Visual display of the Ecology of giant tortoises (Geochelone elephantopus) in the Galapagos Islands

				

161


This conforms to Glasser's (1984) description of a faculta-

tive strategist.    Schluter (1981)v however, showed that in

15  studies of various species    including fish,    reptiles,

birds*   and mammals,    only 4  showed an   increase in diet

breadth with diminishing   food supplies, while 8    showed an

increase in specialization.   It may be that some facultative

strategists perform in   the opposite way from   that proposed

by Glasser (1984).

    The diet of herbivores is generally made up of a variety

of food items varying in   quality.   Large generalist herbi-

vores should optimize   the nutrient mix within   a given food

supply which is constrained by availability of foods only at

rather low levels of availability (Westoby 1974).     However,

if a specific nutrient is limiting   in the diet,   the herbi-

vore's diet may reflect preferences for food items with high

levels of that nutrient, regardless of other quality charac-

teristics. Although nitrogen has been noted as a major fac-

tor in  determining food preferences (Sinclair    19759 White

19785 Mattson 1980),   dietary habits of animals in arid eco-

systems appear more often determined by water content of the

food  than by   energy or  protein  content (Noy-Meir   1974).

Desert tortoises in Utah obtain much of their water directly

from their food or supplementally as metabolic water derived

from food (Woodbury and Hardy 1948).

    Gal~apagos tortoises have been considered not very selec-

tive in their   feeding, eating almost any    green vegetation

they encounter (Slevin .1935, Hintz (1972?], Pritchard 1971,

 

					
				
					

162


1979a).  The first quantitative study of their diet was done

on Volcan Alcedo in 1980 (Fowler   1983, Fowler de Neira and

Johnson 1985).    Prior to that, the majority of descriptions

of their diet consisted of general comments or lists of food

items either seen eaten or found   in their stomachs or feces

(Table 5.1).

    Cactus (primarily Opuntia, though    not always so noted)

and grasses were often named as the principal components of

the tortoise diet (Table 5.1).    The importance of cactus as

a source of water in arid areas and in times of drought was

also stressed (Darwin 1860, Hemsley 1895, Beck 19039 Heller

19039 Slevin 19359 Dawson 1966, Hendrickson 1966,    Pritchard

1979a, Fritts 1983).    Tortoises are known to obtain a great

deal  of fluid   from their  plant diet   (fahmoud and  Klicka

1979).   Cloudsley-Thompson (1971) suggested     that all land

tortoises can exist indefinitely on a vegetarian diet with-

out free water.

    After grasses and cactus,   fruit  was the most mentioned

food item for Galapagos tortoises    (Table 5.1).    The small

fruits of the endemic Psidium    alayaoeium (known locally as

guayavilla) were    a large portion  of the tortoise   diet on

Volcan Alcedo during   garua season (Fowler 1983,    Fowler de

Neira and  Johnson 1985).    Slevin  (quoted in   Van Denburgh
1914, and Fritts and Fritts 1962) thought that the fruits of

HiD~omane mancinella    were preferred over cactus.   Tortois-

es may spend  up to 1 h   feeding solely on this   fruit (Rod-

house et al. 1975).   However, Slevin also stated (Fritts and

Fritta 1962):