Data from: Mutualists or parasites? Context-dependent influence of symbiotic fly larvae on carnivorous investment in the Albany pitcher plant

Carnivorous plants allocate more resources to carnivorous structures under nutrient-limited conditions, and relative investment can also be influenced by animals (infauna) that live in association with these plants and feed on their prey. We investigated these effects within a population of the pitcher plant Cephalotus follicularis containing varying densities of larvae of the fly Badisis ambulans. For plants with a relatively high proportion of adult pitchers, increasing larval density was associated with lower relative leaf allocation to new pitcher buds. For plants with relatively few adult pitchers, however, there was greater relative leaf allocation to pitcher buds with increasing larval density. In a field experiment, there was no significant effect of experimental larval presence or absence on the change in carbon to nitrogen ratio (C/N) of plants. Although the direction of the correlation between B. ambulans larvae and relative investment in carnivorous and non-carnivorous structures depends on the relative number of mature structures, whether the larvae enhance or reduce nutrient stress under different conditions remains unclear. The change in C/N was, however, less variable for pitchers that contained larvae, suggesting a stabilising effect. Eighteen of 52 experimental pitchers were damaged by an unknown species, causing the pitcher fluid to drain. These pitchers were significantly more likely to survive if they contained larvae. These results suggest that the relationship between infauna and host varies with the initial resource status and environmental context of the host plant.

食虫植物在养分受限条件下会将更多资源分配至食肉结构，而其资源的相对投入还会受到与之共生并取食其猎物的动物（infauna）的影响。我们以种群内携带有不同密度斑蝇（Badisis ambulans）幼虫的土瓶草（Cephalotus follicularis）为研究对象，探究了上述效应。对于成年捕虫瓶占比较高的植株，幼虫密度升高与新生捕虫瓶芽的叶片相对投入降低存在关联；但对于成年捕虫瓶数量较少的植株，幼虫密度升高则会提升新生捕虫瓶芽的叶片相对投入。在野外实验中，是否添加实验用幼虫对植株的碳氮比（C/N）变化无显著影响。尽管斑蝇幼虫与食虫、非食虫结构相对投入之间的相关方向取决于成熟结构的相对数量，但幼虫在不同条件下是加剧还是缓解养分胁迫仍不明朗。不过，带有幼虫的捕虫瓶的碳氮比变化幅度更小，这提示幼虫具有稳定化效应。52个实验捕虫瓶中有18个被未知物种破坏，导致捕虫瓶内的液体流失；若这些捕虫瓶内带有幼虫，则其存活概率显著更高。上述结果表明，共生动物与宿主之间的关系会随宿主植物的初始资源状态与环境背景发生变化。