Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong) Variety of Life and Relation of Organisms with their Environment ...... Page 54 Energy flow and the nutrient cycling in an ecosystem An ecosystem comprises biotic and abiotic components which interact extensively with each other. Based on their ecological roles, the biotic components of an ecosystem can be classified as : Producer : they are the green plants which absorb solar energy to synthesize complex organic compounds from simple inorganic substances by photosynthesis, they act as the ultimate food source to all the heterotrophs : other producers are green algae and blue green algae, they are mainly found in aquatic habitat, such as freshwater and marine water, they are the most important producers in earth (as 70% of the earth surface is covered with water). Consumer : they are heterotrophs which ingest other organisms or organic particles, they are mainly animals a) primary consumer : they are the herbivores which feed on plants e.g. pond snail, insect larva and zooplanktons b) secondary consumer : they are the carnivores which feed on primary consumers e.g. water beetles, tigers, etc. c) tertiary consumer : they are large carnivores which feed on the secondary and primary consumers as well as producers, e.g. man d) detritus consumer : they are detritivores (detritus feeder / scavengers) which feed on detritus that refer to the particulate organic matter involved in the decomposition of dead organisms, e.g. earthworm and crab etc. Decomposer : they are mainly bacteria, fungi and some flagellates : by means of their saprophytic activities, they decompose the eliminated products of animals and the dead bodies of the organisms into simple compounds : these compounds are absorbed as nutrients by the green plants again : they enable the nutrients to be used continuously in a cyclic form in the ecosystem : they are most abundant in the soil or water bottom where the dead bodies of plant and animals accumulate : when the temperature conditions are favourable, decomposition occurs rapidly Energy and essential materials are therefore transferred from producers to consumers through the feeding processes. Eventually, decomposers break down the organic matter and release inorganic materials back to the environment. These inorganic materiasl are used by the producers as nutrients again. Food chain : the transfer of food energy from producers through a series of organisms with repeated eating and being eaten Primary secondary Tertiary Ecosystem Producer consumer consumer consumer freshwater pond green algae → protozoa → mosquito larva → fish rocky pond sea weeds → molluscs → starfish → sea birds Grassland grass → grasshopper → lizard → snake Woodland green plants → caterpillars → sparrow → hawk Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong) Variety of Life and Relation of Organisms with their Environment ...... Page 55 Food web : in general, the food chains in an ecosystem are not isolated, but are interconnected with one another, i.e. an herbivore may feed on several species of plants, and/ or be consumed by many consumers and so on, such a number of interconnected food chains is known as food web Trophic level : organisms in a food chain occupy different trophic levels, which indicate their place in the energy flow through the community : organisms whose food is obtained from plants by the same number of steps in the food chain belong to the same trophic level producers - first trophic level primary consumers - second trophic level secondary consumers - third trophic level tertiary consumers - fourth trophic level BS I 3rd ed. p300 fig 10.2 Fig. 35 A simple schematic comparison of terrestrial and aquatic ecosystems Energy flow in the ecosystem I. Productivity : Refers to the amount of energy or living materials fixed in a population, or a trophic level, or an entire ecosystem in a given time a) gross primary productivity : rate of dry matter production by photosynthesis in an ecosystem, it does not represent the actual amount of food potentially available to heterotrophs because some of the organic matters are used to meet plant respiration and metabolism Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong) Variety of Life and Relation of Organisms with their Environment ...... Page 56 b) net primary productivity : it is the biomass which is incorporated into a plant community during a specific time intervals, minus the part respired c) secondary productivity : it is the rate of incorporation of biomass at the consumer levels during a specific time interval [Note] Production is the difference in biomass within a certain time interval. II. Energy flow : • as solar radiation passes through the biosphere, large parts are used to perform other functions, only small parts can be used in photosynthesis Energy Dissipation Percent / % Reflected 30 Direct conversion to heat 46 Evaporation, precipitation (drives hydrological 23 cycle) Wind, waves and currents 0.2 Photosynthesis 0.8 Total 100 BSI 3rd ed. p302 fig10.4 Fig. 36 Flow of energy and cycling of materials through a typical food chain. • the energy supplies for the grass, only a small part is used in the synthesis of organic materials, much are lost • part of the primary productivity is used in the respiration of the producers, of the net primary productivity, a portion is eaten by the primary consumers, the rest remains unused and is passed as dead plant materials • of the food eaten by the primary consumers, some is assimilated and incorporated into the body tissue while others are unabsorbed and are discarded in the form of faeces and other wastes • the same loss of energy occurs when the energy is transferred from the primary consumers to the secondary consumers III. Ecological pyramids : The feeding relationships between organisms at different trophic level within a community can be represented by ecological pyramids. The producers (green plants) form the base of the pyramids can be structured according to numbers of organisms, total biomass, or total energy flow at each trophic level. Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong) Variety of Life and Relation of Organisms with their Environment ...... Page 57 a) Pyramid of numbers : the progressive loss of energy at each trophic level of a food chain puts a natural decrease on the total weight and total number of living organisms that exist at each successive level in the chain : the number of organisms in each trophic level is usually smaller than that of the one they are feeding on thus can be expressed in the form of a pyramid, called the pyramid of numbers : for diagrammatic purposes the number of organisms in a given trophic level can be represented as a rectangle whose length is proportional to the number of organisms in a given area 4 th trophic level 3 rd trophic level 2 nd trophic level 1 st trophic level Examples : Normal A tree is the A tree which is Grasses are pyramid producer, on infested with eaten by a cow which worms parasites and which is are the lived, the latter are infested with that are eaten parasitized by parasites. by birds. further parasites. b) Pyramid of biomass : the graphical representation of the trophic structure for a community of organisms in terms of the biomass [Note] Biomass is defined as the total dry weight of the total amount of living materials presented at a trophic level Fig. 37 Changes in standing crop biomass of producers and primary consumers and in certain environmental variables in a lake during one year. BSI 3rd ed. p307 fig 10.8 Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong) Variety of Life and Relation of Organisms with their Environment ...... Page 58 : two forms of pyramids of biomass can be constructed, the upright pyramid and inverted pyramid : if the producers support herbivores, and herbivores support carnivores then upright pyramid of biomass is the one we might expect : however an inverted pyramids of biomass is found in open water and deep water ecosystem where the producers are small and short-lived, the phytoplanktons (producers) with smaller biomass can support the zooplanktons (primary consumers) with larger biomass because of their much rapid rate reproduction (high turnover rate) [Note] The data are collected only over a limited duration, thus the pyramids of biomass only indicates the amount of material presents over a very short period of time, and gives no indication of the total amount of material produced or the rate at which that material is being produced c) Pyramid of energy : at each transfer of energy from one trophic level to the next higher, there is always a loss of energy from the system : energy transformation is never 100% efficient and the living organisms in each trophic level required certain amount of energy for maintaining basal metabolic rate, for growth and reproduction and movement : it overcomes the difficulty encountered in pyramids of number and biomass, i.e. inverted pyramid never occurred here : it shows the total amount of energy utilized by the organisms in different trophic level in a square metre, over a given period of time, i.e. it shows the amount of new tissues of organisms produced in a unit time (productivity) : it permits comparison of trophic structure of different ecosystem e.g. a desert and a tree forest Exercise : (90 I 7b) Distinguish between production and biomass. [2 marks] (98 I 11) Why is the pyramid of energy always upright whereas the pyramid of biomass can sometimes be inverted ? [4 marks] IV Biogeochemical cycles : The fundamental difference between the flow of energy and the flow of materials in an ecosystem is that the latter can flow through the ecosystem and be recycled again to be available to producers, e.g.