Resource availability and population size in cactophilic Drosophila

1. Four species of Drosophila, Drosophila nigrospiracula (Patterson & Wheeler 1942), Drosophila mettleri (Heed 1977), Drosophila pachea (Patterson & Wheeler 1942), and Drosophila mojavensis (Patterson & Crow 1940) are endemic to the Sonoran Desert of North America and breed in different species of necrotic columnar cacti. Differences in resource availability have been suggested to explain the interspecific variability in fly population biology, but resource availability for these species has not been quantitatively assessed thoroughly in either spatial or temporal terms. The resource availability was quantified quarterly at three sites for 3 years and population sizes for each Drasophila species were estimated. 2. Spatial and temporal availability of resources differed significantly among species of host cacti, with organpipe cactus (Stenocereus thurberi) being the least abundant and senita (Lophocereus schottii) the most abundant spatially. 3. Drosophila species differed significantly in population size. The largest population sizes were found for D. nigrospiracula and D. mojavensis and smallest for D. pachea. Populations of D. mettleri were intermediate to these. 4. Population size was greatest for fly species utilizing host species having the largest and longest lasting necroses. 5. Resource availability does not explain the reduction of fly populations in the summer Necroses were most abundant when flies were absent.