Communities at the extreme: aquatic food webs in desert landscapes

Studying food webs across contrasting abiotic conditions is an important tool in understanding how environmental variability impacts community structure and ecosystem dynamics. The study of extreme environments provides insight into community-wide level responses to environmental pressures with relevance to the future management of aquatic ecosystems. In the western Lake Eyre Basin of arid Australia, there are two characteristic and contrasting aquatic habitats: springs and rivers. Permanent isolated Great Artesian Basin springs represent hydrologically persistent environments in an arid desert landscape. In contrast, hydrologically variable river waterholes are ephemeral in space and time. We comprehensively sampled aquatic assemblages in contrasting ecosystem types to assess patterns in community composition and to quantify food web attributes with stable isotopes. Springs and rivers were found to have markedly different invertebrate communities, with rivers dominated by more dispersive species and springs associated with species that show high local endemism. Qualitative assessment of basal resources shows autochthonous carbon appears be a key basal resource in both types of habitat, although the particular sources differed between habitats. Food-web variables such as trophic length, trophic breadth and community isotopic niche size were relatively similar in the two habitat types. The basis for the similarity in food-web structure despite differences in community composition appears to be broader isotopic niches for predatory invertebrates and fish in springs as compared with rivers. In contrast to published theory, our findings suggest that the food webs of the hydrologically variable river sites may show less dietary generalisation and more compact food-web modules than in springs.

研究不同非生物条件（abiotic conditions）下的食物网（food web），是解析环境变异如何影响群落结构（community structure）与生态系统动态（ecosystem dynamics）的重要研究手段。对极端环境的研究能够揭示群落整体对环境压力的响应规律，可为未来水生生态系统（aquatic ecosystems）的管理提供科学参考。在澳大利亚干旱区的艾尔湖盆地（Lake Eyre Basin）西部，存在两类特征鲜明、生境条件迥异的水生栖息地：泉水与河流水潭。永久隔离的大自流盆地（Great Artesian Basin）泉水，是干旱荒漠景观中水文条件稳定的生境；与之相对，水文条件多变的河流水潭则在时空尺度上呈现暂时性特征。我们针对两类差异显著的生态系统类型开展了全面的水生群落采样，以解析群落组成模式，并借助稳定同位素技术量化食物网属性。研究结果显示，泉水与河流水潭生境的无脊椎动物（invertebrate）群落存在显著差异：河流水潭生境以扩散能力更强的物种为优势类群，而泉水生境则以具有高度地方特有性（local endemism）的物种为典型代表。对基础资源（basal resources）的定性分析表明，尽管两类生境的具体基础碳源存在差异，但自生源碳（autochthonous carbon）均为二者的关键基础资源。两类生境的食物网变量，如营养长度（trophic length）、营养宽度（trophic breadth）及群落同位素生态位（community isotopic niche）面积，整体表现出较高的相似性。尽管群落组成存在显著差异，但两类生境的食物网结构却呈现相似性，其核心原因在于：泉水生境中的捕食性无脊椎动物与鱼类的同位素生态位较河流水潭生境更为宽泛。与已发表的相关理论相悖，本研究结果表明：相比泉水生境，水文条件多变的河流水潭生境的食物网可能表现出更低的饮食泛化程度，且食物网模块更为紧凑。