Trophic niche differentiation between invasive and native frog species in urban wetlands of Kunming, China

Non-native species often proliferate and become invasive in urban ecosystems. Knowing the in-teractions between the overlapping trophic niches of invasive and native species is imperative, and answering this question of how they succeed in these ecosystems is an area of active research. Here, frogs were collected from eight sites in the suburbs of Kunming, including 74 non-native American bullfrogs (Lithobates catesbeianus), 115 native Yunnan pond frogs (Nidirana pleuraden, YPF) and 26 non-native black-spotted pond frogs (Pelophylax nigromaculatus, BPF). The diversity indices and niche widths of the sampled frogs' stomach contents were determined, and delta 13C and delta 15N isotopes in their muscle tissue measured for a subset of samples. At sites where multiple species occurred, the prey Shannon-Wiener diversity index and niche width of the bullfrog were higher than those of BPF yet lower than those of YPF. The prey diversity and niche width of YPF decreased significantly at the level of individual populations. The overlap between the trophic niches of bullfrog and sympatric frogs ranged from 0.3 to 0.37 based on their stomach contents, while that between the YPF and BPF was 0.68. These results demonstrate that bullfrog and YPF eat some items in common, for which competition is likely, but this is not as pronounced as between YPF and BPF. Plotting values of delta 13C (x-axis) against delta 15N (y-axis) isotopes, the standard ellipse areas at the LaoY site, where all three species co-occurred, were 23.75, 12.9, and 11.95 for bullfrog, YPF, and BPF, respectively. The maximal standard ellipse area of YPF across the sites without bullfrog invasion was 4.74. These results showed that bullfrog's prey diversity did not exceed that of YPF in the co-distributed environments, yet its isotopic niche was nonetheless larger. Stomach contents can only convey short-term dietary composition whereas carbon iso-topes can reveal relatively long-term nutritional trends, so we conclude the trophic niche width is larger for Bullfrog than YPF. Their degree of overlap is large, which suggests both species have similar ecological functions and experience intense interspecific competition.