Data_Sheet_1_Disentangling the simultaneous effects of habitat degradation and introduced species on naturally depauperate riverine fish communities.docx

IntroductionGlobal change is a multi-faceted issue putting many species at risk. The broad range of potentially interacting environmental stressors is problematic for effective and efficient conservation and management. In freshwater systems, habitat degradation and introduced species have been repeatedly recognized for their extensive impacts on native ecosystems. However, the simultaneous impacts of these environmental stressors on naturally depauperate and inherently vulnerable communities are poorly understood.
MethodsIn southern New Zealand, the fish communities in 14 tributaries of three lowland lakes were surveyed to quantify the within- and between-community changes along gradients of habitat complexity and abundance of introduced species, specifically brown trout (Salmo trutta Linnaeus) and redfin perch (Perca fluviatilis Linnaeus).
ResultsStable isotope analyses identified that trophic diversity increased with habitat complexity and an abundance of native eels (Anguilla spp.) but was unaffected by introduced species. Within each community, only perch exhibited distinct dietary shifts along all environmental gradients, whereas trout and the native fish had consistent, generalist diets. When supported with length–weight regressions, these impacts became increasingly size-dependent. For example, among the native fish, only the larger eels were unaffected by habitat and achieved greater body conditions with increased numbers of eels and perch; however, more trout were detrimental to eel body condition. In contrast, the smaller bodied natives, including elvers, all had improved body conditions from increased habitat complexity and reduced numbers of trout and perch. For the introduced species, perch weights were consistent regardless of the local environment due to their variable diet, but larger trout generally increased in weight with reduced habitat complexity and greater numbers of introduced fish, although high eel densities were detrimental.
DiscussionOverall, our results highlight how the responses to environmental stressors, even in depauperate communities, are complicated and generally species-specific. Nonetheless, habitat degradation had the most wide-ranging negative impacts on native fish, with perch numbers only affecting the smaller bodied natives and trout only affecting one native species. We conclude that focusing on habitat restoration in conservation strategies will provide the most efficient and effective use of resources, although the realized benefits for native species will be limited if introduced species are overly abundant.

引言
全球变化是一个多维度的问题，使众多物种面临生存风险。众多存在潜在交互作用的环境胁迫因子，给高效且精准的保护与管理工作带来了挑战。在淡水生态系统中，生境退化与外来物种对本土生态系统造成的广泛影响已被多次证实。然而，目前对这些环境胁迫因子同时作用于自然物种贫乏且先天脆弱的群落所产生的影响，尚缺乏深入认知。

研究方法
本研究在新西兰南部，对3个低地湖泊的14条支流中的鱼类群落开展调查，以量化生境复杂度梯度、外来物种（特指褐鳟（Salmo trutta Linnaeus）与红鳍鲈（Perca fluviatilis Linnaeus））丰度梯度下，群落内部与群落间的结构变化。

研究结果
稳定同位素分析显示，营养多样性随生境复杂度与本土鳗类（Anguilla spp.）丰度的提升而升高，但不受外来物种丰度的影响。在各群落内部，仅红鳍鲈表现出沿所有环境梯度的显著饮食结构转变，而褐鳟与本土鱼类则保持稳定的广食性饮食模式。结合体长体重回归模型分析后，上述影响呈现出显著的体型依赖性。例如，在本土鱼类中，仅体型较大的鳗类不受生境条件影响，且随鳗类与红鳍鲈数量的增加，其身体状况更佳；但褐鳟数量增多则会对鳗类的身体状况产生不利影响。与之相反，包括鳗苗在内的小型本土鱼类，其身体状况均随生境复杂度提升、褐鳟与红鳍鲈数量减少而得到改善。对外来物种而言，红鳍鲈的体重不受局部环境影响，这与其饮食结构多变有关；而体型较大的褐鳟体重通常随生境复杂度降低、外来鱼类数量增多而增加，但高鳗类密度会对其产生不利影响。

讨论
综合来看，本研究结果表明，即便在物种贫乏的群落中，生物对环境胁迫因子的响应机制依然复杂，且整体上具有物种特异性。尽管如此，生境退化对本土鱼类造成的负面影响最为广泛；红鳍鲈数量仅会影响小型本土鱼类，而褐鳟仅会对一种本土鱼类产生影响。综上，我们认为在保护策略中优先开展生境修复，能够实现资源的高效利用；但若外来物种数量过多，本土鱼类所能获得的实际收益将十分有限。