In order for an organism to live in any given habitat, it must have the necessary materials that it needs for growth and reproduction. Anything that is essential for an organism’s survival, and for which there is competition, is called a limiting factor. A deficiency or an over-abundance of any kind can limit the survival of an organisms in a particular habitat. For example, cattails growing along the shore of a lake requires a marshy condition where the water is not too deep. In a pond where the bottom is soft and the water is shallow, cattails cannot survive. However, they can survive at the pond’s edge.
Some limiting factors have to do with living things. These limiting factors are called biotic factors. Food is a good example of biotic factors because animals can live only where the kinds of food they eat are available. If limiting factors have to do with nonliving things such as air, temperature, sunlight and water, it is called an abiotic factor. At this time we will examine several biotic and abiotic limiting factors and discuss their affect on the habitats of plants and animals that exist within the pond ecosystem.
Most aquatic organisms have a narrow tolerance to change in temperature and therefore cannot live where drastic temperature changes occur. However some animals are equipped with special mechanisms that helps them to adjust to temperature change. A primary example being cold blooded animals because they can adjust by temporarily reducing their metabolism which results in a suspended state of animation. Once normal temperatures reoccur the animal returns to its normal activity.
Some plants and animals adjust by developing special hibernation phases or forms. Some aquatic plants produce winter buds which breaks off and drop to the bottom of ponds. These winter buds realistically are terminal shoots in which leaves are very close together which acts as a stored food supply. Among animals, various devices are common in order for them to survive the wintry cold waters. Some examples include: winter eggs of certain microscopic animals such as rotifers, cladocerans and rhabdocoels and the cysts of many protists, copeds and annelids.
Other animals may adapt by moving into a different environment. Fishes and snails move away from the from the shore into the deeper water. Most insects go to the bottom of the pond and burrow into the substratum to hibernate. Reptiles and amphibians do the same, but some dig holes in the mud, trash and debris. The majority of freshwater fishes and some amphibians, can withstand fluctuations in temperature and survive temperatures well below freezing in winter and fairly high temperatures in the summer.
The amounts of light that is able to penetrate water is restricted by the amount of suspended material in the water and the color of the water. Light is essential to all green plants in order for them to make food. Likewise, green plants and their stored energy are needed by animals in order to survive.
Excessive algae growth and turbid waters can have a devastating affect on a pond because it hinders the amount of light that can pass through the water. Since the green plants can not receive the necessary amount of sunlight, they will die off, thus causing a decrease of available oxygen and food in the water which will kill animal life in the water.
C. Carbon Dioxide
Most of the carbon dioxide in water comes from the decay of organic materials and from respiration that occurs in both plants and animals. Carbon dioxide is also supplied to ponds dissolved in groundwater and rain. The amount of dissolved carbon dioxide is usually higher at the bottom of ponds. Carbon dioxide is used by plants during photosynthesis. Photosynthesis is the process in which organisms containing chlorophyll are able to use the energy from the sunlight to unite carbon dioxide and water to form the sugar glucose, and other compounds that derived from it, and a source of energy. In doing this they use oxygen and release carbon dioxide in respiration. It is the source of carbon found in proteins, fats, and carbohydrates, the basic food substances of animals.
Carbon dioxide plays an important role in determining the pH, the degree of alkalininty or acidity, of water. It combines with water forming weak carbonic acid which reacts with limestone or dissolved lime to form carbonates and bicarbonates. These compounds serve as buffers that regulate pH. The pH of water often determines what types of aquatic life are found in ponds.
There are two ways in which oxygen enters the water. One method is through photosynthesis of green plants in the water. The second method is by diffusion of oxygen from the atmosphere. The amount of oxygen that the waters retain is dependent upon the temperature, pressure, decomposition and pollution. Clean shallow surface water averages an oxygen content of five to ten cubic centimeters per liter. Cold water will hold more oxygen than warm water; rapid moving waters contain more oxygen than stagnant waters. Water containing decaying organic matter will show a drop in oxygen content because the aerobic bacteria involved in decomposition uses the oxygen.
As the sunlight penetrates the water during the daylight, plants give off oxygen as a by-product of photosynthesis more rapidly than it is used in respiration by living things. Therefore there is a reserve of oxygen built up. At night when photosynthesis ceases, both plants and animals use the reserved oxygen. The oxygen content in ponds fluctuates greatly within a twenty-four hour period. If the oxygen content of water is suddenly reduced, fish will tend to come to the surface in order to gulp air.
E. Nutritional Relationships
Most of the biotic limiting factors involve food. The autotrophs require inorganic nutrients from the environment to synthesis organic compounds. These autotrophs are called food producers. There are three different types of food producers found in the pond: (l) the emergent, rooted plants like water lilies, and cattails found near the shore; (2) the submerged plants like hornworts and eelgrass; and (3) the suspended algae which form the phytoplankton. Microscopic examination of a few drops of pond water usually reveals hundreds of one-celled algae.
Phytoplankton may increase so much that lake water turns to a dark green color. Small crustaceans such as ostracods and copepods feed well on the algae blooms. Since these animals feed on plants, they are called herbivores and are first food consumers of an ecosystem. The energy synthesized and stored by phytoplankton is transferred to the protoplasm of the herbivores. The carnivores, flesh-eating animals are divided into two groups: (l) the first-level carnivores, which eat and use the energy of the herbivores; and (2) the second-level carnivores, which prey on the first-level carnivores
The scavengers feed on dead organisms. They are important in the cycling of chemicals and the transfer of energy to the animals in the ecosystem that feed on them. Crayfish, some snails and fish are some examples of scavengers. The bacteria and yeast are representatives of decomposers in the pond. They break the excretions and tissues of organisms into simpler substances through the process of decay.
Other bacteria present in the mud bottom of the pond and in the soil convert the simpler substances left by the decomposers in to nitrogen compounds, which are used by plants. The bacteria that do this are called transformers. Both decomposers and transformers return nitrogen, phosphates, and other substances to the soil or water so that the plants can begin the cycle again. Matter would not be available for recycling, or reuse, in an ecosystem if decomposers did not exist.
Small amounts of organic pollution may increase the fertility of a body of water if it can be decomposed without seriously reducing the oxygen content. However, if the oxygen content is seriously reduced, anaerobic decomposition will be initiated and undesirable gases will form. Even though ponds may be able to survive some forms of organic pollutions, it is practically impossible for a pond to survive inorganic pollutants such as those from chemical, radioactive wastes, etc... because of their potency.
Sticklebacks, carps, eels, stoneworts and duckweed can survive in polluted waters. Other animals show a wide range of tolerance to organic pollution. Most animals that can survive in highly polluted waters have a high tolerance level in the presence of waters with a reduced oxygen supply. Many of the protists can live in these type of conditions. Many of the bivalves and snails have a high tolerance level for pollution. However, the absence of these and other mollusks from ponds is often a warning of pollution.
People are often misled when they use certain physical conditions of ponds as indicators for pollution. Foamy area in ponds, fluorescence or distinctive odors may be good evidence that a pond may be polluted. However, foam may be due to certain harmless inorganic salts or some species of phytoplankton; the odors and fluorescence may be due to the presence of certain protozoans.