Using molecular diet analysis to inform invasive species management: A case study of introduced rats consuming endemic New Zealand frogs

The decline of amphibians has been of international concern for more than two decades, and the global spread of introduced fauna is a major factor in this decline. Conservation management decisions to implement control of introduced fauna are often based on diet studies. One of the most common metrics to report in diet studies is Frequency of Occurrence (FO), but this can be difficult to interpret, as it does not include a temporal perspective. Here, we examine the potential for FO data derived from molecular diet analysis to inform invasive species management, using invasive ship rats (Rattus rattus) and endemic frogs (Leiopelma spp.) in New Zealand as a case study. Only two endemic frog species persist on the mainland. One of these, Leiopelma archeyi, is Critically Endangered (IUCN 2017) and ranked as the world's most evolutionarily distinct and globally endangered amphibian (EDGE, 2018). Ship rat stomach contents were collected by kill-trapping and subjected to three methods of diet analysis (one morphological and two DNA-based). A new primer pair was developed targeting all anuran species that exhibits good coverage, high taxonomic resolution, and reasonable specificity. Incorporating a temporal parameter allowed us to calculate the minimum number of ingestion events per rat per night, providing a more intuitive metric than the more commonly reported FO. We are not aware of other DNA-based diet studies that have incorporated a temporal parameter into FO data. The usefulness of such a metric will depend on the study system, in particular the feeding ecology of the predator. Ship rats are consuming both species of native frogs present on mainland New Zealand, and this study provides the first detections of remains of these species in mammalian stomach contents

两栖动物种群衰退问题已引发国际学界关注二十余年，而外来引入动物的全球扩散是导致其衰退的核心因素之一。针对外来引入动物的防控保护管理决策，通常基于食性研究结果制定。食性研究中最常报告的指标之一为出现频率（Frequency of Occurrence, FO），但该指标因未纳入时间维度，往往难以直观解读。
本研究以新西兰入侵性船鼠（黑家鼠，*Rattus rattus*）与特有滑跖蟾属（*Leiopelma* spp.）蛙类为研究案例，探讨基于分子食性分析得到的FO数据能否为入侵物种管理提供科学参考。目前新西兰本土仅存两种特有蛙类，其中阿氏滑跖蟾（*Leiopelma archeyi*）被世界自然保护联盟（IUCN, 2017）列为极危物种，同时在2018年被评为全球进化独特性最高且最濒危的两栖动物（EDGE指数）。
研究团队通过诱杀法捕获船鼠，采用三种食性分析方法（1种形态学法与2种基于DNA的方法）对其胃容物进行检测。我们开发了一套针对所有无尾目物种的新型引物对，该引物对覆盖度良好、分类分辨率高且特异性合理。通过纳入时间参数，我们得以计算每只船鼠每晚的最小摄食事件数，这一指标相较于更为常用的FO更为直观易懂。据我们所知，目前尚无其他基于DNA的食性研究将时间参数纳入FO数据的分析框架。此类指标的实用性取决于研究系统的具体特征，尤其是捕食者的摄食生态习性。
本研究证实，船鼠正取食新西兰本土现存的两种蛙类，同时首次在哺乳动物胃容物中检测到这两种蛙类的残留组织。