Trophic level

The biotic structure is similar in all ecosystems and includes three basic categories of organism-feeding relationships that interact forming a food web  

All other organisms, which are heterotrophs, must consume organic material made by autotrophs in order to obtain energy and nutrients. In this way they may eat plants (herbivores) or animals (carnivores), or decompose the remains of other organisms and their waste products (decomposers). Herbivores are primary consumers, carnivores are secondary consumers, or they may be tertiary consumers if they eat other consumers. 

Decomposers, such as many fungi and bacteria, obtain energy by breaking down complex molecules in tissues of dead plants and animals. These decaying organisms are essential for the health of all ecosystems because they cycle important nutrients. 

Despite their complexity, food webs basically follow a series of steps or levels—from producers to primary consumers, to secondary consumers, and so on. These feeding levels are called trophic levels. All producers belong to the first trophic level all primary consumers belong to the second trophic level, and so on. All feeding relationships can be visualized as a flow of nutrients and energy through a series of trophic levels. There are no more than three or four discernible trophic levels in any ecosystem. 

For instance, a typical food chain in a field ecosystem might be 

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Environmental problems associated with PCBs are the result of a number of factors. Several open uses of PCBs have resulted in thein direct introduction into the environment, eg, organic diluents careless PCB disposal practices have resulted in significant releases into aquatic and marine ecosystems higher chlorinated PCBs are very stable in thein persistence in different environmental matrices and by a variety of processes (Fig. 1) PCBs are transported throughout the global ecosystem and preferentiaHy bioconcentrate in higher trophic levels of the food chain. 

Bioconcentration, Bio accumulation and Biomagnification. These aspects are determined by the physicochemical properties of a chemical, an organism s ability to excrete the chemical, the organism s lipid content and its trophic level. Bioconcentration relates to the difference between the environmental concentration and that of the body tissues. A high bioconcentration factor (BCF) predisposes to bioaccnmulation. The upper limit of bioaccnmulation is determined by lipid levels in the organism s tissues. Whether the resultant body burden causes biomagnification in the food chain depends upon the metabolic capabilities of the exposed organism. 

Biomagnincation- Increase in the concentration of a chemical substance as it is progresses to higher trophic levels of a food chain. 

As shown in Figure 2, when a herbmore eats a plant, only about 10 percent of the energy scored in that plant is converted to animal biomass the rest is used up in everyday activities. The same is true for each succeeding trophic level. Note that the commonly cited 10 percent energy transfer figure is... 

In a temperate forest ecosystem on Isle Royale, Michigan, ecologists found that it takes 762 pounds (346 kg) of plant food to support every 59 pounds (27 kg) of moose, and that 59 pounds of moose are required to support every one pound (0.45 kg) of wolf. The basic point is that massive amounts of energy do not flow from one trophic level to the next energy is lost at each stage of the food chain, so there are more plants than herbivores and more herbivores than carnivores. 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Fish from Lake Erie are generally the least contaminated of all the Great Lakes IS), It has been speculated that contaminants in a more advanced eutrophic system become masked or removed by sedimentation within the food chain and have less opportunity to reach higher trophic levels 24), The management implications of this interaction between nutrient and contaminants needs to be further elucidated. 

Fig. 3-9 Trophic levels in ecosystems. Thin arrows show flow of energy up the food chain (through living biomass) and the broad arrows show the complementary flow of dead organic matter (detritus) back down. R indicates respiration.

Trophic levels are imprecise categories many organisms, such as human beings, obtain their energy from two or more trophic levels. 

The consequences of the massive "loss" of energy accompanying passage from one trophic level to another also include the fact that organisms low in the trophic ladder tend to dominate the cycling of elements through the biosphere. This is especially true on land where vascular plants dominate both the physical structure and... 

The first case is based on a hypothetical stream receiving a metal-poUuted source (for instance the outlet of a metal factory). In this case, metal concentration is expected to be driven by dilution, being higher under low-flow than under base-flow conditions and minimum during floods. Metal accumulation is expected to be maximum under low-flow conditions and proportional to the duration of this water scarcity situation. Chronic exposure will lead to community adaptation, which is often related to changes in species composition. Metals will therefore be bioaccumulated in fluvial biofihns and transferred to higher trophic levels in the fluvial food web. 

Dibutyltin. A larger data set exists for dibutyltin, including both acute and long-term test results. The lowest concentration identified was a chronic NOEC of 0.015 mg/1 for Daphnia magna exposure to dibutyltin chloride. Long-term values were available across three trophic levels, and, therefore, an uncertainty factor of 10 was considered appropriate. 

Bones of 19 individuals were analyzed for strontium, rubidium and zinc. The number of samples was limited by the availability of bone after the stable isotope analyses were completed. Strontium was analyzed in order to test for trophic level, and to compare to other results obtained in the region on prehistoric peoples (Katzenberg 1984). Rubidium is not expected in human bone, so its presence acts as a measure of contamination. The use of zinc as a paleodi-etary indicator has been questioned recently (Ezzo 1994) and we were interested to see if there was any relationship between zinc content in food and bone. 

Sex and age differences in stable isotopes of nitrogen and carbon are not pronounced. There is no evidence that males and females were eating different foods and the only evidence for age differences, higher 8 N in infants, has been explained by the trophic level shift during the time the infant derives most of its protein from, breast milk. The small amount of variation in both and 5 N values supports the historical sources, which indicate that while food was plentiful, the diet was rather monotonous. 

Stepwise enrichment in nitrogen was first observed in marine (coastal) ecosystems (Minagawa and Wada 1984 Miyake and Wada 1967). The latter quote an average A N of -h3.4 l.l%oper trophic level, occurring... 

It can be argued that the gut flora provides an additional trophic level in ruminants (Steinhour et al. 1982). This should result in a larger A N in ruminants than in non-ruminant herbivores. As many domesticated animals are ruminants this is a factor that has to be taken into account. Available 5 N data does not seem to show systematic differences between the categories ruminants and non-ruminants, although systematic species differences exist. 

How do carbon and nitrogen trophic level effects compare We have argued that the cause of shifts in carbon might be diverse, and that these shifts are probably variable in size. Especially because of this ambiguity we feel that preferably the term trophic level effect should be avoided for carbon. Given that nitrogen trophic level effects are much more pronounced and universally fairly similar in size (possibly outside of arid areas), it should be possible to calculate food 5 N values by subtracting the trophic level effect from ancient bone 8 N values. 

All these factors make comparisons with other archaeological indicators of paleodiet more complicated. Nevertheless, nitrogen trophic level effects, together with measurement of the A CapaiU,.coii,g.n spacing, seems to be the best way to quantify proportions of animal protein in the diet. 

Table 3.2 shows the 5 Cu and 5 Cc values of herbivores, omnivores, carnivores and humans. The (climate-corrected) trophic level effect between herbivores and carnivores is 0.90%o. Human values are closer to carnivore and omnivore values than to herbivore 5 Cc values. The human 5 Cc values are on average 0.66%o more positive than the herbivore 5 Cc values, a good estimate for a carnivore effect in humans (see section on trophic level effects, below). The average human 5 Cc value is -19.92 1.28%o,which would indicate that Holocene humans in Europe had a diet that consisted of C3 terrestrial foods, whieh is as might be expected. By looking at the humans separate from the total bone data set, we notice potential human food selection (Fig. 3.3) we can see a non-climatic pattern, which is much less uniform than in the total bone data set (Fig. 3.2b). Italy (6 Cc = -21.3%o) has a much more negative 8 Cc value than the Czech Republic (8 Cc =-18.7%o), Spain (8 Cc = -19.3%o) and Greece (-18.9%o but the 8 N of 9.0%odoes not indicate marine food), while the northern European coimtries are closer to a 5 Cc value of-20%o. What the actual causes are for this pattern in the human samples is not clear to better understand these variations it is best to consider, where possible, the 8 N values with the 8 Cc values. 

Hesslein, R.H., Capet, M.J., Fox, D.E. and Hallard, K.A. 1991 Stable isotopes of sulphur, carbon, and nitrogen as indicators of trophic level and fish migration in the lower Mackenzie River Basin, Canada. CanadianJoumal of Fisheries and Aquatic Science 48 2258-2265. 

Hobson, K.A. 1990 Stable isotope analysis of marbled murrelets evidence for fieshwater feeding and determination of trophic level. Condor 92 897-903. 

Schoeninger, M.J. 1985 Trophic level effects on and ratios in bone collagen and... 

These isotopic differences between tissues within individuals between species and trophic levels can be used to check the preservation of the isotopic signatures in Pleistocene samples of cold temperate and arctic enviromnents. 

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