VARIATION IN THE DEMOGRAPHY OF GUPPY POPULATIONS - THE IMPORTANCE OF PREDATION AND LIFE-HISTORIES

We compare the demography of natural populations of guppies (Poecilia reticulata) coexisting with two different communities of predators. In some populations, guppies co-occur with Rivulus hartii, a small, gape -limited predator; in others, guppies co-occur with larger species of fish that also prey on guppies. We found an association between the de mography of the resident guppy population and the species of predator( s) present at a site. Populations of guppies that co-occurred with Riv ulus had fewer small, immature guppies and more mid- and large-sized, mature guppies than populations of guppies that lived in the community of larger predators. The sex ratio of adult guppies did not differ be tween predator localities but it was significantly more variable among sites with the small predator. Ninety percent of all samples had a fe male-biased sex ratio. Season (wet vs. dry season) did not have a stat istically significant influence on sex ratio or on stage or size distr ibutions. The observed variation in guppy demography could be simply a result of differences in the prey size preferences of the predators. However, guppies from the two types of localities show considerable di fferences in life history traits, including age at maturity and fecund ity. This variation in traits could be contributing to the differences in demography. Indeed, results from an individual-based computer mode l suggest that fecundity, offspring size, and individual growth rates were most influential in producing interlocality differences in size d istributions and the proportions of the populations that were immature . Mortality rates. and individual growth rates seem to make the greate st contribution to variation in adult sex ratio. Therefore, the observ ed patterns in the demography of these populations appear to result fr om complex interactions between the mortality schedules and life histo ry traits of their constituents.