Priority Effects in Experimental Pond Communities: Responses of Hyla to Bufo and Rana

Abstract

The timing of hatching of two early spring breeding frogs, Rana sphenocephala and Bufo americanus, was controlled in a 3 x 3 factorial experiment (each species introduced early, introduced late, or not present) in a randomized block design replicated three times in an array of 27 experimental ponds. After nearly all Bufo had metamorphosed and left the ponds, but before Rana began metamorphosis, 500 hatchlings of a summer—breeding species, Hyla chrysoscelis, were added to each of the 27 ponds and to 3 more ponds that had recently been filled. Hyla had a higher survival rate, shorter mean larval period, and larger mean size at metamorphosis in the newly filled ponds than in any other treatment. The older ponds without either spring species were the next best habitat. These “old” controls were used to assay the effects of the spring—breeding species on Hyla. Bufo had a significantly adverse effect on Hyla even though the two species were not contemporaneous, this was interpreted as being due to an impact by Bufo on the trophic structure of the ponds, an impact that persisted throughout the summer. Rana had an even stronger negative effect on Hyla than did Bufo. The 2 x 2 subset of treatments consisting of Hyla with early or late Rana and early or late Bufo was analyzed separately. The combination of late Rana and late Bufo had the strongest adverse effect on Hyla and the other three combinations were about equal in their effects. These experiments demonstrate the advantages to anurans of ovipositing soon after pond filling and of colonizing ponds that have not been used previously in the season by other species. The detection of historical effect reveals that our experimental communities were not governed by stationary first—order Markov processes and that perturbations had persistent effects on their trajectories. Our results suggest that optimal seasonal patterns for reproduction exist in animals with complex life cycles that exploit temporary habitats.