3.5.3 Energy and Ecosystems

Ecosystems

• An ecosystem includes all organisms living in a particular area and all the abiotic conditions
• In all ecosystems, there are producers (organisms that make their own food)
• In photosynthesis plants use energy and carbon dioxide to produce glucose and other sugars
• Some sugars produced by photosynthesis are used in respiration, for growth
• The rest of the glucose can make other biological molecules, (e.g. cellulose) which make up a plants biomass
• Energy is transferred through organisms via food chains, when organisms eat other organisms

Measuring Biomass with a Colorimeter

• Biomass can be measured in terms of the mass of carbon that an organism contains or the dry mass of its tissue per unit area per time
• Dry Mass is the mass of an organism with the water removed. This can be measured using a bomb calorimeter
• To measure dry mass, firstly dry the sample in an oven at a low temperature. Then weigh the sample at regular intervals. Once the mass is constant, all water has been removed
• If needed, the result from the sample can be scaled to give the biomass of the total population/area under investigation.
• Mass of Carbon present is generally 50% of the dry mass
• Biomass tends to change over time, so its useful to measure it over time.
• The amount of heat given off tells you how much energy is present. This is measured by the temperature change of the water

GPP and NPP

• GPP is the total amount of Chemical energy converted from light energy by plants, in a given area in a given time
• 50% of the gross primary production is lost to the environment as heat when plants respire. This is known as respiratory loss
• Remaining chemical energy is the net primary production (NPP)
• NPP=GPP-R
• NPP is the energy available for plant growth and reproduction . It is also the energy available for organisms at the next stage of the food chain, including herbivores and decomposers

Calculating Consumer Net Production

• Consumers store chemical energy in their biomass
• Consumers get energy by ingesting plant material, or animals that have eaten plant material
• Not all chemical energy stored in the consumers food is transferred to the next trophic level, as roughly 90% is lost in various ways;
• -not all food is eaten
• -from what is ingested, some are egested as faeces and some is lost to the environment via respiration or excretion of urine
• energy that remains after this is stored in the consumers biomass and is available to the next trophic level (this is consumers net production)
• Net Production of Consumers can be calculated using the following formula:
• N=I-(F+R)

Food Chains and Food Webs

• They show how energy is transferred through an ecosystem
• Food chains show simple lines of energy transfer. Each of the stages in a food chain is called a trophic level
• Food webs show lots of food chains in an ecosystem and how they overlap
• Decomposers are part of food webs, as they break down dead or undigested material, thus allowing nutrients to be recycled

Farming Practices

• Aim to increase amount of energy available for human consumption. This can be done in 2 ways; energy lost to other organisms (by simplifying food webs) and energy lost via respiration (by reducing respiratory losses)
• Simplifying food webs:
• *Getting rid of pests:
• -Chemical Pesticides can be used in the form of insecticides, that kill pests that eat and damage crops. This means less biomass is lost from crops, so they grow to be larger, as productivity is greater. They can also be in the form of herbicides, that kill weeds. This eliminates direct competition with the crop for energy from the sun, as well as preferred habitats or food sources of the insect pests, which helps to further reduce their numbers and simplify the food web
• -Biological agents can also reduce the number of pests, so crops lose less energy and biomass, thus increasing energy transfer to humans. There are 2 types of Biological agents; Parasites and Pathogenic bacteria and viruses. Parasites lay their eggs on a pest insect, thus either killing it or reducing its ability to function. Pathogenic bacteria and viruses just produce toxins.
• -Integrated systems can be used, to reduce pest numbers
• Reducing respiratory losses:
• *Controlling the conditions that they live in, so more energy is used for growth and less is lost through respiration;
• -Keeping animals in warm pens to restrict movement and ensuring less energy is wasted by generating body heat. This produces more biomass and stores more chemical energy, so productivity increases and the efficiency of energy transfer to humans increases.
• -one advantage is that more food can be made quicker at lower cost, however this method raises ethical issues, as some people believe the conditions cause the animals distress, pain and restricts their natural behaviour