With accelerating species introductions in an era of globalization, co-occurring alien species have become increasingly common. Understanding the combined ecological impacts of multiple invaders is not only crucial for wildlife managers attempting to ameliorate biodiversity loss, but also provides key insights into invasion success and species coexistence mechanisms in natural ecosystems. Compared with much attentions given to single-invader impacts, little is known about the impacts of multiple co-occurring invaders. The American bullfrog (Lithobates catesbeianus = Rana catesbeiana) and the red swamp crayfish (Procambarus clarkii) are two aquatic invasive species in many different areas of the globe. They coexist with native anurans in a variety of permanent lentic waters, which provide an ideal model system to explore the combined effects of multiple invaders from different trophic levels on native species. Based on a global diet analysis covering 34 native and invasive bullfrog populations, and data from 10-year field surveys across 157 water bodies in the Zhoushan Archipelago, China, we observed a reduced impact of bullfrogs on native anurans at high crayfish densities when the two invaders co-occurred. The global diet analysis showed that crayfish occurrence reduced the number of native anuran prey consumed by bullfrogs in both native and invasive populations. After accounting for pseudoreplication of different observations among water bodies, islands, and survey time, model averaging analyses based on GLMMs showed a negative relationship between bullfrog density and native anuran densities for field observations of invasive bullfrogs alone and co-invaded observations with low crayfish density. However, this negative relationship disappeared when the two invaders co-occurred with high crayfish density. Structural equation modelling (SEM) analyses further validated that the impacts of bullfrogs on native frogs were mitigated by the negative interactions between crayfish and bullfrogs. Our results provide novel evidence of a density-dependent antagonistic effect of two sympatric invaders from different trophic levels on native species. This study highlights the importance of considering complex interactions among co-invaders and native species when prioritizing conservation and management actions and will facilitate the development of a more precise framework to predict invasion impacts.

Summary1. Shifts in the body size of insular vertebrates have been an interesting theme in ecological and evolutionary studies. Four primary factors, including predation pressures, resource availability, inter-species competition and immigrant selection, have been proposed to explain the trend in insular body size. Life-history theory predicts that body size, average age, the proportion of old-aged members and the density of insular populations are negatively correlated with predator species richness, and that body size and population density are positively related to resource availability. The niche expansion hypothesis argues that a positive relationship is expected to exist between insular body size and prey size, which varies in response to extinction due to small or large competitors. The immigrant hypothesis predicts that insular body size is positively correlated with distance to the mainland. 2. We tested these hypotheses by using populations of rice frogs Rana limnocharis on 20 islands in the Zhoushan Archipelago and two sites of nearby mainland China. 3. The body size (snout-vent length) of rice frogs on half of the islands was larger before and after the variable of age was controlled for; rice frog density and prey availability was higher and prey size was larger on most of the islands as compared to the two mainland sites. On the islands, the body size and other features [e.g. average age, the proportion of old-aged frogs (ages 3 and 4) and density] of the rice frogs were negatively associated with predator species richness; female body size and other features were positively associated with prey availability. The inference of multivariate linear models based on corrected Akaike Information Criterion (AIC c ) showed that the relative importance of predator species richness on body size and each of the other features was larger than that of prey availability, prey size and distance to the mainland. In addition, the parameters for predator species richness were all negative. 4. The results provided strong support for the life-history theory of predation pressures, but weak evidence for the life-history theory of prey availability, the niche expansion or the immigrant hypothesis. The reduced predator species richness was a dominant factor contributing to the body gigantism of rice frogs on the islands.