RESOURCE AVAILABILITY AND POPULATION-SIZE IN CACTOPHILIC DROSOPHILA

1. Four species of Drosophila, Drosophila nigrospiracula (Patterson & Wheeler 1942), Drosophila mettleri (Heed 1977), Drosophila pachea (Pat terson & Wheeler 1942), and Drosophila mojavensis (Patterson & Crow 19 40) are endemic to the Sonoran Desert of North America and breed in di fferent species of necrotic columnar cacti. Differences in resource av ailability have been suggested to explain the interspecific variabilit y in fly population biology, but resource availability for these speci es has not been quantitatively assessed thoroughly in either spatial o r temporal terms. The resource availability was quantified quarterly a t three sites for 3 years and population sizes for each Drasophila spe cies were estimated. 2. Spatial and temporal availability of resources differed significantly among species of host cacti, with organpipe ca ctus (Stenocereus thurberi) being the least abundant and senita (Lopho cereus schottii) the most abundant spatially.3. Drosophila species dif fered significantly in population size. The largest population sizes w ere found for D. nigrospiracula and D. mojavensis and smallest for D. pachea. Populations of D. mettleri were intermediate to these. 4. Popu lation size was greatest for fly species utilizing host species having the largest and longest lasting necroses. 5. Resource availability do es not explain the reduction of fly populations in the summer Necroses were most abundant when flies were absent.