Temporal Overlap, Competition, and Priority Effects in Larval Anurans

Interspecific competitors often colonize communities at different times, but few studies have experimentally tested whether the strength of interspecific competition hinges on the order or temporal separation of species' arrivals. We added hatchlings of two sympatric anuran species (Hyla[=Pseudacris] crucifer and Bufo woodhousii) to artificial ponds on three different dates to manipulate the order and temporal separation of their arrival. Measurement of the growth and survival of each species in ponds where the second species arrived simultaneously (day 0), or after a delay of 7 or 14 d, indicated whether differences in the history of introductions affected interspecific competition. Other ponds contained H. crucifer alone, introduced on the same three dates, of B. woodhousii alone, introduced on the first and second date. These provided controls for seasonal differences in performance in the absence of competition from the other anuran species. Introductions of tadpoles at different times produced small differences in growth rates and larval periods when each anuran species occurred alone. In ponds containing both species, differences in the order and temporal separation of introductions had complex effects on the intensity of interspecific competition. When Bufo preceded Hyla, Hyla had prolonged larval periods and reduced mass and growth. When Hyla preceded Bufo, it was unaffected by Bufo. Hyla had no effects on Bufo, whether its introduction preceded or followed the introduction of Bufo. Asymmetric priority effects were not easily explained as a simple size—dependent competitive advantage of earlier arrivals. Early arrival increased the competitive impact of Bufo on Hyla, but failed to generate a competitive effect of Hyla on Bufo. Bufo tadpoles are more active than Hyla, and so may consume resources at higher rates that cannot be offset by a temporal advantage. Competition from Bufo was strongest when Hyla arrived 7 d later, suggesting that Hyla arriving 14 d after Bufo benefited from a longer period of reduced competition after Bufo metamorphosed from the ponds. The natural pattern, where Hyla breeds before or simultaneously with Bufo, permits Hyla to minimize competition from Bufo, while Bufo suffers no measurable cost from the size advantage obtained by competitively weaker Hyla tadpoles. Bufo may be prevented from breeding any earlier in the season by physiological constraints acting on eggs or breeding adults.