Background/Question/Methods
A long history of ecology has demonstrated that interactions among species can play an important role in determining population sizes and fitness components of individual species.  Mounting evidence, however, suggests that the timing at which species arrive into a community can also have important effects on other members in the community.  Such priority effects can occur if early arriving species have a competitive advantage or consumptive advantage over late arriving species.  Although it is recognized that priority effects can have important effects in ecological systems, the mechanisms through which these effects are produced are not always clear.  Differences in the developmental time of frog tadpoles provide an opportunity to examine mechanisms through which priority effects may influence fitness components of frogs.  Specifically, tadpoles of the southern leopard frog (Rana) can often require more than a year to complete metamorphosis so they overwinter in a pond and compete with newly deposited tadpoles in the spring.  We conducted an experiment in artificial ponds to evaluate different mechanisms through which overwintering Rana tadpoles influence fitness components of southern toad (Bufo) tadpoles deposited into ponds during the spring.  We did this by altering the times at which overwintered Rana were present in a pond. 

Results/Conclusions
The effect of Rana on Bufo depended on the time at which Rana enter a pond.  Although the presence of newly hatched Rana in the spring had no impact on Bufo, overwintered Rana present in a pond during winter and spring caused a reduction in Bufo survival and mass at metamorphosis.  This priority effect is not simply the result of overwintered Rana achieving a size advantage.  Overwintered Rana did not affect Bufo survival or mass at metamorphosis when Rana was only present during the spring.  Instead, overwintered Rana appear to reduce Bufo mass at metamorphosis by changing the environment prior to Bufo arrival.  In contrast, overwintered Rana only caused a reduction in Bufo survival when Rana was present in a pond during winter and spring but had no impact on Bufo survival when Rana was only present in either the winter or spring .  This result suggests that overwintered Rana suppresses Bufo survival via priority effects produced by a synergy in the impact of Rana on the environment at two different times (prior to the arrival of Bufo and while Bufo is present) of the year.