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Ecological Isolation in a Rich Tropical Avifauna

R. E. Moreau
Journal of Animal Ecology
Vol. 17, No. 2 (Nov., 1948), pp. 113-126
DOI: 10.2307/1473
Stable URL: http://www.jstor.org/stable/1473
Page Count: 14
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Ecological Isolation in a Rich Tropical Avifauna
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Abstract

1. In a tropical African area of about 3000 sq. miles, nine families of birds are represented by 172 species in 92 genera, 41 of which show more than one species. Considered family by family these birds provide in all 173 cases of theoretically possible ecological overlap between species of the same genus and 1474 cases between other species. 2. Thirty-one habitat subdivisions are adopted and each species is allocated to the subdivisions in which it forages. The Ploceidae (weavers) present a different ecological picture from the other families (which are frugivorous and insectivorous). 3. Excluding the Ploceidae, within genera only 16% of the theoretically possible cases of habitat overlap are realized and in only one-third of these is there apparent overlap in diet and consequently ecological overlap. Moreover, some of the `cases' are qualified by the fact that one or both the species concerned are present in such small numbers that `competition' is unlikely. 4. Of the congeneric species 94% are ecologically isolated from each other, since they are distinct in habitat or in diet or in both. The corresponding figure for the other species in the same families is 98%. It follows that the isolating mechanism is at least as effective between the non-congeneric species as between the congeneric; and it may provisionally be inferred that generic relationship is not of overriding importance in determining the composition of a community. 5. The grounds for concluding that most of the cases of habitat overlap are not cases of diet overlap (and consequently not cases of ecological overlap) are almost entirely (1) differences in size (of body or beak), and (2) differences in details of food-seeking methods or of range within the habitat. These two means of ecological separation are about equally prevalent. 6. Among the Ploceidae, which have been excepted from the foregoing remarks, the subfamily Ploceinae shows a much higher percentage of both habitat overlap and ecological overlap than either the other Ploceid subfamily, the Estrildinae, or the non-Ploceid families. This result is due to the fact that numerous Ploceine species feed together on the temporarily superabundant graminaceous food produced in, for example, flood-plains. As the dry season progresses and the seed supply disappears, so do the Ploceinae that have been feeding on it. It seems likely that they move from one focus of temporary superabundance to another. 7. Mixed foraging parties, so prominent in Africa, often contain more than one species of the same family or the same genus. Preliminary indications are that in these cases the respective foraging habits are complementary. 8. The inference from the foregoing is that the forces that minimize ecological overlap between related birds are highly effective. This derives confirmation from a number of cases cited where a widespread African species is, in one or more localities, inexplicably absent from its niche. Its place is then often taken by another species of the same family, sometimes of the same genus, and usually, but not always, from the same biome.

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