The treefrog’s deadline: a pond must last long enough

Why timing matters

In the Pinelands, shallow ponds can fill and dry within a year. The interval with standing water is a pond’s hydroperiod. If water disappears after eggs are laid but before young frogs complete metamorphosis, a breeding attempt can fail. The Pine Barrens treefrog is a relatively late breeder compared with the spring peeper and southern leopard frog. That timing was the reason the 2010 Pinelands Commission study compared the three species.

The Commission’s long-term monitoring program began frog-calling surveys in 1996 along a forest-to-developed-land gradient in the western Mullica watershed. Calling records show where adults attempted to breed. The later pond study went further by checking eggs, larvae, developmental stages and the appearance of metamorphs. These are different kinds of evidence: a calling adult does not establish that young frogs survived to leave the pond.

Three ponds, two kinds of record

Researchers selected three old excavated ponds on state land whose water patterns resembled natural intermittent ponds, while their clearer margins made larval sampling more feasible. They measured pond depth over roughly a decade and sampled eggs and developing frogs in 2005 and 2006. Across those two seasons they collected and staged 4,545 larvae, pre-metamorphs and metamorphs of five species, including 1,251 Pine Barrens treefrogs. The counts are captured samples, not estimates of all frogs in the ponds.

The treefrog’s breeding signal was stronger than its egg record: adults called at the three ponds in both intensive-study years, but researchers found treefrog eggs on only one sampling date, June 1, 2006, at one pond. They found treefrog metamorphs at two ponds in each year. Egg masses could have been missed between visits, so the single observed date is a limit on inference, not proof that breeding occurred only once.

A model of earlier drying

The team did not pump these ponds dry. It subtracted hypothetical water-depth reductions of 5 to 50 centimeters from measured levels, then asked whether ponds would have dried before observed developmental milestones. In the two-year comparison, reductions of 5 or 10 centimeters generally produced little or no modeled effect; noticeable effects began around 15 centimeters. Using the longer water-level record changed the picture: modeled effects on the first pre-metamorphs and metamorphs began around 10 to 15 centimeters, and effects on initial egg deposition around 20 centimeters. The largest reduction, 50 centimeters, would have shortened pond life enough to nearly eliminate successful larval development in all three focal species in these scenarios.

Across the simulations, the estimated impact was greatest for the Pine Barrens treefrog. Its later breeding leaves less time before seasonal drying. But these are modeled water-depth differences at three particular ponds, not a statewide pumping limit or a prediction that every pond loses the same number of frogs after a given change.

What the study leaves open

The authors caution that pre-metamorphs and metamorphs of some species were missing from particular pond samples, and counts were low in places. The one observed treefrog egg date was later than calling records would lead them to expect. Their study links water duration to possible reproductive outcomes, but it does not measure the effect of an actual new groundwater withdrawal on a whole population. The archive therefore maps this work to the northwestern Mullica watershed in prose, without assigning a pond-specific pin.