untitled

the case where the deer had a much greater opportunity to develop detailed know-

ledge of the landscape. Similar patterns have been recorded in cattle (Laca et al. 1993;

Distel et al. 1995) and dorcas gazelles (Gazella dorcas) (Ward and Salz 1994).

Large herbivores, particularly grazers, might also have good reason to avoid using

patches of dense vegetation. The reason is that taller plants often have more cellu-

lose and lignin than shorter plants to provide support for their height and weight.

Consequently, a herbivore which grazed tall plants would obtain less nutritious and

less digestible food than one which concentrated on younger growth forms. How-

ever, at very low plant sizes, the rate of cropping is very low, and this can also com-

promise rates of food intake. As a result, grazers should benefit best by feeding on

intermediate height and biomass of grasses (Fig. 5.8). Several experimental studies

have shown that large herbivores show grazing preference for swards of inter-

mediate grass height and biomass, including cattle (de Vries et al. 1999), elk

(Wilmshurst et al. 1995), bison (Bergman et al. 2001), red deer (Langvatn and Hanley

1993), and Thomson’s gazelles (Fryxell et al. 2004). On the other hand, reindeer on

68 Chapter 5

96

80

64

40

32

16

0

0 8 16 24 32

Travel time (s)

Time in patch (s)

Fig. 5.7Observed

patch residence times

by great tits foraging

in an experimental

aviary (shown by

filled circles) versus

the patch residence

times predicted by

the marginal value

theorem (solid curve).

(After Cowie 1977.)

50

40

30

20

10

18

16

14

12

10

0 20406080100

Grass biomass (g / m^2 )

Thomson’s gazelles / km

2

Energy gain (MJ/day)

Fig. 5.8Relative daily

energy gain (
) and

observed population

densities of Thomson’s

gazelles () in relation

to the biomass of grass

in particular patches in

Serengeti National Park.

(After Fryxell et al.

2004.)

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