 : A symbiotic relationship in which one partner benefits and the other partner is unaffected. ? Parasitism寄生 : A symbiotic relationship in which one partnerthe parasiteobtains nutrients at the expense of the otherthe host. ? Coevolution共同進(jìn)化 : the interdependent evolution of two interacting species. 蜜旋木雀 ,半邊蓮 共同進(jìn)化 紅雪松 菌根真菌 互利共生 附生植 偏利共生 寄生 Predation 捕食 ? Predation is the consumption of one species, the prey獵物 , by another, the predator捕食者 . Predation includes both animals eating other animals and animals eating plants. Predation has resulted in and ‘a(chǎn)rms race’, with the coevolution of predator strategiesmore efficient wary to catch preyand prey strategiesbetter ways to escape the predator. ? A predator that is more efficient at catching prey exerts a strong selective force on its prey, and over time the prey reduces the probability of being captured. The countermeasure acquired by the prey in turn may act as a strong selective force on the predator. Competition 競(jìng)爭(zhēng) ? Competition occurs when two or more individuals attempt to use an essential mon resource such as food, water, shelter, living space, or sunlight. Because resources are often in limited supply in the environment, their use by on individual decreases the amount available to others. If a tree in a dense forest grows taller than surrounding trees, it is able to absorb more of the ining sunlight. Less sunlight is therefore available for nearby trees that are shaded by the taller tree. ? Competition can occur among indiv