2'f 



ones in particular situations. It is suggested that newly-disturbed 

 areas open to either reproductive mode would be occupied by bisexual 

 species until the origin of a parthenogenetic clone, which would then 

 displace the bisexuals due to its higher intrinsic rate of increase. 

 Selection would promote the survival of clones more or less specially 

 adapted to unique communities of perpetually disturbed areas or areas 

 not occupied by bisexual species. Termination of the physical or cli- 

 matic conditions promoting and maintaining recurrent disclimax ecolo- 

 gies would cause the extinction of the parthenoforms and the reinvasion 

 of climax communities. Assuming that hybridization gave rise directly 

 to parthenogenetic species is cautioned against. The acceptable corol- 

 lary to this, in the opinion of this reviewer, is that not every hybrid- 

 ization event leads to the establishment of a successful parthenoform 

 (i.e. _C. perplexus ); the cytogenetic and ecological factors must syn- 

 ergize. 



63. — . 1979. On the ecology of coexistence in parthenogenetic and 

 bisexual lizards of the genus Cnemidophorus . AMER. ZOOL. 19: 773-786. 



The question of why so many congeneric species of this genus are 

 found together and exactly what their ecological and geographical re- 

 quirements are remains virtually unanswered. Sympatry among 7 species 

 of Cnemidophorus ( exsanguis , inornatus , neomexicanus , tesselatus , tig - 

 ris, uniparens and velox ) is documented and discussed for several lo- 

 cal itlesTmostly in New Mexico), and the first field experiment dealing 

 with competitive interactions between a parthenogenetic and a bisexual 

 species is reported. Short-term habitat alteration and collecting 

 pressures are implicated as factors affecting the interactive demo- 

 graphies of the above species; in some cases the only species involved 

 are parthenogenetic. Collecting can apparently wipe out populations of 

 several of the parthenospecies if sustained and steady over a period of 

 years. The field experiment involved the bisexual species _C. tigris 

 and the unisexual species _C. uniparens . The former species dominates 

 in mesquite-creosote habitats near Elephant Butte Reservoir whereas the 

 latter occurs in all habitats from the river to the foothills of the 

 San Mateo Mountains. _C. tigris is absent from all habitats in the Rio 

 Grande floodplain. The study site is a weedy field between Tamarix and 

 Populus forests which is adjacent to and interdigitates with _C. tigris 

 habitat; _C. uniparens is dominant here. This species was selectively 

 removed for a period of several days in each of the years 1975, 1976 

 and 1978. _C. tigris failed to invade this habitat although it could do 

 so easily; it was instead repopulated by _C. uniparens from the adjacent 

 gallery forests. The mean size of individuals of this species declined 

 as did the number of reproductives, however. Only 3 individual _C. tig - 

 ris were seen, indicating that this species is actively avoiding the 

 riparian zone. The area was visited again in 1979, when this paper was 

 in proof, and the C. uniparens population was identical in density and 

 mean individual size to the previous year. Individuals predominated on 

 the edges of the field rather than the center, indicating invasion from 

 the periphery. 14 different individuals of _C. tigris were seen distri- 



