Community factors influencing biomagnificationExpert Group __
Several population or community factors may influence the concentrations of toxins in a given species or individual, independent of the general trend of higher trophic level organisms having higher concentrations of toxins. Age, size, and feeding behaviors all influence food sources, and therefore ingestion, of toxins.
In general, larger organisms feed higher on the food chain, though there are many exceptions. Since most organisms get larger through their life span, older organisms will tend to be larger, feed higher on food chains, and have higher concentrations of toxins found in their food supply. Older organisms have also been living and eating longer, giving them more time to accumulate toxins.
Some organisms go through significant changes through their life histories, perhaps changing their place in a food chain from, for example, a larval to an adult stage. Omnivores naturally shift among food sources from different trophic levels. Individuals within a species may vary in their primary food source in the same system, resulting in variation within the population.
Finally, some members of the biotic community, decomposers, may play roles in the availability of toxins. For example, mercury exists in different forms in the ecosystem, and some forms are more easily taken up by organisms than other forms. Some forms of mercury are less abundant and may reside in sediments, thereby kept out of the food chain. Bacteria play a role in changing mercury to a form more easily absorbed by and therefore more likely to accumulate in organisms, increasing the potential that mercury will be magnified up the food chain. The bacteria convert mercury under anoxic (low oxygen) conditions. Therefore, both the members of the biotic community and abiotic environmental conditions can influence the degree of biomagnification in a given situation.
Expert Discussion – Guiding question
- For your topic, list at least five factors or conditions that might create complexity, or cause the expected relationship between trophic levels and biomagnification to vary.
Physiological factors influencing biomagnificationExpert Group __
Several factors at the physiological level of the organisms may influence the concentrations of toxins in a given species or individual, independent of the general trend of higher trophic level organisms having higher concentrations of toxins. These may include where the toxin is stored in the organism, the toxic response of the organism, and possible multiple pathways for ingestion of the toxin.
Most toxins have a particular point in the body where they are stored, such as in fat tissue, or in an organ like the liver. Therefore, if the entire organism is not consumed, only a portion of the toxin in the prey may move up the food chain. Also, when we sample only particular tissues withouttaking this differential storage into consideration, we can underestimate toxin concentrations.
Toxins accumulate in organisms through both direct and indirect pathways. Primary producers may directly assimilate the toxins from the soil (i.e., rooted plants) or water (i.e., algae). Consumers may assimilate toxins directly through respiration (e.g. fish accumulate toxins through their gills as they respire) or indirectly through consumption of contaminated prey. Once heavy metal toxins are assimilated, they are not excreted and instead accumulate as the organism ages. As toxin concentrations increase in the body, organisms are increasingly susceptible to disease, neurotoxic damage, organ failure and a variety of other health problems depending on the particular toxin. Older cohorts within a population feed higher on the food web, and may therefore experience increased mortality rates as a result of prior exposure.
Mercury (Hg) is a heavy metal that can damage the nervous system. In areas where humans regularly consume fish or birds highly contaminated with mercury, neurotoxicological effects of poisoning have been well documented. For pregnant women that ingest contaminated foods, the developing nervous system of the fetus may be particularly susceptible to damage from mercury. Mothers with no symptoms of mercury poisoning may give birth to infants with severe disabilities, or may expose infants to mercury through breast feeding. These effects extend beyond childbirth; children exposed to mercury while in utero have been shown to score lower on standardized tests and to experience delayed development.
Cadmium (Cd) accumulates in the human kidneys, and low level exposure over long periods can result in kidney damage. Cadmium has also been shown to produce osteomalacia in children, a painful bone disorder similar to rickets. Cadmium is also a carcinogen.
Expert Discussion – Guiding question
- For your topic, list at least five factors or conditions that might create complexity, or cause the expected relationship between trophic levels and biomagnification to vary.
Ecosystem and landscape factors influencing biomagnificationExpert Group __
Several landscape or ecosystem-level factors may influence toxin concentrations in an individual or species, independent of the general trend of higher trophic level organisms having higher concentrations of toxins. The location of toxin sources influences their distribution across the landscape; sources can be either “point source” (having identifiable points of origin, such as in a storm drain or factory drain pipe) or “nonpoint source” (diffuse points of origin, such as runoff from land or atmospheric deposition). The highest toxin concentration will be found closest to a source, with higher concentrations near point sources compared to nonpoint sources. Plants and animals living close to toxin sources are likely to have higher toxin concentrations compared to organisms living further from these sources.
Plants and animals can also change the way a toxin is spread across the landscape. For example, wetland plants and sediments surrounding a body of water can absorb toxins entering in runoff before they reach the water body itself, in a process known as “buffering”. A buffered lake, therefore, should have lower toxin concentrations in its waters compared to a lake without these buffers. The organisms living in a buffered lake should have lower toxin concentrations in their tissues than organisms from an unbuffered lake. Even a lake with a point source of a toxin could have low toxin concentrations in its waters if it is surrounded by buffering plants and sediments. Therefore, one must take multiple factors into account when considering the likelihood of high toxin concentrations and biomagnification in a landscape or ecosystem.
Two toxins that are unevenly spread across the landscape are cadmium (Cd) and mercury (Hg). Cadmium enters the environment from both natural and anthropogenic (human-caused) sources. The natural sources of Cd include the weathering and mining of rocks and minerals that contain cadmium. Anthropogenic sources of Cd include the mining and electroplating industries, which either use or produce Cd, as well as cigarette smoke. Cadmium enters water bodies or the atmosphere as a result of these sources, and organisms are exposed directly in the water, through inhalation, exposure from atmospheric deposition on soils or other surfaces, or through the consumption of plants or animals that have accumulated Cd in or on their tissues.
Mercury (Hg) is emitted into the atmospherethrough anthropogenic sources, such as manufacturing or burning coal for fuel, and from natural sources, such as volcanoes. Atmospheric Hg can be transported over a range of distances, potentially resulting in deposition on local, regional, continental and/or global scales. The availability of Hg for uptake by organisms is dependent upon the conversion of inorganic Hg into methylmercury by bacteria. Once Hg is converted into methylmercury, it can accumulate up the food chain in fish, fish-eating animals, and people.
Expert Discussion – Guiding question
- For your topic, list at least five factors or conditions that might create complexity, or cause the expected relationship between trophic levels and biomagnification to vary.