Head starting is an essential tool for conserving and restoring the critically endangered dusky gopher frog, as it provides a buffer for years in which there is zero natural juvenile recruitment from the natural population. While it is known that growth and survival of Rana sevosa tadpoles is higher in open canopy vs. closed canopy ponds, we wanted to explore the mechanisms behind this to help shape current restoration efforts. We used outdoor tanks to test if litter from open canopy ponds were better than litter from closed canopy ponds for captive rearing of R. sevosa tadpoles. Our closed canopy litter was a mixture of longleaf pine (Pinus palustris) and sweetgum (Liquidambar styraciflua) leaves, and our open canopy litters were juncus (Juncus repens) and maidencane (Panicum hemitomon). We tested each litter individually and in combinations to examine the benefits of litter diversity vs. monoculture use. We concurrently examined the effectiveness of feeding tadpoles supplemental algal wafers (0, 3, 6 or 9 algal wafers/1000 L tank/week), to find the optimal level of wafer addition.
Results/Conclusions
Survival, snout-vent length (SVL), and mass at metamorphosis were generally smaller in R. sevosa raised in closed vs. open canopy litters. Tadpole survival for maidencane was 40% lower than for juncus and juncus/maidencane when wafers=0. SVL (36.5mm) and mass (4.70g) at metamorphosis for maidencane was lower than for juncus (38.0mm, 5.28g) or the combination (37.8mm, 5.09g), averaged across algal treatments. Average SVL and mass increased with increasing wafers. Metamorphs from the juncus/maidencane treatment with no wafers had an average SVL (33.9mm) and mass (3.42g) similar to those for natural metamorphs from 1997-2015 (32.5mm, 3.67g), and for 3-wafer maidencane tanks (34.9mm, 3.84g). Open canopy litters increased survival and growth, and mixtures of litters were better than monocultures, but addition of algal wafers compensated for less productive litter. Our current recommendation for head starting programs is to use maidencane and juncus in combination with an addition of 6 wafers/week in order to maximize growth and survival and decrease time to metamorphosis, but not exceed observed natural sizes at metamorphosis.