from: Wildlife Society Bulletin 28(4):895-901 (Winter 2000)
USING EGG-MASS COUNTS TO MONITOR WOOD FROG POPUALTIONS
William B. Crouch, Department of Natural
Resources Science, University of Rhode Island, Kingston, RI 02881,
Peter W. C. Paton, Department of Natural Resources Science, University
of Rhode Island, Kingston, RI 02881, USA
Calling surveys are used to monitor trends in frog populations for
most North American species. However, wood
frogs (Rana sylvatica), one of the most abundant species
in New England, may not be detectable using calling surveys at breeding
ponds. We assessed efficacy of using egg mass counts to monitor
wood frog populations in southern Rhode Island from 1997 to 1999.
We completely encircled 8 breeding ponds with drift fence arrays to
quantify adult wood frog population size and then censused egg masses.
Egg mass counts were very correlated with number of females
(r = 0.97, P < 0.001) and males (r = 0.95, P < 0.001), and accounted
for approximately 92% of females captured by drift fence arrays. Counts
of egg masses at the same breeding ponds by 2 observers varied by
12%. Based on studies of egg deposition chronology, breeding
ponds should be surveyed over a 3-week period to ensure that all egg
masses are censused. In smaller breeding ponds with <100 egg masses,
16% of egg masses may be deposited at sites away from the communal
aggregation. Therefore, the entire breeding pond should be searched
to ensure all egg masses are counted. Egg mass surveys are more
cost effective than drift fence arrays and provide more information
than calling surveys. We suggest that egg mass counts may be
an effective means to monitor wood frog populations, as it is a relatively
accurate and precise survey technique.
The research areas were
completely surrounded by driftnet fencing and ex-large coffee cans
(pitfall traps) were placed in the ground to capture migrating animals.