How has the remnant population of the threatened frogLeiopelma pakeka(Anura: Leiopelmatidae) fared on Maud Island, New Zealand, over the past 25 years?

Abstract

Despite widespread global reports of declining amphibian populations, supporting long-term census data are few, limiting opportunities to study changes in numbers and survival over time. However, in New Zealand, for the past 25 years (1983–2008), we studied Leiopelma pakeka, a threatened, terrestrial frog that inhabits rocky boulder banks under forest on Maud Island. Using night sampling at least annually on two 12 × 12 m plots, we had 5390 captures of 1000+ individuals, 327 on one plot (grid 1), 751 on the other (grid 2). The mean (±SE) number of frogs found per night was 11.3 (±0.6) on grid 1 and 25.6 (±1.4) on grid 2. We used capture-recapture models to estimate population size, proportion of animals remaining beneath the surface and survival rate. The mean (±SE) population estimate was 131 (±14.7) frogs on grid 1 and 367 (±38.7) on grid 2. Over 25 years the estimated population increased on grid 1 and fluctuated on grid 2. Some frogs were captured on most sampling visits, others less often, evidently failing to emerge from cover each visit. Using a combination of open and closed population models, we estimated the mean (±SE) proportion remaining underground was 0.63 (±0.12) on grid 1 and 0.53 (±0.07) on grid 2. Our research represents one of the longest-run population studies of any frog, and we recorded significant longevity, two males reaching 35+ and 37+ years, a female 34+ years. No significant differences occurred between mean annual survival rates of apparent females and males, or between the two sites. The number of toes clipped for individual identification had little influence on the return rate, once the effect of time of first capture was removed.