Pollutants, Community Structure & Population Dynamics

A pollutant is any substance introduced by human activities into an ecosystem which produces harmful effects on resident populations thus altering community structure.
Pollutants may alter the abiotic environment thus indirectly affect population dynamics. 1) Increases in atmospheric CO₂ --- “Greenhouse” effect; Chicago-Gary air pollution and precipitation at LaPorte, IN. 2)Hot water --- waste heat --- Thermal pollution. 3) Nutrient enrichment --- Eutrophication.
Pollutant may be toxic (effect survival and /or reproduction of individuals) thus directly affect population dynamics. 1) Radioactive atoms (radioisotopes) --- I¹³¹, St⁹⁰ , etc. 2) Heavy metal compounds --- Cu, Hg, Pb, etc. 3) Man-made organic molecules --- DDT, PCB’s, Dioxine, 2-4-5-T, etc.
Of course, human activities can effect community structure and population dynamics in other ways --- “Environmental Alteration”.

Trophic Transfer, Biomagnification and Toxic Substances

The movement of compounds (molecules) through trophic levels is called trophic transfer. Toxic substances, as well as nutrients, can be transferred through trophic levels. Substances which are not transformed are particularly suitable for trophic transfer, as are radioactive atoms. Biological magnification is the increase in concentration of a substance in successive members of a food chain. Toxic substances may accumulate in members of higher trophic levels as a result of biomagnification. Some examples: 1) Strontium⁹⁰ (a radioactive atom); 2) DDT (a man-made organic compound).

Trophic Transfer and Biomagnification of Strontium⁹⁰

The concentration of Strontium⁹⁰ tends to increase at successive trophic levels. Biomagnification of St⁹⁰ is due to : 1) physiological similarity with calcium .. a mineral nutrient retained by plants and animals (e.g. bone and shell); 2) biomass transfer through food chains. St⁹⁰ is a radioactive material with a 1/2 life of 28.1 years and was produced during atmospheric nuclear testing. Like other radioactive matter, St⁹⁰ causes mutations ... random changes in DNA(genes).

Trophic Transfer and Biomagnification of DDT

The concentration of DDT tends to increase in food chains from one trophic level to the next. Biomagnification of DDT is due to : 1) high solubility in tissue fats -- low solubility in water; 2) very low rate of DDT metabolism in tissues -- low rate of loss from body; 3) transfer of biomass from one trophic level to text -- as a “rule to thumb”, 10-20 grams of producers are required to maintain 1 gram of primary consumers and so on .......

Nutrients, Eutrophication and Community Structure

Eutrophication is the accumulation of nutrients in aquatic ecosystems. The addition of phosphorus and nitrogen in sewage (organic matter) brings about changes in abundance of plant and animal populations found in lakes, ponds, streams, rivers, and estuaries. Added nutrients stimulate growth of algae and plant populations to high densities, after which rapid declines in population sizes create large quantities of detritus (organic matter). Bacterial populations rapidly decompose detritus, thus lowering dissolved oxygen. Fish and other consumer populations “die off” as the result of low O₂ levels. See Dr Poulson’s hypothesis on the chains of indirect events leading to cultural eutrophication on page 66 of Murray and Nyberg, 2000.

Water Chemistry and Eutrophication

Temperature -- Max. temperature tolerated by carp (37^o C) and rainbow trout (24.5^o C). Thermal pollution can change species composition.
pH -- Below 5 and above 8 harmful to many animal species.
Dissolved O₂ -- 8-15 mg per liter for game fish. Decomposition lowers O₂ concentration.
Carbon dioxide -- 10 mg/ liter or more fatal to fish. Decomposition lowers CO₂ concentration.
Ammonia -- < 1 mg per liter in natural bodies of water. Decomposition increases NH₃ (or NH₄^-) concentration. High levels stimulate algal blooms and are toxic to fish.
Nitrates/Nitrites -- < 1- 10 mg per liter in natural bodies of water. Decomposition increases nitrates and nitrites. Nitrates stimulate algal blooms.
Phosphates -- Often < 0.1 mg per liter in natural bodies of water. High levels stimulate algal blooms.

Changes in Biomass Due to Eutrophication

The biomass of producers in the eutrophic ecosystem is greater, but by far the biggest change is in the relative contributions of the green food web and the brown food web. In eutrophic ecosystems, a smaller proportion of net primary productivity is passed up the food chain to consumers. Instead, this biomass dies and becomes detritus, supporting scavengers, detritivores, and decomposers. See the details of change given in the diagram on page 68 of Murray and Nyberg, 2000.

Changes in Water Chemistry and Biota Following Adding Sewage to a Stream

Read the play-by play example of the events leading to eutrophication on pages 62-65 in Murray and Nyberg, 2000.