Dispersal of Desert-Adapted Drosophila: The Saguaro-Breeding D. nigrospiracula

• Published in: The American naturalist 1976-07, Vol.110 (974), p.629-651
• Main Authors: Johnston, J. Spencer, Heed, W. B.
• Format: Article
• Language: English
• Subjects: Desert insects; Drosophila; Evolutionary genetics; Insect genetics; Population genetics; Population structure
• ISSN: 0003-0147; 1537-5323
• DOI: 10.1086/283095

Dispersal of Drosophila nigrospiracula on naturally occurring necrotic saguaro cacti occurs at rates up to three times higher than those observed for any other Drosophila. In eight separate experiments, a total of 3,034 marked flies were captured at 22 cacti. Overall immigration rates averaged 6.83 % over 371 m. Immigration from a single cohort averaged 3.67% over all distances. Effective-population sizes at a single cacti are estimated from 361 to 3,999, while the overall population of D. nigrospiracula is estimated at 100 billion flies. The genetics of large, mobile populations has been considered, using island, stepping-stone, and isolation-by-distance models. All three models suggest that D. nigrospiracula is so mobile that drift plays a minor role. The population acts as if it were panmictic. This view is supported further by behavioral considerations. Immigrants are an effective part of mating populations. Barriers to dispersal are nonexistent. Adverse habitat conditions increase dispersal rates. The high dispersal rates of D. nigrospiracula were not anticipated by earlier Drosophila studies. Three explanations are proposed. First, immature adults may be more mobile than mature flies. This paper presents the first evidence of this in Drosophila. Second, dispersal may be increased by food stress. If so, baited studies, which considerably enrich the environment, underestimate dispersal rates in adverse environments. Third, dispersal rates are an evolutionary response to habitat predictability. Those species (such as D. nigrospiracula at cacti) which occupy short-lived and widely-spaced habitats are more mobile than species (such as D. melanogaster at a garbage dump) which occupy a small, rich, constantly renewed habitat.