Denis F. Owen

Grasses and Grazers: Is There a Mutualism?

tion: population studies of desert grasshoppers on creosote bushes. J. Anim. Ecol. 44: 29-54. Owen, D. F. 1980. How plants may benefit from the animals that eat them. Oikos 35: 230-235. and Wiegert, R. G. 1976. Do consumers maximize plant fitness? Oikos 27: 488-492. and Wiegert, R. G. 1981. Mutualism between grasses and grazers: an evolutionary hypothesis. Oikos 36: 376-378. Rockwood, L. L. 1973. The effect of defoliation on seed production of six Costa Rican tree species. Ecology 54: 1363-1369. Rosenthal, G. A. and Janzen, D. H. (eds.), 1979. Herbivores, their interaction with secondary plant metabolites. Academic Press, New York. Stephenson, A. G. 1980. Fruit set, herbivory, fruit reduction, and the fruiting strategy of Catalpa speciosa (Bignoniaceae). Ecology 61: 57-64. Tinbergen, N. 1969. El estudio del instinto. Siglo Veintiuno Eds., Mexico. Translated by J. Almela. Tullock, G. 1979. On the adaptive significance of territoriality: comment. Am. Nat. 113: 772-775. Watt, A. S. 1962. The effect of excluding rabbits from grassland A (Xerobrometum) in Breckland, 1936-60. J. Ecol. 50: 181-198. Wilson, E. 0. 1975. Sociobiology. Belknap Press, Cambridge, Mass.

Polymorphism in the land snail Cepaea hortensis in Iceland

In Iceland, the polymorphic land snail Cepaea hortensis Mull. is confined to a relatively small area in the extreme south. Sampling 19 populations revealed the presence of a considerable range of phenotypes, the occurrence and frequency of which varies from population to population. A method of expressing the total genetic diversity of the phenotypes in different populations is suggested. The effective absence of the song thrush Turdusphilomelos Brehm, a bird that is known ol prey selectively on Cepaea elsewhere, suggests that other factors promote and maintain the polymorphism. In one area rodents were found to be feeding upon hibernating snails, and it is possible that they exert a considerable selection pressure.

Beating the Walnut Tree: More on Grass/Grazer Mutualism

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The white snails of Africa: the significance of Man in the maintenance of a striking polymorphism

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