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birds (13) and lizards (14) on some oceanic islands compared to that in similar habitats on nearby continents. These insular Department of Zoology,
University of Texas, Austin 78712
References and Notes
1. R. H. MacArthur, Geographical Ecology (Harper & Row, New York, 1972), pp. 111-207; T. W.
Schoener, Science 115, 27 (1974); E. R. Pianka,
Evolutionary Ecology (Harper & Row, New
York, 1974), pp. 132-155, 230-244; M. L. Cody
and J. M. Diamond, Eds., Ecology and Evolution of Communities (Harvard Univ. Press,
Cambridge, Mass., 1975).
2. R. M. Hansen and D. M. Ueckert, Ecology 51,
640 (1970); A. R. E. Sinclair, J. Anim. Ecol. 44,
3. J. H. Brown, J. J. Grover, D. W. Davidson, G. A.
Lieberman, Ecology 56,987(1975).
4. M. B. Fenton and T. H. Fleming, Biotropica 8,
5. Overlap (a) may be quantified on a scale of 0 to 1
by the formula
A 6. 7.
8. 9. 10.
12. 13. 14.
15. Pi, where aij is the overlap of taxonj on taxon i, and
pi is the unweighted utilization of a particular
seed size category by species i relative to its utilization of the other size categories [R. H. MacArthur and R. Levins, Am. Nat. 101, 377 (1967)].
When this is done, the overlap of rodents on ants
is 0.59 and the overlap of ants on rodents is 0.50.
Analysis of seeds from O. J. Reichman [U.S./Int.
Biol. Program Desert Biome Res. Memo., in
press]. Analysis of annuals from J.H.B. and
0. J. Reichman, Ecology, in press.
D. W. Davidson, ibid., in press. The differential
response of the Pheidole spp. may also be owing
to the fact that these ants are exceptionally good
J. H. Brown, Ecology 54, 775 (1973); in Ecology
and Evolution of Communities, M. L. Cody and
J. M. Diamond, Eds. (Harvard Univ. Press,
Cambridge, Mass. 1975), p. 315.
Data on ants in the north-south gradient from
R. A. Bernstein,Am. Nat. 108,490 (1974); W. G.
Whitford and G. Ettershank, Environ. Entomol.
W. G. Whitford (U.S./Int. Biol. Program Desert
Biome Res. Memo., in press) has independently
performed a series of granivore-exclusion experiments in the desert near Las Cruces, N.M. His
results are similar to ours. Densities of annual
plants were 2.1 times greater on plots in which
rodents and ants were absent than on those in
which rodents, ants, or both were present. Ant
colonies increased 49 percent on plots from
which rodents were excluded relative to controls. He did not test for the response of rodents
to the absence of ants.
K. L. Crowell, Ecology 43,75 (1962); R. H. MacArthur, J. M. Diamond, J. R. Karr, ibid. 53, 330
(1972); J. M. Diamond, inEcology and Evolution
ofCommunities, M. L. Cody and J. M. Diamond,
Eds. (Harvard Univ. Press, Cambridge, Mass.
1975), p. 342.
T. J. Case,Ecology 56, 3 (1975).
This work was supported by grants GB 8765 and
GB 39260 from the National Science Foundation
and by grant 556 from the Desert Biome, U.S./
International Biological Program. We thank W. G.
Whitford and 0. J. Reichman for making available data from reports in press. 29 October 1976; revised 14 December 1976
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