Herbivores fractionation

Ambrose and Norr (1993) and Tieszen and Fagre (1993) have shown that 5 C of carbonate in bone apatite (6 C,p) is the most accurate measure of the whole-diet composition (Ambrose and Norr 1993 28). The actual 5 C of total diet is related to that of apatite by an isotopic offset (fractionation) which Ambrose and Norr estimate to be 9.5 0.6%o. Other estimates range from 9.6 0.1%o for small mammals on controlled diets (DeNiro and Epstein 1978) to 12%o for large herbivores on natural diets (Lee-Thorp et al., 1989). The origin of this offset is of some concern to us here. We can only use 5 Cap as a measure of total diet if we know A,p.j,e, and also know that this fractionation is a constant, at least for a given species, and does not itself depend on the quality of the diet. 

Several authors, beginning with Kmeger and Sullivan (1984), have noted a smaller 8 C fractionation between collagen and apatite (A,p.co) in carnivores as compared to herbivores. These authors presented a model to accoimt for this, and the observation was confirmed in studies by Lee-Thorp and van der Merwe (1991) of populations of carnivores and herbivores that differed widely in their intake of C3- and C4-based foods. Here 1 shall show how this effect might be accounted for as a consequence of partial blocking of AA synthesis from lipids. 

Most accounts of the larger A,p.,.o in carnivores have attributed this effect to higher proportion of lipids in the diet of carnivores. This arises because carnivores obtain all or most of their nutrition from the flesh of other animals, a significant part of which is composed of lipid. By contrast, lipids make up a much smaller fraction of the total carbon pool in the diet of herbivores, particularly mminants which get much of their energy from digestion of cellulose. Humans who selectively use seeds and grains as food sources obtain a... 

Figure 10.2. Schematic diagram showing how restricted conversion of fatty acids to amino acids influences the fractionation between collagen and CO3 of bone apatite LI = lipid component, PR = protein, T = total isotopic composition AP = COj component of apatite, a) Herbivorous diet (Cj plants only) b) Carnivorous diet, assuming rj = 1 (no barrier to fatty acid conversion to AAs) c) Carnivorous diet, assuming ri < 1 note that carbonate-collagen fractionation is smaller.

Figure 10.4. Effect on apatite-collagen isotopic fractionation due to inhibition of amino acid production and preferred use of exogenous amino acids. Carnivore and herbivore, both based on C3 plants, have similar bulk isotopic composition of total edible tissues (T), leading to similar 5 C for apatite carbonate (AP). Collagen (CO) of carnivore is more enriched in Cthan that of herbivore, because of preferential utilization of amino acids derived from protein (P) of herbivore flesh in construction of carnivore s proteins. C ss = assimilated carbon.

Plants are the base of the terrestrial food chain. The body tissue of herbivores feeding on these plants, and subsequently of omnivores and carnivores, is dependent on the isotopic composition of the plant material ingested, and therefore reflects the 813C values of either a C3 or C4 biome. The exact values are modified by metabolically induced fractionation as the food is digested and incorporated into different body tissues. At its simplest, it has been assumed... 

Figure 4. The fractionation of carbon between diet and different tissues in mammalian herbivores...

There have been numerous studies examining the selection of data for an SSD. Forbes and Calow (2002) made the point that only a fraction of the species going into the SSD determines the effects threshold. With all species being weighted equally, the loss of any species is of equal importance to the system, while keystone or other important species are assumed to be randomly distributed in the SSD. For example, the ecologically realistic distribution of species by trophic level was 64% primary producers, 26% herbivores (invertebrates), and 10% carnivores (fish), compared to the mean ratio from SSDs for different chemicals of 27.5, 34.7, and 37.8%, respectively. Such variations were shown to alter the SSDs by as much as 10% (Duboudin et al. 2004). A sensitivity analysis performed on available data for chromium (VI) in marine waters (Table 4.8) shows how additional data points, or selective removal of data, have an impact on the derived 5th percentile (HC5). The effects are relatively small but can be higher for the 1st percentile data (HC1). Our view is that, provided the data set includes numbers of sensitive and insensitive species equal to or above the minimum data set, it is considered to be adequate. 

A theoretical model for carbon isotope fractionation between herbivore diets and herbivore bone is shown in Figure 2A This model shows the isotopic relationship of gelatin to that of hydroxyapatite and the relationship of both to the original plant food diet The gelatin seems to reflect the growth portion of the diet while the apatite seems to reflect the energy portion of the diet The difference of - 1 o/oo between apatite values and gelatin values is due in part to the fact that blood bicarbonate has a enrichment produced by the transfer of CO from blood plasma to expired air This effect has been experimentally verified and will be described elsewhere ... 

Figure 2. Top, theoreticaJ. model for carbon isotope fractionation between diet and bone in herbivores and bottom, isotopic analyses of contemporary herbivore bones compared to the theoretical model (solid line).

Herbivores may obtain a small fraction of their energy needs from amino acids. 

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