Ch19 Ecosystem PDF - HKDSE, HKCEE, and HKALE Past Paper

19 Ecosystems Shek O rocky shore Natural environment in Hong Kong Think about......

19 Ecosystems Shek O rocky shore Natural environment in Hong Kong Think about... 1 What are the other types of Hong Kong has a wide variety of natural wildlife living in different natural environment in Hong environments. For example, there are many different types of Kong? organisms in rocky shores. The organisms interact with one another 2 What are the features required and with their surrounding environment, forming a self-supporting to maintain a self-supporting and stable system. and stable system in nature? Watch more (Answers on p. 61) Acknowledgements and Important Notice: All questions from the HKDSE, HKCEE and HKALE are reproduced by permission of the HKEAA. Unauthorized use of the aforementioned questions in this electronic version is prohibited. II Organisms and Environment 19.1 Basic concept of ecology A What is ecology? Cross-link There is a wide variety of organisms on earth. Organisms may live in Details about the variety of various places such as forests, grasslands, deserts, rivers, the ocean and organisms will be discussed in Bk 4, Ch 28. the polar regions. The place where organisms live is called their habitat*. Each habitat has a well-defined set of physical conditions (e.g. soil types and temperature). Fig 19.1 shows a few examples of organisms and their habitats. a b c Freshwater pond is the habitat of the Desert is the habitat of the cactus. Coral reef is the habitat of many kinds frog and the lotus. of sea animals. Fig 19.1 Examples of habitats of different organisms In order to survive, organisms show different ways of adapting to the specific conditions of their habitats. In the habitat, the organisms do not live independently, but interact with one another and with their The German word of environment. Ecology* is the study of these interactions, i.e. ‘ecology’ is ‘Ökologie’, which comes from the Greek word the inter-relationships between organisms; and ‘oikos’, meaning a house or living place. the inter-relationships between organisms and their environment. Cross-link Ecological studies provide us with information to better understand the Details about how we natural processes on earth. The information can also help us improve can improve the natural environment will be the quality of the natural environment, manage our natural resources, discussed in Bk E2, Ch 2. and protect human health. B Levels of organization in ecological studies We can study the inter-relationships between organisms, and those between organisms and their environment by considering them at different levels. The levels of organization in ecological studies are shown on the next page. ecology 生態學 habitat 生境 19– 2  19 Ecosystems Species* Species is the lowest level of classifying organisms. It consists of organisms that can interbreed and produce Population* zebra, its fertile offspring. species name is Population refers to a group of organisms of Equus quagga the same species living in the same habitat. Community* Community refers to all the populations of different species living in the same habitat. a population of zebras Ecosystem* An ecosystem is a natural unit of the living components (i.e. all the organisms in a a community composing of zebras, community) and the non-living components (i.e. elephants, antelopes*, etc the physical environment). These components interact to form a self-supporting, stable, and yet dynamic* system in a particular area. Biome* A biome is a complex of communities that occupies a large geographical area. It is characterized by a dominant type of plant, which is determined by the climatic conditions. For example, the savanna biome the savanna* ecosystem is located in Africa, India, the northern part of Australia and the middle part of South America. Biosphere* Biosphere refers to the entire space on the earth’s surface where organisms exist. It consists of the land, savanna the water and the lower part of the atmosphere. a biosphere distribution of biomes on earth antelope 羚羊 biome 生物羣系 biosphere 生物圈 community 羣落 population 種羣 dynamic 動態的 ecosystem 生態系 savanna 稀樹草原 species 物種 19– 3 II Organisms and Environment What studies can we carry out at different levels of organization? At the species level… At the population level… At the community level… We can study how the body features We can study how environmental We can study the interaction among of the individuals of a species adapt factors affect the size of a different species in a habitat. to their environment. population. At the ecosystem level… At the biome level… At the biosphere level… We can study the cycling of We can study how changes in We can study the effect of human materials between organisms and world climate affect organisms activities on ecological processes their environment. in a biome. in the whole biosphere, such as the issue of global warming. In this chapter, we will focus on the study of ecology at the ecosystem level. C Basic features of an ecosystem All ecosystems are self-supporting, stable and yet dynamic. Being stable and dynamic means that the mean density of the population of each species is approximately constant because the birth rate is balanced by the death rate. The following three features are required to maintain such properties: Cross-link 1 Cycling of materials The cycling of carbon and nitrogen in the ecosystem Useful elements (e.g. carbon and nitrogen) inside organisms are not will be discussed in lost from the ecosystem. Instead, they are cycled through the ecosystem Section 19.5. when the organisms carry out photosynthesis, respiration, excretion, egestion or when they die. Thus, an ecosystem does not require the supply of extra materials. This explains why an ecosystem is stable and self-supporting. Cross-link 2 Interactions of organisms with one another and The interactions will be discussed in Sections 19.2 with the environment and 19.3. Interactions of organisms with one another and with the environment make an ecosystem dynamic. The interactions that lead to cycling of materials are usually related to feeding, where the materials are transferred from one organism to the other in the form of food. 19– 4  19 Ecosystems Inorganic materials in the environment may be absorbed by the organisms and may be converted to organic matter through autotrophic nutrition. Materials may be released back to the environment through respiration, excretion and decomposition of dead organic matter. Besides feeding, organisms may develop other relationships with one another. For example, some species may compete for common resources, such as living space. Organisms show different adaptations in order to live in specific environments. On the other hand, organisms also affect their physical environment. For example, they may take up substances and release waste into the environment. They may also modify their physical environment by speeding up erosion or stabilizing the substratum (Fig 19.2). Fig 19.2 Roots of trees hold soil in place on hillside 3 Energy flow Unlike materials, energy cannot be cycled. As the survival of organisms requires a continuous supply of energy, there must be a continuous input of energy into the ecosystem. Usually, the input of energy comes from the sun. The sun’s energy is absorbed by photosynthetic organisms to build organic matter from inorganic materials. After that, energy flows from one organism to another in the form of food when organisms are eaten by others. Energy is eventually lost as heat from the ecosystem. Cross-link The energy flow in an heat ecosystem will be discussed in Section 19.4. sun organisms materials light energy Key: energy flow cycling of materials physical environment Fig 19.3 Cycling of materials and energy flow in an ecosystem 19– 5 II Organisms and Environment D Major types of ecosystems in Hong Kong All ecosystems possess the three basic features described on p. 4 and 5, but they differ in their physical and living components. The major types of ecosystems in Hong Kong are shown below. Can you identify the main differences among them? Freshwater streams* Hong Kong has a hilly landscape with many hill streams. There is a rich variety of fish, amphibians and insect species in the upper and middle course of the streams. However, the lower course of many streams is facing serious threats due to human activities. Mangroves* Mangroves are located in sheltered shores where freshwater streams run into the sea. Although influenced by the tides, most mangroves are sheltered from wave action. They provide the breeding ground for many organisms. Woodlands* In Hong Kong, woodlands are mainly found in country parks. Evergreen broad-leaved trees* are the major plant species in woodlands. Other tree species have been introduced to restore the damaged woodlands in early years. evergreen broad-leaved tree 常綠闊葉樹 freshwater stream 淡水河流 mangrove 紅樹林 woodland 林地 19– 6  19 Ecosystems New Territories Kowloon Hong Kong Island Lantau Island Key: freshwater stream Lamma mangrove Island woodland grassland rocky shore Fig 19.4 Locations of major ecosystems in Hong Kong Rocky shores* Grasslands* Rocky shores are marine coastal areas Many hillsides in Hong Kong are covered covered with large rocks. They are exposed by grasslands. Short grass species are the to strong wind and waves. During low tide, major plant species in grasslands. Frequent the shores become exposed to air. Organisms exposure to strong wind limits the growth of that live in rocky shores experience daily larger plants on the hillsides. fluctuations in environmental conditions. grassland 草地 rocky shore 岩岸 19– 7 II Organisms and Environment 1 What is ecology? Ecology is the study of the inter-relationships between organisms, and the inter-relationships between organisms and their environment. 2 What are the levels of organization in ecological studies? In ecology, organisms and their environment can be studied at different levels of organization: species → population → community → ecosystem → biome → biosphere 3 What make the ecosystem self-supporting, stable and yet dynamic? In an ecosystem, the cycling of materials, interactions between organisms and their physical environment lead to the flow of energy. These features maintain a self-supporting, stable, and yet dynamic ecosystem. 4 What are the major types of ecosystems in Hong Kong? The major types of ecosystems in Hong Kong are freshwater streams, mangroves, woodlands, grasslands and rocky shores. Level 1 1 For each of the level of organization in ecological studies listed in column 1, select from column 2 one phrase that matches it. Put the appropriate letter in the space provided. (3 marks) Column 1 Column 2 Community A Made up of a group of organisms of the same species living in the same habitat Ecosystem B Made up of groups of organisms of different species living in the same habitat Population C Made up of organisms and the physical environment they are living in p. 3 Level 2 2 Which of the following statements about an ecosystem is correct? A An ecosystem is made up of different biomes. B Continual input of materials is needed for a stable ecosystem. C In an ecosystem, the mean density of the population of each species is approximately constant. D Energy can be recycled in an ecosystem. p. 4, 5 19– 8  19 Ecosystems 19.2 Component of an ecosystem: the abiotic factors An ecosystem is made up of two components: the abiotic factors, which are the non-living components and the biotic community, which is the living component (Fig 19.5). In this section, we will focus on the abiotic factors in different ecosystems. example temperature, light, rainfall and humidity, abiotic factor* wind speed, water current, oxygen level, (non-living) salinity, nature of substratum ecosystem biotic community* including interactions among organisms (living) (to be discussed in Sections 19.3 and 19.4) Fig 19.5 Components of an ecosystem A Some common abiotic factors 1 Temperature The distribution and behaviour of organisms are greatly affected by temperature. This is partly because metabolic reactions in the cells are catalysed by enzymes, which only work best within a narrow range of temperatures. There is a lower diversity of organisms in areas of extreme temperatures (e.g. deserts and polar regions). 2 Light Light intensity mainly affects the rate of photosynthesis in plants. The duration of light in a day (day length*) also affects the flowering of plants. Light also affects the behaviour of animals. Diurnal* animals are active during the day and rest at night. Nocturnal* animals are active at night. Fig 19.6 The Masked Palm Civet* is a nocturnal animal They may develop good night vision or other special sensory methods suitable for a dark environment. abiotic factor 非生物因子 biotic community 生物羣落 day length 日照時間 diurnal 晝行的 Masked Palm Civet 果子狸 nocturnal 夜出的 19– 9  II Organisms and Environment 3 Rainfall and humidity Both rainfall and humidity affect the amount of water available in a habitat. Organisms in habitats with limited water supply develop special mechanisms to retain water in their body and reduce water loss. Fig 19.7 The cactus has spiny leaves to reduce water loss 4 Wind speed and water current In terrestrial ecosystems, strong wind may blow the soil away, making the soil not suitable for the growth of large plant species. In aquatic ecosystems, strong winds may generate strong waves that wash immobile or slow-moving animals away. Organisms living in windy Cross-link habitats or fast-flowing water develop body structures to hold themselves Refer to p. 13 and 16 for firmly onto the substratum*. They may have a streamlined* body form the adaptive features of to reduce their resistance when moving in water. organisms living in habitats with high wind speed and fast water current. Water current influences other abiotic factors. When the speed of the current is high, the oxygen level in water tends to be higher. The particle size of the substratum tends to be larger. Less fine sediment is accumulated. 5 Oxygen level The oxygen level in aquatic habitats depends on a number of factors: Factor Effect on oxygen level Reason Temperature The oxygen level decreases The solubility of oxygen when temperature increases. in water decreases as the temperature increases. Water current The oxygen level increases Splashes increase the contact when water flows more of water with oxygen. quickly. Organic matter The oxygen level decreases The decomposition of in the presence of organic organic matter uses oxygen. matter. streamlined 流線形的 substratum 基層 19– 10  19 Ecosystems 6 Salinity Salinity* refers to the concentration of dissolved salts and ions in water. It directly determines the water potential and hence the availability of water and minerals to the organisms. The salinity of seawater in the open ocean is relatively constant. The daily fluctuation of salinity in soil water can Fig 19.8 Mangroves are located in be large in the intertidal zone*. the intertidal sheltered shores 7 Nature of substratum Substratum refers to the material that the organisms are growing on or attaching to. It varies from bare rock to soil of different particle sizes (Fig 19.9). It also includes any stable surface provided by organisms, such as tree trunks and coral reefs. In soil with small particles, the air spaces among the particles are small (Fig 19.10) and the concentration of oxygen in the soil is lower. The soil is easily flooded with water (i.e. becomes water-logged*). gravel* (>2.0 mm) sand* (0.05–2.0 mm) clay* (< 0.002 mm) silt* (cannot be (0.002– observed at 0.05 mm) this scale) smaller soil particles larger soil particles Fig 19.9 The types of soil are classified according to Fig 19.10 The air spaces in soil with particle size (in terms of diameter) smaller particle size are smaller Besides the difference in particle sizes, different types of soil may also vary in pH value, oxygen, water, nutrient and humus* contents. Humus (Fig 19.11) is the major source of nutrients in soil for plant growth. It is formed from the decomposition of organic matter (including dead plants, animal parts and animal droppings) by microorganisms in soil. Humus can also hold a large amount Fig 19.11 Rotten leaves are gradually turned of moisture, and therefore increases the soil’s capacity to withstand into humus drought conditions. clay 黏土 gravel 砂礫 humus 腐殖質 intertidal zone 潮間帶 salinity 鹽度 sand 砂土 silt 粉砂 water-logged 鎖水 19– 11  II Organisms and Environment B Key abiotic factors in different ecosystems People often think that it is the abiotic factors which affect the organisms, but not vice versa. In the following, we will see that, in some cases, the abiotic factors in an ecosystem are greatly altered because of the organisms living there. 1 In woodlands Woodlands are composed of a variety of plants of differing heights (Fig 19.13). light and heat trees rain wind canopy layer* shrubs and small trees ferns and shrub layer* grasses Fig 19.12 Different layers of plants in a herb layer* woodland ground layer* humus soil air, soil water and minerals Fig 19.13 Composition of a woodland Light intensity and temperature differ at different layers of a woodland. The canopy layer forms a shelter against strong wind and heavy rain. Runoff refers to movement This helps prevent soil erosion* and reduce runoff *. of water across the soil surface. It occurs when The tree roots are deep. The extensive root system holds the soil irrigation, rain or snow-melt adds water to a surface faster particles together. This helps prevent soil erosion and reduce runoff. than it can enter the soil. A thick layer of humus on the ground layer maintains fertility of the soil. Fig 19.14 The canopy of trees in a Fig 19.15 Ground layer of woodlands is woodland covered with fallen leaves canopy layer 樹冠層 ground layer 地面層 herb layer 草本植物層 runoff 徑流 shrub layer 灌木層 soil erosion 土壤侵蝕 19– 12  19 Ecosystems 2 In coastal ecosystems Coastal ecosystems are located in intertidal zones. The substrata in these ecosystems are periodically exposed and submerged. Rocky shores and mangroves are two major types of coastal ecosystems. i) Rocky shores VR 19.1 Rocky shores have a hard substratum exposed to strong waves. The shores can be divided vertically into different zones based on the overall average exposure of the zones. The zones closer to the water surface are exposed to greater changes in abiotic factors like temperature and light intensity. Specific groups of organisms are found in each zone. This is called zonation* (Fig 19.16). Rock pools* may form in hollows on the hard rocks as the tide recedes. Marine organisms may be trapped. lichens* splash zone* periwinkles* high tide zone* limpets*, chiton*, rock oysters* mid tide zone* pool sea anemone*, fish algae*, sponges*, low tide zone* crabs, barnacles* sea urchins*, subtidal zone* sea cucumbers*, tube worms* Fig 19.16 Vertical section of a rocky shore showing the zonation of Fig 19.17 Zonation pattern shown on organisms the rocks Many organisms in the rocky shores develop ways to attach themselves firmly to the rocks. Most of them also have a hard shell for protection (Fig 19.18). Some organisms stay away from strong waves by hiding themselves in rock crevices. periwinkles chiton limpet barnacles rock oysters Fig 19.18 Organisms commonly found on rocky shores alga 藻 barnacle 藤壺 chiton 石 high tide zone 高潮帶 lichen 地衣 limpet 䗩 low tide zone 低潮帶 mid tide zone 中潮帶 periwinkle 濱螺 rock oyster 石蠔 rock pool 岩池 sea anemone 海葵 sea cucumber 海參 sea urchin 海膽 splash zone 濺浪帶 sponge 海綿 subtidal zone 潮下帶 tube worm 管蟲 zonation 成帶 19– 13  II Organisms and Environment ii) Mangroves VR 19.2 Most mangroves are located in sheltered bays. They are subject to tidal actions. There is a continuous exchange of water from the river and the sea. Therefore, the salinity of the mud fluctuates throughout the day. Mangroves are protected from wave action. Organic matter can easily accumulate in the soft mud and the oxygen level in the mud is low. Specialized forms of roots are developed in some mangrove trees for obtaining oxygen from the air. The roots are raised above the mud (Fig 19.19). Some mangrove trees can excrete excess salt absorbed through their leaves (Fig 19.20). a b Fig 19.19 (a) The knee joint* of Kandelia* and (b) the pneumatophores* of Black Mangrove* are raised above the mud to obtain oxygen from the air salt secreted Fig 19.20 The leaves of Aegiceras corniculatum* possess salt glands to excrete the excess salt Aegiceras corniculatum 蠟燭果 Black Mangrove 海欖雌 / 白骨壤 Kandelia 秋茄樹 / 水筆仔 knee joint 膝狀根 pneumatophore 出水通氣根 19– 14  19 Ecosystems The large amount of organic matter that accumulates in the substratum of mangroves provides a food source for a diversity of animals such as mudskippers, crabs, snails and bivalves (Fig 19.21). The distribution of the animals in the mud is affected by the oxygen level and particle size of the substratum. mudskipper* fiddler crab* bivalve* Inn Keeper Worm* sand snail* Fig 19.21 Some animals found in mangroves 3 In freshwater ecosystems VR 19.3 Hill streams are typically fast-flowing at the upper course. The water current gradually reduces as they flow downstream towards the lowlands. In the upper course, the oxygen level is usually higher. The particles of the substratum are usually larger. Fig 19.22 The water flow is more rapid upstream (left) and slower downstream (right) Strong water currents may wash animals away. Animals in fast-flowing water have different adaptations to overcome this problem (Fig 19.23 on the next page). bivalve 雙殼類 fiddler crab 招潮蟹 Inn Keeper Worm 絳體管口 mudskipper 彈塗魚 sand snail 灘棲螺 19– 15  II Organisms and Environment a b sucker on toe Some animals (e.g. Mayfly nymph*) are Some amphibians (e.g. Hong Kong flattened in shape so that they can hide Cascade Frog*) have suckers on their under rocks and reduce their resistance toes so that they can hold themselves when moving in water. firmly to the rocks. Fig 19.23 Adaptations of aquatic animals in fast-flowing water Plants along stream bank provide food for many aquatic organisms in the stream. They also serve important functions including regulating water temperature, purifying water by removing sediments and contaminants, reducing the risk of flooding and reducing erosion on stream bank. What are the abiotic factors in an ecosystem? Abiotic factors are the non-living components of an ecosystem. Examples include temperature, light intensity, day length, rainfall, humidity, wind speed, water current, salinity, oxygen level and the nature of the substratum. Level 1 Level 2 Questions 1 to 3: State whether the statements 4 Which of the following is correct about the are true or false. effect of fast water current on the abiotic 1 Abiotic factors in an ecosystem may change factors in a freshwater stream? throughout the day or the year. Oxygen level Particle size p. 9–11 in water of substratum 2 Abiotic factors affect the number of A increases increases organisms in ecosystems. p. 9–16 B increases decreases 3 Mangroves, rocky shores and freshwater C decreases increases streams are located in the intertidal zone. D decreases decreases p. 13–16 p. 10, 11 Hong Kong Cascade Frog 香港湍蛙 Mayfly nymph 蜉蝣若蟲 19– 16  19 Ecosystems 19.3 Component of an ecosystem: the biotic community A Describing the biotic community The biotic community is the living component of an ecosystem. There are some common terms used to describe the biotic community. 1 Niche and habitat The habitat can be A habitat is the place where an organism lives. It provides the analogous to a person’s conditions that the organism needs to survive. address, while the niche can be analogous to the job A niche* is the role that an organism plays in its environment. the person does. It refers to the way of life of an organism in its natural habitat. In a habitat, each species occupies a specific niche. The niche of a species is determined by the temperature range in which the organisms of the species live, the types of food they eat, and the space they occupy. Populations of two species cannot coexist if they have the same niche. Fig 19.24 On a mudflat, different birds occupy different niches and they can coexist in the same mudflat niche 生態位 19– 17  II Organisms and Environment 2 Species diversity Species diversity* refers to the variety of organisms that make up a community. It is determined by: species richness, which refers to the number of species in the community, and relative abundance of different species, which refers to the proportion of different species within the community. For example, consider the number of individuals of four species A, B, C and D in communities 1 and 2: Species A B C D Community 1 25 24 28 23 2 94 2 2 2 The species richness is the same in both communities. However, when the relative abundance of each species is considered, the four species are present in similar proportions in community 1 and we can easily notice the four different species. On the other hand, we might only notice the most abundant species A (the dominant species) in community 2. We say that ‘the species diversity of community 1 is higher than that of community 2’. 3 Dominant species Among the different species in a community, there are usually a few species that exert a strong influence on the composition and diversity of the community. These species are called the dominant species*. In terrestrial ecosystems, the dominant species are usually the commonest types of plants (Fig 19.25). The types of animals in a community greatly depend on the types of plants living in the same area. Fig 19.25 Evergreen broad-leaved trees are the dominant species in the woodlands in Hong Kong. dominant species 優勢種 species diversity 物種多樣性 19– 18  19 Ecosystems DSE B Interactions among organisms in a 12(IA)Q13, 14, 14(IA)Q31, 14(IB)Q5, community 17(IB)Q8, 19(IB)Q9 Organisms in a community interact with one another in different ways, leading to different results. Organisms may gain benefit from (+), be harmed by (–) or not be affected by (0) their relationships with other organisms. 1 Predation + / – Predation* is the interaction between two species in which one species hunts, captures and kills the other for food. The organism that eats the other is the predator. The one eaten by the predator is the prey. In this relationship, the predator gains benefit (+) while the prey is harmed (–). The populations of both the prey and the predator sometimes follow a cycle. The change in the population of the predator is always lagging behind that of the prey (Fig 19.27). Fig 19.26 An owl (predator) caught a Key: mouse (prey) for food. ❶ prey* predator* ❸ population ❷ ❹ ❺ time Fig 19.27 The predator-prey cycle Graph reading ❶ The prey has plenty of food. It breeds and increases in number. ❷ An increase in the prey population means that more food is available for the predator. So the predator population increases. ❸ A large number of predator feed on a large number of prey. So the prey population decreases. ❹ As the food supply becomes limited, the predator population decreases. ❺ The number of prey recovers and the cycle repeats itself. predation 捕食 predator 捕食者 prey 獵物 19– 19  II Organisms and Environment 2 Competition – / – Competition* means fighting for some common needs, e.g. food, space, shelter, sunlight and mates. All organisms involved in competition are harmed (–). There are two major types of competition: a Interspecific competition*: This refers to competition among organisms of different species. The stronger competitor would be able to occupy a more favourable area within the habitat. b Intraspecific competition*: This refers to competition among organisms of the same species. It is usually more intense than interspecific competition because individuals of the same species have the same niche and hence the same needs. Fig 19.28 shows the distribution of two species of barnacles (A and B) on the rocky shore. Both species feed on the same kinds of food. I II III Barnacle A can live in Barnacle B prefers to live When barnacles A and B both shallow and deep in deep water. coexist on the rocky shore, water on a rocky shore. barnacle A is mainly found in shallow water. Barnacle B is found in deep water. Fig 19.28 Distribution of two species of barnacles (A and B) on the rocky shore Cross-link In cases I and II, there is intraspecific competition among the Intraspecific competition is individuals of each species of barnacles. The individuals compete for actually beneficial for the species as a whole. It is food and space. They do not cluster together. a part of natural selection, allowing the best genes of In case III, there is interspecific competition between barnacles A and the species to be preserved B. Barnacle B is a stronger competitor so it can occupy the deep water and passed onto the next generation (refer to Bk 4, region. Barnacle A is restricted to shallow water. Ch 30). competition 競爭 interspecific competition 種間競爭 intraspecific competition 種內競爭 19– 20  19 Ecosystems 3 Commensalism + / 0 Two organisms may live together in such a way that one gains benefits (+) while the other is not affected (0). This type of relationship is called commensalism*. If too many barnacles are a Some barnacles live on the body of whales or sea turtles. The attached to the body, the barnacles benefit from being taken to different places by the whales whale may have difficulties in swimming. The relationship or sea turtles. The whales or sea turtles are generally not affected. between the barnacles b Some egrets stay on the back of elephants or cattle. The egrets and the whale is no longer commensalism in this case. benefit from feeding on insects stirred up by the animal as they graze in grassland. The elephants or cattle are unaffected. c Epiphytes grow on tree trunks to obtain more sunlight. They do not harm the trees as long as they are not too heavy. a b c barnacles sticking on a whale’s body an egret* staying on the back epiphytes* growing on tree of a cattle trunks Fig 19.29 Examples of commensalism 4 Mutualism + / + Two organisms may live in such a way that both gain benefits (+). This kind of relationship is called mutualism*. a Certain sea anemones live on sea anemones living on the shell of a hermit crab* the shells of hermit crabs. The sea anemones benefit from the movement of the hermit crabs and also from their food remains. The hermit crabs gain protection from the stinging cells* on the tentacles* of the sea anemones. Fig 19.30 Example of mutualism: sea anemones and hermit crab commensalism 偏利共棲 egret 鷺 epiphyte 附生植物 hermit crab 寄居蟹 mutualism 互利共生 stinging cell 刺細胞 tentacle 觸手 19– 21  II Organisms and Environment b Remora fish attach to the body of sharks. The remora fish eat food scraps dropped by the sharks. They also gain transportation and shelter from the sharks. The sharks benefit when the remora fish eat up the parasites* on their skin. shark remora fish* Fig 19.31 Example of mutualism: remora fish and shark c Sometimes, the mutualistic relationship between two organisms is so important that the organisms cannot survive if they are separated from one another. The lichen is an example. Lichens consist of algae and fungi living together. The algae make food by photosynthesis for both organisms while the fungi form a sponge-like body that holds water and provides protection and Fig 19.32 Lichens on anchorage for both (Fig 19.33). Together, they can grow on places barren rock where neither could survive alone, e.g. on tree trunks and barren* rocks. densely packed fungal hyphae layer of algae loosely packed fungal hyphae densely packed fungal hyphae (×180) Fig 19.33 Composition of lichen barren 光禿 parasite 寄生物 remora fish 短 19– 22  19 Ecosystems 5 Parasitism +/– Some organisms live on or inside other organisms, obtaining benefits (+) from them but causing them harm (–). This kind of relationship is called parasitism*. The organism which benefits is called the parasite. The organism being harmed is the host*. Cross-link a Tapeworms are parasites affecting some mammals (e.g. cats, pigs, Some organisms are human cattle and humans). They live inside their hosts’ intestines and parasites and can cause disease. Refer to Ch 23 for absorb digested food from them. The tapeworms benefit by obtaining more examples. nutrients from the hosts, while the hosts are harmed through the loss of nutrients. b Parasitic wasps lay eggs in the body of caterpillars. The larvae of the wasps (the parasite) feed on body tissue of the caterpillars (the host). The caterpillars then die. c Dodder is a parasitic plant. It grows by winding itself around the stem of another plant (the host) and absorbs nutrients from it. It does not need to have leaves, roots and chlorophyll. a b electron micrograph of a human tapeworm* a caterpillar covered with cocoons* of (×12) parasitic wasp* c dodder node stem of host plant dodder* winding around a host plant photomicrograph showing a dodder growing nodes to absorb nutrients from its host plant (T.S.) (×60) Fig 19.34 Examples of parasitism cocoon 繭 dodder 菟絲子 host 寄主 parasitic wasp 寄生蜂 parasitism 寄生 tapeworm 絛蟲 19– 23  II Organisms and Environment All organisms living in the same habitat compete with one another. Organisms living in the same habitat do not compete with one another if they use different resources. For example, in woodland areas, animals do not compete with plants for sunlight. Apple snails, an invasive species in Hong Kong Apple snails* were introduced to Hong Kong from South America in the early 1980s. They are widely distributed in freshwater habitats such as streams, ponds, semi-aquatic vegetable fields and wetlands in the New Territories. They feed on rice plants and vegetables like watercress* and water spinach*. a b Fig 19.35 (a) Apple snails and (b) their eggs on rice plants Apple snails were once raised as human food in Hong Kong. However, the raising soon stopped because the snail meat is tough and parasites may grow in it. Abandoned snails soon established large populations in various freshwater habitats. The apple snail is a strong invasive species*. They reproduce and grow quickly. They are resistant to a number of environmental conditions including high temperature and desiccation. They out-compete many local Cross-link invertebrates. In addition, they release waste to the water which becomes The impact of the nutrients for phytoplankton and algae. These may change the conditions overgrowth of algae on the of local ecosystems in a way that disturbs the living of local organisms. ecosystem will be discussed in Bk E2, Ch 1. Learn more about other invasive species in Hong Kong at: http://www.nature.edu.hk/glossary/invasive-species Choose one of the species as the topic and conduct a project on it. apple snail 福壽螺 invasive species 入侵物種 watercress 西洋菜 phytoplankton 浮游植物 water spinach 通菜 19– 24  19 Ecosystems 1 What is the habitat and niche of an organism? The habitat is the place where an organism lives. The niche of an organism is the role played by the organism in its community. It is determined by the temperature range in which it lives, the types of food it eats, and the space it occupies. 2 What are species diversity and dominant species in a community? Species diversity refers to the variety of organisms that make up a community. It is determined by the species richness and the relative abundance of different species in the community. Dominant species are the species in a community that exert strong control over the composition and diversity of the community. In terrestrial ecosystems, they are usually the commonest types of plants. 3 How do organisms interact with one another in an ecosystem? Organisms interact with one another and they may gain benefits (+), be harmed (–) or not be affected (0). Type of relationship Species 1 Species 2 Predation + (predator) – (prey) Competition – – (interspecific or intraspecific) Commensalism + 0 Mutualism + + Parasitism + (parasite) – (host) Level 1 Level 2 1 Refer to the graph below. What is the 2 Mikania micrantha* is a plant that grows possible relationship between species X rapidly on the neighbouring plants, thus and Y? reducing the amount of light that reaches species X them. Dodder is another plant that grows population size on Mikania and absorbs nutrients from it. Which of the following correctly matches species Y the relationship of Mikania with the other plants? time Neighbouring plants Dodder A predation A competition mutualism B competition B competition parasitism C mutualism C parasitism competition D parasitism p. 19 D parasitism parasitism p. 20–23 Mikania micrantha 薇甘菊 19– 25  II Organisms and Environment DSE C Ecological succession 17(IA)Q12, 18(IA)Q33 The variety of species of a community changes gradually as a result of interactions between organisms and the physical environment. The series of changes in the composition of a community over a period of time is called ecological succession*. During ecological succession, organisms modify their environment in ways that allow other species to come in and replace them. The process repeats itself and species diversity progressively increases as more and more species appear in the community. Species diversity reaches a maximum when a climax community* is reached. The changes in two types of ecological succession are shown below. Primary succession* starts with the colonization of a barren area where there were no soil or organisms before. 0–15 years 15–35 years 35–80 years 80–115 years 115+ years trees mosses, ferns shrubs and grass barren area with some lichens Fig 19.36 Primary succession Secondary succession* takes place in areas that once supported life but are now barren. 0–2 years 2–20 years 20–70 years trees 70+ years area that once supported life shrubs and grass and small trees ferns Fig 19.37 Secondary succession climax community 頂級羣落 ecological succession 生態演替 primary succession 原生演替 secondary succession 次生演替 19– 26  19 Ecosystems Animation 19.1 1 Primary succession Let us look at the process of primary succession on barren rocks after a glacial retreat*. Another example of I After the retreat of the glacier, barren rocks are exposed (Fig 19.38). barren area where primary As a result of weathering*, the rocks are broken up into smaller succession can occur is the land covered with cooled particles. Lichen spores are brought to the rocks by wind. The lava* after a volcanic lichens grow and form the pioneer community*. Lichens are often eruption*. the first group of species to colonize a barren area because they are able to grow in adverse, dry conditions. The dead bodies of the lichens are decomposed to become organic matter. The small rock particles and the organic matter gradually form the soil. II Mosses, ferns and grass start to grow in the soil (Fig 19.39). They become established over most part of the rocks and become the dominant species. They provide food and shelter for plant-eating animals. Fig 19.38 Barren rocks are left behind after the Fig 19.39 Lichens and mosses growing on the retreat of a glacier rocks III Dead, decaying plants and animals, as well as animal waste form humus which further enriches the soil. The soil becomes thick enough for the growth of shrubs. The shrubs grow over the mosses, ferns and grass and become the dominant species. More animals are attracted to the habitat because there are more food and shelter. IV Finally, the soil becomes thick and rich in nutrients. It can support the growth of trees which grow over the shrubs, and the trees become the dominant species. The habitat turns into a woodland that attracts increasing numbers of animals. A climax community is reached. glacial retreat 冰川後退 lava 熔岩 pioneer community 先鋒羣落 volcanic eruption 火山爆發 weathering 風化 19– 27  II Organisms and Environment The climax community has the following characteristics: It has the largest variety of species. A maximum biomass and the greatest organism interactions are maintained. Cross-link The energy input to the ecosystem is balanced by the energy Refer to p. 34 for the flow consumed. of energy in an ecosystem. If a climax community is disturbed by some external forces (e.g. a hill fire), it will redevelop until it attains stability again. Secondary succession takes place in this case. 2 Secondary succession Secondary succession can take place on hillsides after a hill fire or abandoned fields (Fig 19.40). a hillside after a hill fire an abandoned field Fig 19.40 Examples of areas where secondary succession takes place The stages that take place in secondary succession are similar to those in primary succession except the following: The pioneer community of lichens is not needed as soil is already present. Grasses and ferns form the pioneer community. Secondary succession usually develops more quickly than primary Cross-link succession. This is because seeds, roots and underground Refer to Bk 2, Ch 12 for how vegetative organs of plants may still survive in the soil. They a new plant is developed from an underground can grow and develop into new plants rapidly once the conditions vegetative organ. become favourable. What is the main abiotic factor that distinguishes primary succession from secondary ? succession? 19– 28  19 Ecosystems What are the differences between primary succession and secondary succession? Primary succession Secondary succession Where it In barren areas where In areas where organisms takes place no organisms and soil were present before but is were present before now barren Pioneer Lichens Grasses and ferns because community soil is already present Time needed Longer Shorter because seeds, roots to reach and underground vegetative climax organs of plants are present community in the soil Level 1 1 Which of the following does not happen during primary succession? A The species diversity increases. B The soil becomes more fertile. C The biomass increases. D More light reaches the substratum. p. 27 2 When grasses and ferns are growing on the hillside after a fire, what is the relationship between the grasses and the ferns? A mutualism B commensalism C competition D parasitism p. 28 Level 2 3 After a hill fire, grasses and ferns usually form the pioneer community in secondary succession. Which of the following is/are the correct explanation(s)? (1) Their seeds and spores are resistant to high temperatures. (2) They can grow rapidly. (3) They are difficult to burn. A (1) only B (1) and (2) only C (2) and (3) only D (1), (2) and (3) p. 28 19– 29  II Organisms and Environment Watch this to prepare for your class and answer the 19.4 Energy flow in an ecosystem questions. An ecosystem functions as a self-supporting unit in which there are energy flow and cycling of materials between its physical environment and biotic community. In this section, we will discuss in detail how Video Questions energy flows in an ecosystem. A The sun as the ultimate source of energy Cross-link In most ecosystems, energy ultimately comes from the sun. Light You will learn about how energy is captured by green plants through photosynthesis. It is then sunlight is captured for photosynthesis in Ch 20. converted into chemical energy (stored in food and body material) in the plants. The energy is transferred to animals when the animals feed on the plants. It is further transferred when the animals are eaten by other animals. In other words, energy flows in the form of food in an ecosystem. fox is eaten by wolf energy from the sun grass captures sunlight for photosynthesis chemical energy rabbit is eaten in fox by fox chemical energy chemical energy in grass in rabbit grass is eaten by rabbit Fig 19.41 Energy flow in the form of food Cross-link The processes that lead to energy lost are not shown Energy is lost from the organisms through different processes. Therefore, in Fig 19.41. They will be discussed in more detail on a continual input of energy from the sun is essential for maintaining a p. 33. stable ecosystem. 19– 30  19 Ecosystems DSE B Food chains and food webs 18(IA)Q14, 29, 32 We can construct food chains* and food webs* to represent the feeding relationship among organisms. 1 Food chains The feeding pattern in Fig 19.41 can be represented by the food chain below: grass rabbit fox wolf ( means eaten by) A food chain shows the following: the sequence of organisms in a particular feeding relationship; the direction of energy and material flow from one organism to another. Organisms in a food chain may act as producers*, consumers* or decomposers*, depending on their roles in the energy flow. i) Producers Most food chains start with producers. They are organisms which can make their own food (i.e. autotrophs*). Most of them are photosynthetic, e.g. green plants, algae and photosynthetic bacteria. Fig 19.42 Algae are the major producer They capture light energy and convert it into chemical energy stored in in the sea organic compounds. ii) Consumers Organisms that depend on other organisms for food are consumers. Heterotrophs can be further They are also called heterotrophs*. They feed on other organisms. divided into three groups By feeding, they transfer energy along food chains in the form of food. according to what they eat. carnivores*: eat meat only Depending on their positions in a food chain, consumers are classified herbivores*: eat plants only into: omnivores*: eat both plants and animals primary consumers*, which feed on producers; secondary consumers*, which feed on primary consumers; tertiary consumers*, which feed on secondary consumers. In the food chain above, the rabbit is the primary consumer. The fox is the secondary consumer, while the wolf is the tertiary consumer. autotroph 自養生物 carnivore 肉食性動物 consumer 消費者 decomposer 分解者 food chain 食物鏈 food web 食物網 herbivore 草食性動物 heterotroph 異養生物 omnivore 雜食性動物 primary consumer 初級消費者 producer 生產者 secondary consumer 次級消費者 tertiary consumer 三級消費者 19– 31  II Organisms and Environment 2 Food webs Most animals feed on more than one type of organism. At the same time, a particular type of organism can be eaten by more than one type of organism. Therefore, the feeding relationships in an ecosystem are often complicated. We can connect related food chains together to form a food web. Fig 19.43 shows all the possible feeding relationships among selected organisms in a forest ecosystem. bear An organism may play different roles in different food chains. For example, in Fig 19.43, the fox is a secondary consumer when it eats the rabbit. It becomes wolf a tertiary consumer when it eats the bird. fox When drawing a food web, pay attention to the following points: bird Producers should be put at the bottom of the food web. deer rabbit grasshopper caterpillar Organisms at the same feeding level, e.g. primary consumers, should be put at the same level. grass oak tree Crossing of arrows should be avoided. Fig 19.43 A food web in a forest where grass and oak tree are producers DSE C Energy lost along trophic levels 12(IB)Q6, 16(IA)Q30, 17(IA)Q34, 35 A trophic level* is a level in a food chain. It is the feeding level of an organism depending on what it feeds on, and by what organism it is eaten. Energy flows from lower to higher trophic levels. For example: Organism in the food chain Role in the food chain Trophic level Direction of energy flow Wolf Tertiary consumer Fourth ↑ Fox Secondary consumer Third ↑ Rabbit Primary consumer Second ↑ Grass Producer First trophic level 營養級 19– 32  19 Ecosystems A large portion of energy is lost to the surroundings when it is transferred from one trophic level to another. There are four main ways in which energy is lost (Fig 19.44): 4 Respiration* 3 Excretory products* energy to next trophic level 1 Uneaten materials of producer faeces Key: 2 Undigested and energy lost egested materials Fig 19.44 Ways of energy loss from producer to primary consumer 1 Uneaten materials Some organisms can escape from their predators all through their lives. Chemical energy stored in the uneaten body material is not passed to higher trophic levels and is lost from the food chain. 2 Undigested and egested materials When organisms are eaten by organisms at a higher trophic level, usually a large proportion is indigestible and unabsorbed. For example, cellulose in plants, and bones and hair in animals are hardly digested. The undigested and unabsorbed materials will be passed out as faeces (i.e. egested). Therefore, not all the energy stored in the eaten organisms can be transferred to organisms at the next trophic level. 3 Excretory products Energy is lost through the excretory products (e.g. carbon dioxide and urea) of organisms. The energy inside the excretory products cannot be transferred to organisms at higher trophic levels. 4 As heat during respiration All organisms carry out respiration. Much of the chemical energy in the food eaten is released as heat or used to support various body functions. Only a very small proportion of energy (usually less than 10%) is transferred from one trophic level to the next. Therefore, less energy is available to the organisms at a higher trophic level (Fig 19.45 on the next page). excretory product 排泄物 respiration 呼吸作用 19– 33  II Organisms and Environment dead bodies, egested materials heat loss due fourth trophic level and excretory products to respiration (tertiary consumers) dead bodies, egested materials and excretory products heat loss due third trophic level to respiration (secondary consumers) decomposers heat loss dead bodies, egested materials due to