Impacts of deforestation-induced warming on the metabolism, growth, and trophic interactions of an afrotropical stream fish

1. In ectotherms, anthropogenic warming often increases energy requirements for metabolism, which can either impair growth (when resources are limiting) or lead to higher predator feeding rates and possibly stronger top-down trophic interactions. However, the relative importance of these effects in nature remains unclear because: 1) thermal adaptation or acclimation could lower metabolic costs; 2) greater prey production at warmer temperatures could compensate for higher predator feeding rates; and/or 3) temperature effects on trophic interactions via altered biological rates could be small relative to other, temperature-unrelated human impacts on food webs. 2. Here, we examined effects of deforestation-associated warming on the minnow Enteromius neumayeri, occurring in both forested (cool) and deforested (warm) streams located inside or nearby an afrotropical rainforest. Combining approaches from physiological and community ecology, we quantified impacts of anthropogenic warming on the metabolism, growth, and trophic interactions of this tropical ectotherm. We then compared these effects with impacts of land use unrelated to temperature. 3. In a long-term laboratory acclimation experiment quantifying the temperature-dependence of growth and metabolism in E. neumayeri, warming increased metabolic rates and decreased growth (at a limited ration). We found no evidence of local (thermal) adaptation, with warming affecting farm and forest populations similarly. 4. Then, using mark-recapture methods to quantify impacts of warming on performance in situ, we found similar growth rates in fish from deforested and forested streams despite their distinct thermal environments. This suggests higher prey consumption at deforested sites to compensate for greater metabolic costs, which could strengthen fish-invertebrate interactions. 5. Finally, we developed a bioenergetics model to estimate fish-invertebrate interaction strength and quantify temperature-related and unrelated impacts of land use on this interaction. We found that although warming increased fish consumption, it apparently increased invertebrate production even more and thus had a net weakening effect on estimated interaction strength. Most importantly, variation in both fish and invertebrate density not directly related to temperature had a much stronger influence on estimated interaction strength than temperature effects on predator consumption and prey growth.
6. We conclude that ectotherms can sometimes offset the metabolic costs of warming with a small increase in consumption that hardly effects food web interactions compared to non-metabolic impacts of anthropogenic disturbances. Future research should assess whether this is a common feature of heavily-impacted ecosystems facing multiple stressors.

1. 变温动物（ectotherm）的新陈代谢能量需求往往会因人为变暖（anthropogenic warming）而升高，该效应既可能在资源受限条件下抑制个体生长，也可能提升捕食者的摄食率，进而增强自上而下的营养级相互作用（trophic interaction）。但目前学界仍不清楚这些效应在自然生态系统中的相对重要性，原因包括：①热适应（thermal adaptation）或温度驯化（acclimation）可降低代谢成本；②升温环境下更高的猎物生产力可补偿捕食者升高的摄食需求；以及/或③相较于其他非温度相关的人类活动对食物网的影响，温度通过改变生物速率对营养级相互作用产生的效应相对微弱。
2. 本研究以栖息于非洲热带雨林（afrotropical rainforest）内部及周边林溪（低温环境）与毁林溪流（高温环境）中的鲤科小型鱼类Enteromius neumayeri为研究对象，探究了与毁林相关的变暖对其产生的影响。结合生理生态学（physiological ecology）与群落生态学（community ecology）的研究方法，我们量化了人为变暖对该热带变温动物的代谢、生长及营养级相互作用的影响，并将其与非温度相关的土地利用活动的影响进行了对比。
3. 在一项量化Enteromius neumayeri生长与代谢温度依赖性的长期实验室驯化实验中，我们发现升温提升了其代谢速率，并在限量饵料条件下抑制了个体生长。未检测到本地热适应的证据，升温对森林种群与农田种群的影响并无显著差异。
4. 随后，我们利用标记重捕法（mark-recapture methods）量化了升温对鱼类原位生存表现的影响，结果发现尽管栖息环境的温度存在显著差异，毁林溪流与林溪中的鱼类个体生长速率并无显著差异。这表明毁林溪流中的鱼类摄食率更高，以补偿其因高温升高的代谢成本，进而可能强化鱼类与无脊椎动物（invertebrate）之间的相互作用。
5. 最后，我们构建了生物能量模型（bioenergetics model），以估算鱼类与无脊椎动物之间的相互作用强度，并量化土地利用活动中与温度相关及无关的效应对该相互作用的影响。结果显示，尽管升温提升了鱼类的摄食率，但其对无脊椎动物生产力的促进作用更为显著，因此对估算得到的相互作用强度产生了净削弱效应。尤为关键的是，与温度无直接关联的鱼类与无脊椎动物密度变异，对相互作用强度的影响远大于温度对捕食者摄食率及猎物生长的效应。
6. 综上，变温动物有时可通过小幅提升摄食率来抵消升温带来的代谢成本，但相较于人为扰动的非代谢性影响，该补偿效应对食物网相互作用的影响微乎其微。未来研究应进一步评估，这一现象是否是面临多重胁迫的重度受损生态系统的普遍特征。