Competition between organisms can influence both the abundance of species and the function of ecosystems. Here, I report the results of a field-based aquatic microcosm experiment, where the timing of arrival and abundance of two herbivores, tadpoles (Limnodynastes tasmaniensis) and mosquito larvae (Ochlerotatus notoscriptus), were manipulated to determine their impact on invertebrate community structure and ecosystem processes. Although successful establishment decreased with experimental time, there was no evidence that interactions between tadpoles and mosquitoes decreased the other species' subsequent survival. However, there were negative effects of tadpole addition on other invertebrates, with decreases in the abundance of zooplankton (Moina australiensis) and dipterans (Ephydridae and Chironomus oppositus). The addition of both tadpoles and mosquito larvae also increased ecosystem productivity. The negative effect of tadpoles on invertebrate abundance may result from competition for food or space, while complementary tadpole and mosquito effects on ecosystem processes are likely to result from changes in the dominant pond state. Tadpoles and mosquito larvae might influence the development of the dominant pond state through preferential consumption of edible phytoplankton and bacteria, reducing the competitive pressure on relatively inedible metaphyton, which forms dominating mats.