Photosynthetic Pathway Impacts Herbivores

Megafaunal changes are correlated with a shift between C3- and C4-dominated ecosystems. Several lines of evidence suggest relationships between turnover of mammalian grazing taxa and the shifts between C3/C4 vegetation types. Cerling et al (1998a) reported abrupt changes in mammalian lineages in East Africa asso-

8180 paleosol trend

10-point average

613c paleosol trend

10-point average (f) -10

8180 paleosol trend

10-point average

613c paleosol trend

10-point average (f) -10

Percent G. bulloides (

Percent G. bulloides (

FIGURE 9 Faunal Change Index from the Pakistan, represented by the number of first (nf) and last {n{) occurrences, including only occurrences (n0), normalized to species richness (nsr). The Faunal Change Index is normalized to 1.0 for the total data set. Adopted from Cerling et al (1998b).

dated with the transition from C,-dominated to C3/C4-dominated ecosystems. During the same time period, Cerling et al (1998b) showed that abrupt changes in faunal diversity of Pakistani mammals occurred at the same time as the emergence of C4-dominated ecosystems in Pakistan (Fig. 9). Evolutionary relationships between C3/C4 and horses appear to be somewhat different (Mac-Fadden and Cerling, 1996; MacFadden et al, 1996). Evolution of the modern horse is associated with the transition from "browsing" to "grazing" horses, which is typically marked by the lengthening of the Ml molar, creating the high-crowned tooth (Fig. 10). However, the evolution of the M1 tooth and the increased diversity of horse taxa was not associated with the global expansion of C4 ecosystems, because these changes occurred in a C3 world. However, the crash in biological diversity of horses at 6 Ma is associated with the expansion of C4-dominated ecosystems into regions that once contained only C3 plants (Fig. 10).

Modern mammalian herbivores exhibit strong preferences for C, versus C4 diets (Fig. 11), with only limited numbers of examples of mixed C3 /C4 feeding (Figure 3). While it may be difficult to quantify the exact percentages of C3/C4 within the diets of some mammals, it is possible to classify the extreme grazers and browsers: hypergrazers with nearly 100% C4 grass and hyperbrowsers with nearly 100 % C3 browse. It is remarkable that the herbivore mammals of the savannahs and grasslands of Africa falls into such distinct C3/C4 dietary categories, with extreme hypergrazers such as the wildebeest standing distinct from grazers such as the oryx and zebra (Fig. 11). The modern African elephant

M1 height [mm]

Carbon isotope ratio of tooth enamel |% "

M1 height [mm]

Carbon isotope ratio of tooth enamel |% "

Morphology Change

Diversity Change

Diet Change

FIGURE 10 A chronology of horse evolution. Top plate: morphological changes in the height of the Ml tooth. Middle plate: diversity changes as recorded by the number of extant genera. Bottom plate: carbon isotope ratios of tooth enamel illustrating that the transition from browsing horses to grazing horses was not associated with expansion of C4 ecosystems, but that the loss of genera was correlated in time with C4 expansion. Adopted from Cerling (1999).

Morphology Change

Diversity Change

Diet Change

FIGURE 10 A chronology of horse evolution. Top plate: morphological changes in the height of the Ml tooth. Middle plate: diversity changes as recorded by the number of extant genera. Bottom plate: carbon isotope ratios of tooth enamel illustrating that the transition from browsing horses to grazing horses was not associated with expansion of C4 ecosystems, but that the loss of genera was correlated in time with C4 expansion. Adopted from Cerling (1999).

giraffe

iDH

black rhino

MD

browsers

Grant s gazelle

O

eland

(-Π0

dik-dik

HIH

zebra

white rhino

O

grazers

wildebeest

»

hartebeest

o|o

oryx

FIGURE 11 Ranges in the carbon isotope ratios of diets for African browsers and grazers. Adopted from Cerling et al (1999).

FIGURE 11 Ranges in the carbon isotope ratios of diets for African browsers and grazers. Adopted from Cerling et al (1999).

(.Loxodonta) is often regarded as a grazing animal, yet its isotopic composition strongly shows that these animals are distinctly C3 browsers (Cerling et al, 1999). In contrast, a million years ago elephants were distinctly grazers.

The selective basis for differential C3/C4 herbivory may be related to the differential distributions of leaf protein within C3 and C4 leaves (Ehleringer and Monson, 1993). In C3 plants, relatively high protein levels are found in most mesophyll cells. These cells have relatively thin cell walls, especially when compared to the much thicker bundle-sheath cell walls (Brown, 1977). In contrast, there is relatively more protein within bundle-sheath cells of C4 leaves than in mesophyll cells. Thus, tooth morphology in mammalian grazers would be expected to play a role in determining whether or not animals were able to extract sufficient protein from their C3/C4 diet. Insects such as grasshoppers show a strong preference for C3 or C4 food sources, but typically not for both (Isely, 1946; Caswell et al, 1973; Boutton et al, 1978; Ehleringer and Monson, 1993). Here it is known that there are significant differences in mandible morphology correlated with C3/C4 dietary preference.

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