Stability of small unisexual-bisexual populations of Poeciliopsis (Pisces: Poeciliidae)

All-female species of Poeciliopsis depend upon ~ ~ of closely related species for sperm. This relationship creates a competitive situation in which bisexual ~ ~ compete with unisexual ~ ~ for sperm from ~ ~ of the bisexual species. Earlier studies demonstrated a sufficient mechanism inherent in the behavior of ~ ~ for allowing coexistence in mixed unisexualbisexual populations. Courtship preference of ~ ~ for conspecific ~ ~ is dynamically countered by social interaction among ~ ~ : subordinate males mate with unisexual ~ ~ in impetuous courtships. The Poeciliopsis monacha-Poeciliopsis 2 monacha-lucida complex occurs naturally in small isolated populations where stochastic factors are not negligible. This paper evaluates the stability of small populations of this complex by laboratory population experimentation and stochastic computer simulation. Three small mixed populations persisted in the laboratory for 2 yr but extinction of the bisexual species occurred in four of nine simulated replicates of these experiments. Life tables and fitness values calculated from estimates of birth and death rates showed the all-female species to be much more fit in the laboratory populations than was the bisexual species. Additional simulations done at relative fitness values more characteristic of natural populations indicated that extinctions in small isolates would be rare in nature. Isolated demes can become confluent during the annual rainy season. It was expected that this confluence, by effectively increasing population size, might enhance stability by reducing the effect of stochastic factors; simulations of two and four demes mixed annually did not support this hypothesis.