It's a Small World After All! (Pond Microcosms) Background Information

Related Topics: ecosystems, pollution, change, variables (control vs. treatment), energy, matter, the Earth as a closed system (macrocosm)

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Background Information:

• Microcosm. A microcosm (micro=small, cosm=world) by definition is a self-contained miniature world. The study of microcosms allows students to investigate many facets of a closed system.
• Closed system. A closed system is one where only energy flows in and out of the system. Any matter present continually cycles within the system. A microcosm is an example of a small closed system. The Earth is an example of a large closed system.
• Biodiversity. Biodiversity (bio=living, diversity=variety) is a term which is used to describe the variety or richness of living organisms in an area. An area rich in biodiversity tends to have more complex food web relationships and is therefore more likely to be sustained over time than an area with poor biodiversity.
• Physical Factors. Physical factors such as available space and available oxygen play an important role in this investigation. Life is affected from the moment that organisms are placed in a microcosm. This new system, significantly smaller than the pond ecosystem, does not provide adequate space for plant life needed to sustain larger consumers. In short, it is an "unbalanced system." Over time, the oxygen level is adjusted as plants (algae, duckweed) find their niche. Organisms that can compete in this system survive. Those that cannot are selected out of the system. Eventually a "balanced system" with small plants, small consumers, and reduced oxygen levels evolves.
• Choosing a Pond. When selecting a pond, keep in mind that small naturally occurring ponds tend to be richer in diversity than man-made ponds.
• Healthy Pond Water vs. Healthy Drinking Water.
  • The healthiest pond water has a tremendous amount of biodiversity (living organisms).
  • The healthiest drinking water has NO biodiversity.
• Controlled Investigations. Controlled investigations involve:
  • deliberately changing one variable at a time (e.g., adding rice to one microcosms while leaving a second microcosm unchanged)
  • observing the effect on another variable (e.g., microcosm ecosystem diversity)
  • holding all other variables constant (e.g., same size jars, same amount of light exposure)
• Manipulated, Responding, and Controlled Variables. Note: The information in the chart below applies to the What Is the Effect of Rice on Microcosm Biodiversity? portion of the inquiry.

• Control vs. Treatment Groups. In controlled experiments, scientists deliberately control circumstances in order to obtain evidence. In this experiment, the control group consists of a pond microcosm that is unaltered. The treatment group consists of a pond microcosm with eight grains of uncooked rice added. Throughout the experiment, the two groups are compared and contrasted in order to identify how diversity is affected as a result of rice pollution.
• Replication. Scientists must be able to demonstrate that their results are accurate by replicating (repeatedly getting the same result under the same conditions). Since you will have six or more teams conducting the experiment, this is a perfect opportunity to illustrate the value of replication in drawing conclusions.
• Observations. There is life in virtually every drop of pond water. Without a microscope, the following can be observed in the typical pond microcosm: egg sacks, daphnia, varieties of insect nymphs, larva, algae, dead plant and animal matter, snails. With a microscope, the following can be observed in a typical pond microcosm: embryos in egg sacks, body parts of the daphnia (including a visible heart beating), specific identification of various nymphs and larva, specific identification of algae, rotifers, flatworms, ostrocods (seed shrimp), and more.
• Change. Each microcosm jar is cloudy when first filled due to the mixing of water and soil. In the first 24 hours, much settling of the microcosm occurs. Over time each microcosm will adjust to the space, water, energy, and air limitations of the jar. Specific changes that can be anticipated are outlined below.
  • Adaptation. As larger organisms are selected out due to decreased oxygen levels, certain microorganisms find their niche in this ever-changing environment. Algae, protozoans, rotifers, and bacteria, to name a few, thrive over the first several weeks/months, while larger consumers such as snails and small insects will eventually die out.
  • Treatment Group. Within the first few days, much of the life that can be observed without a microscope dies in the treatment jar. This is due in part to the concentrated energy released by the pollution (rice) in the system. The bacteria that decompose the pollution quickly rob the system of oxygen.
  • Control Group. The control, or non-polluted, microcosm will typically undergo a more gradual change as the system seeks a balance between the living and nonliving components.
  • Hydrogen sulfide, known for its rotten egg or sewage smell can be observed first in the treatment jar, then later in the control jar. Hydrogen sulfide is a waste gas produced by the growing population of bacteria in the system. The increase in bacteria is due to the energy released by the pollution source (treatment jar) or due to the dying organisms that provide additional energy used by the bacteria (both jars).
• Pollution. By definition, a pollutant is a substance that substantially alters the quality of a system. In simpler terms, pollution occurs when there is too much of something in an area. In the treatment microcosm, eight grains of rice are used as a pollutant in order to contrast polluted (treatment) and unpolluted (control) ecosystems. Rice is a concentrated energy source which causes rapid bacterial growth and, consequently, a rapid loss of oxygen to the system. The resulting rapid oxygen loss due to this pollutant is what truly alters the system.
• Energy Flows and Matter Cycles. Two of the great laws of nature are energy flows and matter cycles. Energy is constantly flowing into a system, such as a microcosm, as radiant energy from sunlight or fluorescent light. This energy is then transformed into the energy needed to sustain life for organisms such as plants and animals. This energy is used for life processes such as respiration, photosynthesis, digestion, and reproduction. Once used, it is transformed into heat energy and leaves the system. This heat energy is then dispersed into the atmosphere and, in time, into space. As this energy is lost to the system, new energy flows in to replace it. Matter, on the other hand, is constantly cycling within a system. Plant and animal waste products, as well as dead plants and animals, are broken down by decomposers (bacteria and fungi) and returned as nutrients to the soil, air, and water. As these nutrients are used by plants, they cycle back to the living part of the system once again.
• Interdependence. In any system, interrelationships contribute to the overall balance. A food web can be constructed to show the interdependence of organisms with each other and their energy source (radiant energy) in this system.
• Caution(s). Allow time for students to thoroughly wash hands after working with open microcosms, just as they would after visiting a pond or playing outside.
• Expected Results. 
  • For How Much Biodiversity Exists in a Pond Microcosm?, expect students to find a large variety of consumers and several varieties of producers within their pond microcosm ecosystems.
  • For What is the Effect of Rice on Microcosm Biodiversity?, expect students to see marked differences in the two microcosms within the first week. The treatment microcosm will appear lifeless shortly after the pollutant is added while the control microcosm will undergo more gradual change. Over time, both microcosms will find a balance between living and nonliving components within the system. Expect both microcosms to continue to support microscopic life.