Invasion of a Non-Native Anuran Likely Disrupts Pond Ecosystems

Freshwater ecosystems worldwide are under increasing pressure from multiple threats, including invasive species and climate change, with ponds being particularly vulnerable because of their shallow depth and dynamic hydrology. Australian brown tree frogs Litoria ewingii, introduced to New Zealand in 1875, have spread across both main islands, breeding in a large range of shallow fishless ponds. Since native frogs do not produce aquatic tadpoles, any tadpole influence will be novel, so these ecosystems may be especially vulnerable to such influences. To determine the functional role of L. ewingii tadpoles and to uncover potential effects on pond community structure and function, we conducted two 3-week-long mesocosm experiments. The first crossed tadpole presence with predatory invertebrate presence. Mesocosms containing tadpoles had a lower biomass of periphyton and macrophytes, and lower abundance of two families of small-sized invertebrates (Culicidae and Chironomidae) compared to mesocosms without tadpoles, possibly aas a consequence of grazing, predation or reduced dipteran oviposition. However, predatory invertebrates did not affect tadpoles except at their smallest stage, so L. ewingii probably are not subject to strong top-down control by invertebrates in fishless ponds. We evaluated the effects of tadpole density on ecosystem processes in the second experiment where half the mesocosms also were shaded to simulate permanent pond hydrological status; shading stabilised the temperature fluctuations typical of temporary ponds. In shaded mesocosms tadpole density did not change phytoplankton, whereas in unshaded mesocosms, phytoplankton biomass increased with tadpole density, possibly because tadpole excretion enhanced algal growth in high light conditions. Higher densities of tadpoles also decreased dissolved oxygen concentration regardless of shading during the day-potentially an indirect effect of grazing on macrophytes and periphyton reducing photosynthesis. Overall, these non-native tadpoles are likely to be having large influences on ecosystem processes such as nutrient cycling in small ponds, especially when at high densities. In light of these potential effects on pond ecosystems, and the generalist traits of these frogs which could see them spread to even remote alpine ponds, control measures to prevent further range expansion should be considered in New Zealand.