Module 6 Ecosystems

Question 1

Ecosystems

Answer 1

• Species interact with each other to form communities. Communities interact with each other and the environment they live in to form ecosystems.
• Ecosystems vary greatly in size and complexity and there is a flow of energy and recycling of nutrients through them.
• Biotic factors include predation, competition, disease, parasitism and mankind.
• Abiotic factors include availability of water, pH, temperature, humidity, light intensity and soil composition.

Question 2

Biomass transfers

Answer 2

• Biomass is the mass of living material and chemical energy stored in an organism. It can be measured as dry mass, mass of carbon (generally 50% of the dry mass) or the energy content released when burned in pure oxygen.
• Dry mass is the mass after all water has been removed. It may change over time, perhaps due to seasons. Calorimetry can be used to measure the energy content stored in dry mass. The dry mass should be burnt in a calorimeter and heat a known volume of water. The change in temperature is recorded.
• To find the dry mass the sample should be heated at a low temperature in a crucible until there is no longer a change in mass, as all the water has been lost and the sample is fully dehydrated.
• Limitations of calorimetry include that it takes a long time to fully dehydrate the sample, it requires precise equipment, and heat can be lost to the surroundings, in which case a bomb calorimeter should be used.
• Efficiency of transfers is affected in producers as not all sunlight is converted to chemical energy. Only certain wavelengths are absorbed for photosynthesis and some sunlight falls away from plants.
• Only a small percentage of biomass is transferred from producers to consumers. This is because not all biomass of the producer is eaten or digested. Some biomass is converted to chemical energy by heating or movement.
• Efficiency = biomass transferred/biomass intake x 100.
• Net productivity of producers = gross productivity – respiratory losses.
• Net productivity of consumers = chemical energy ingested – (faeces and urine losses + respiration loses).
• Humans can increase efficiency to maximise agricultural productivity. Arable farmers can use artificial lights and greenhouses, fertilisers and pesticides. Livestock farmers can use good quality feeds, antibodies, reduce competition and predation and use indoor husbandry to reduce movement and energy loss from heating.

Question 3

Nitrogen Cycle

Answer 3

• Nutrients are recycled in ecosystems. Saprobionts are true decomposers like fungi and bacteria which secrete enzymes and absorbs the products by extracellular decomposition. This process is called saprobiotic nutrition. Mycorrhizae represents the relationship between plants and fungi. Fungi have hyphae which greatly increase the surface area of the plant’s roots, meaning they can take up more water and minerals. In turn the fungi receive glucose from the plant.
• Nitrogen is abundant in the air and used in nucleic acids, but it is relatively inert. It must be fixed to be used in living organisms. Nitrogen can be fixed by lightening, the harber process or bacteria.
• Azotobacter that lives freely in the soil and rhizobium which lives in root nodules of leguminous plants fix nitrogen by converting it to ammonium ions (NH4+).
• Nitrifying bacteria – Nitrosomas convert ammonium ions to nitrites (NO2-) and Nitrobacter converts nitrites to nitrates (NO3-).
• Denitrifying bacteria converts nitrates to nitrogen gas through respiration.
• Bacteria and fungi are involved in saprobiotic decomposition.

Question 4

Carbon Cycle

Answer 4

• Carbon is recycled in ecosystems, so the number of carbon atoms remains constant.
• Carbon is stored in the atmosphere, as fossil fuels, dissolved in oceans, soil, vegetation and sedimentary rocks.
• Carbon dioxide is fixed by autotrophs through photosynthesis. CO2 is converted into organic molecules and carbon is removed from the atmosphere. This occurs in the Calvin cycle, when rubisco carboxylates RuBP.
• Sedimentation occurs when plants are not fully decomposed and instead compress to form fossil fuels such as peat and coal.
• All life forms respire, although heterotrophs rely on respiration as their only source of energy. Respiration releases CO2 into the atmosphere through the link reaction and Krebs cycle. Anaerobic respiration also releases CO2 during fermentation.
• Carbon is passed from autotroph to heterotroph and from one consumer to another by feeding.
• Waste matter such as faeces and urine as well as decomposition by saprophytes releases stores of carbon.
• Burning of fossil fuels releases CO2 into the atmosphere at a faster rate than it is taken out, leading to global warming.

Question 5

Succession

Answer 5

• Ecosystems are dynamic and can change from being very simple to complex, as biotic and abiotic factors change over time.
• Primary succession begins when newly formed land is colonised. This may have been formed by a volcanic explosion, or a lake drying up and exposing bare rock.
• Pioneer species arise from seeds and spores carried by the wind to the new land. These are generally moss and lichen. When they die, the dead organic matter forms a humus which forms a soil.
• From there small plants arrive and the soil becomes deeper and more nutrient rich. Roots from these plants hold the soil in place. Larger plants can grow, and more water can be held in the soil.
• Large trees are the dominant species in a complex ecosystem and make up the climax community.
• Each step of succession leaves abiotic factors more suitable for the next species and less suitable for previous species.
• Deflected succession is when human activities prevent or interrupt succession. Livestock may eat plant shoots or grass may be mowed. This may happen because there is a higher level of biodiversity present at intermediate stages of succession. Succession can be prevented by temporarily grazing animals or managed burning.

Question 6

Distribution/abundance

Answer 6

• Sampling measures distribution – where species are found - and abundance – how many species are present - in large and complex ecosystems.
• Random sampling is used when distribution is uniform and there is no clear pattern. Sampling points are chosen due to chance and there is no sampling bias.
• Systematic sampling is used when sampling points are chosen. They may show bias and be unrepresentative of an area.
• Quadrats are used to mark off an area being sampled. They are laid randomly to avoid bias using a random number generator. The abundance of species or percentage cover of the quadrats is measured.
• Belt transects are used when there is a change in physical conditions throughout an area. Systematic sampling is used, and quadrats are placed at regular intervals along a transect.
• Mark-release-capture is used for motile animals. A first sample is taken, counted, marked and released back into the habitat. Time passes before a second sample is taken and the number of marked/unmarked individuals is counted.
• Use N = (n1 x n2)/m2. N = population size. n1 = size of first sample n2 = size of second sample m2 = no. of marked individuals in second sample. The equation assumes that sufficient time has been taken for the population to disperse, marking doesn’t affect survival chances, marking remains visible.
• To measure abundance in difficult habitats, biologists use the qualitative ACFOR rating system.