Land use impacts poison frog chemical defenses through changes in leaf litter ant communities

Much of the world’s biodiversity is held within tropical rainforests, which are increasingly fragmented by agricultural practices. In these threatened landscapes, there are many organisms that acquire chemical defenses from their diet and are therefore intimately connected with their local food webs. Poison frogs (Family Dendrobatidae) are one such example, as they acquire alkaloid-based chemical defenses from their diet of leaf litter ants and mites. It is currently unknown how habitat fragmentation impacts chemical defense across trophic levels, from arthropods to frogs. We examined the chemical defenses and diet of the Diablito poison frog (Oophaga sylvatica), and the diversity of their leaf litter ant communities in secondary forest and reclaimed cattle pasture. Notably, this research was performed in collaboration with two high school science classrooms. We found that the leaf litter of forest and pasture frog habitats differed significantly in ant community structure. We also found that forest and pasture frogs differed significantly in diet and alkaloid profiles, where forest frogs contained more of specific alkaloids and ate more ants in both number and volume. Finally, ant species composition of frog diets resembled the surrounding leaf litter, but diets were less variable. This suggests that frogs tend to consume particular ant species within each habitat. To better understand how ants contribute to the alkaloid chemical profiles of frogs, we chemically profiled several ant species and found some alkaloids to be common across many ant species while others are restricted to a few species. At least one alkaloid (223H) found in ants from disturbed sites was also found in skins from pasture frogs. Our experiments are the first to link anthropogenic land use changes to dendrobatid poison frog chemical defenses through variation in leaf litter communities, which has implications for conservation management of these threatened amphibians.全球绝大多数生物多样性均蕴藏于热带雨林之中，而这类生态系统正日益受到农业活动的割裂破碎。在这些受威胁的生态景观中，诸多生物通过摄取食物获取化学防御能力，进而与当地食物网形成紧密关联。树棘蛙科（Dendrobatidae）的毒蛙便是其中一例：它们以落叶层中的蚂蚁和螨类为食，从中获取基于生物碱的化学防御物质。目前学界尚不明确生境破碎化如何影响从节肢动物到蛙类的各营养级的化学防御机制。为此，我们针对迪阿布利托毒蛙（Oophaga sylvatica），研究了其化学防御与食性，并调查了次生林及复垦牛牧场生境中该蛙类所在落叶层的蚂蚁群落多样性。值得注意的是，本研究与两个高中理科课堂合作完成。研究结果显示，林栖蛙生境与牧场栖蛙生境的落叶层蚂蚁群落结构存在显著差异。同时，林栖蛙与牧场栖蛙的食性及生物碱谱亦存在显著差异：林栖蛙体内的特定生物碱含量更高，且捕食的蚂蚁数量与体积均更为可观。此外，蛙类食物中的蚂蚁物种组成与周边落叶层的蚂蚁群落相似，但蛙类食性的变异性更低，这表明蛙类在各自生境中倾向于取食特定的蚂蚁物种。为进一步明确蚂蚁如何影响蛙类的生物碱化学谱，我们对多种蚂蚁物种开展了化学谱分析，结果显示部分生物碱在多种蚂蚁中普遍存在，而另一些则仅局限于少数蚂蚁物种。在受干扰生境的蚂蚁体内检出的至少一种生物碱（223H），同样在牧场栖蛙的皮肤提取物中被发现。本研究首次通过落叶层群落的差异，将人类土地利用变化与树棘蛙科毒蛙的化学防御机制建立关联，该发现对这类受威胁两栖动物的保护管理具有重要参考价值。