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Birds as Agents of Long-Distance Dispersal for Disjunct Plant Groups of the Temperate Western Hemisphere
Robert William Cruden
Vol. 20, No. 4 (Dec., 1966), pp. 517-532
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2406587
Page Count: 16
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Recent interest in disjunct plant distributions makes a discussion of birds with respect to long-distance dispersal both timely and valuable. Birds have seemingly migrated for a long enough period of time to account for the transport of propagules of temperate American disjuncts from one hemisphere to the other. Although birds carry propagules, both internally and externally, only seeds carried externally are likely to be carried across the tropics in one jump. Internal transport of seeds is not plausible, as the passage of seeds through a bird takes but a few hours, and a month at least is necessary for the birds to make the trip from California to Chile. External transport of plant parts is not likely either. It seems unlikely that seeds carried externally would survive the numerous preenings, etc., they would be subject to, before the birds reached a favorable habitat in the other hemisphere. The disjunct plant groups are not especially qualified for long-distance dispersal. They live in open, unstable habitats; their seeds or fruits often lend themselves to animal dispersal; and they are, as far as is known, self-compatible and often autogamous. As Baker (1955) has pointed out, only a single seed need be dispersed for such a species to become established. The species in question are closely related morphologically and are few in number compared to the total flora. This is surely an indication of the recentness and perhaps rareness of their dispersal. Although a large number of birds in terms of species, migrate from North America to temperate South America, only five are considered likely agents of long-distance dispersal. Even these birds can be expected only rarely to come in contact with seeds of a majority of the disjunct plants, as they frequent wet areas and 60 per cent of the plants are of dry grassland or wooded habitats. Various workers have noted a seeming correspondence between flyways of the birds and distribution of the disjunct plants. I have indicated the correspondence, at least in South America, is actually between wintering areas and the plant distributional ranges. While the introduction of a single propagule of self-compatible plants is sufficient to start a population, the great variability and diversification of some taxa indicate a large segment of the parental gene pool was transported across the tropics. This is inconsistent with the assumption that autogamous populations tend to become homozygous, and that it takes but a single seed to establish a new population. I suggest that mountain hopping be given greater consideration as a means of explaining some disjunct distributions of essentially autogamous plants with great morphological and ecological diversity. Means of dispersal other than shorebirds could thus account for stepwise dispersal across the tropics. Mountain hopping provides a reasonable explanation for the movement of a large segment of the parental gene pool across the tropics through the buildup of large intermediate populations.
Evolution © 1966 Society for the Study of Evolution