Wednesday, April 11, 2012

How are frogs impacted by the early spring?

The early spring could benefit or harm the frogs. We have several frog species that start calling (and breeding) as soon as the ice-melts in wetlands and small ponds.

The earliest breeder of these is the wood frog (Rana sylvatica), which sounds like quark-quark. This species emerges from hibernation as soon as the small wetlands they breed in have open water. Wood frogs can benefit from the early spring by breeding earlier, which gives their tadpoles longer time to grow over spring and summer. Bigger tadpoles turn into bigger froglets, which increases their likelihood of survival through the fall and following winter.
Adult wood frog
HOWEVER, early springs are often associated with low snow pack, which means that these small, usually temporary, ponds start the season with much lower water levels than in a typical spring. As these ponds are generally fed by snowmelt and precipitation (no, or little, connection to groundwater), the water levels at wood frog breeding time are directly tied to snowmelt, and maintaining adequate water levels for tadpoles to develop depends on spring and early summer rains. If the ponds dry up in June (even for just a few days), before the tadpoles metamorphose into frogs, the entire generation of frogs from the pond is lost.

Another impact of the early spring is the possibility of extended calling for wood frogs (who usually just call for a few days to a week) which could greatly exhaust the males as they wait for females to arrive in the ponds to lay eggs. Male frogs arrive in ponds first and beginning calling individually or in groups. Females arrive later, select a mate (based on factors we don't fully understand) and lay eggs. In an early spring, male wood frogs can start calling WEEKS before the eggs are laid - especially if the weather shifts between warm and cold several times a week. Frog calling is very energetically demanding activity, and male wood frogs complete breeding before foraging for food on the forest floor. If the warm spring weather starts and stops (as it has this year), the male wood frogs could get really worn out and deplete their energy reserves while calling on and off for multiple weeks while they wait for the weather to be consistently warm enough for the females to arrive and be ready to lay eggs.

(as posted on Ask NRRI)


  1. This is a very interesting question.
    Did you see either of the effects that you describe in Minnesota?
    I didn't see either of these effects at some long-term sites I work at in Michigan and Ohio. The very wet summer and fall led to water levels being much higher than normal. Additionally, the wood frog calling season was actually much shorter than normal. Mike

    1. Mike, you raise a great point. What I have described is what COULD happen, not what WILL happen. The actual impacts of any early spring on frogs depends greatly on local conditions, including rain later in the summer as well as the preceeding fall and winter.

      So far this year, the wood frogs have started and stopped calling several times as temperatures swung from record highs to sub-freezing. Some egg-laying occurred during the second round of calling, but not in all ponds that typically have good numbers of breeding wood frogs. I will continue to monitor these ponds over the next month to see what happens.

      A few years ago during weekly monitoring of Northern leopard frog breeding habitat during an early spring, I observed males calling for 4 weeks straight without seeing a single female and only finding 1 egg mass. A week or so after the last male was heard, we started seeing females at these same ponds. Without intensive trapping we cannot confirm that this was a complete mis-match in males and female leopard frogs at these pond, but no additional eggs were found after several more weeks of monitoring.

      This spring, water levels in many ponds around Duluth are extremely low after a nearly snow-less winter. If we do not receive good rainfal this spring and early summer, I expect they will dry up before tadpole development is complete. Spring 2009 was very similar, and the ponds I monitor dried up mid-June before any wood froglets emerged.

      Thanks again for pointing out that this can be highly variable.