Age dependant effects of rat control on Archey’s frog (Leiopelma archeyi) at Whareorino, New Zealand

Predation by introduced mammals is considered a primary threat to New Zealand’s native frog populations. Rats are known predators of the terrestrial Archey’s frog (Leiopelma archeyi), New Zealand’s smallest Leiopelmatid frog. During a 12 year study in Whareorino Conservation Area, we investigated effects of sustained rat control on survival, number of independent juveniles per adult, and abundance of Archey’s frog. Frogs were monitored following a capture-recapture robust design at four grids, split between a 300-ha ‘nontreatment’ area and a 300-ha rat control ‘treatment’ area. Low rat abundances were achieved in the treatment area for much of the study (rat tracking index threshold: < 10%), although, mouse abundances were higher at times there. Adult frog survival rates were greater in treatment (0.74–0.78) compared to nontreatment (0.53–0.55) areas, indicating rat suppression reduces adult frog mortality; however, juvenile and subadult frog survival was lower in treatment (0.03–0.27) than nontreatment areas (0.26–0.61). This may be due to increased vulnerability of smaller frogs to other predators e.g. mice. Population modelling showed ongoing rat suppression has a positive effect on the rate of independent juveniles produced per adult frog and on adult abundance over time, revealing a significant increase in frog abundance in treatment (annual rate of increase, adult frogs: 10.75, 95%; CI [4.62, 17.24]) compared to nontreatment areas where frogs declined (annual rate of decline, adult frogs: −5.73, 95% CI [−10.95, −0.21]). We show that suppressing rat populations is successful for frog population management at Whareorino Conservation Area. However, rat control alone may not be sufficient to recover depleted populations elsewhere or grow newly translocated populations on the mainland. We recommend further work to substantiate the above findings and research to investigate both the impact of mice on native frogs and elucidate age class dependant parameters for Leiopelmatid frogs in mainland and island populations.