Meta-data from Top-down response to spatial variation in productivity and bottom-up response to temporal variation in productivity in a long-term study of desert ants

Under the Ecosystem Exploitation Hypothesis ecosystem productivity predicts trophic complexity, but it is unclear if spatial and temporal drivers of productivity have similar impacts. Long-term studies are necessary to capture temporal impacts on trophic structure in variable ecosystems such as deserts. We sampled ants and measured plant resources in the Simpson Desert, central Australia over a 22-year period, during which rainfall varied 10-fold. We sampled dune swales (higher nutrient) and crests (lower nutrient) to account for spatial variation in productivity. We asked how temporal and spatial variation in productivity affects the abundance of ant trophic guilds. Precipitation increased vegetation cover, with the difference more pronounced on dune crests; seeding and flowering also increased with precipitation. Generalist activity increased over time, irrespective of productivity. Predators were more active in more productive (swale) habitat, i.e. spatial impacts of productivity were greatest at the highest trophic level. By contrast, herbivores (seed harvesters and sugar feeders) increased with long-term rainfall; seed harvesters also increased with as seeding increased. Temporal impacts of productivity were therefore greatest for low trophic levels. Whether productivity variation leads to top-down or bottom-up structured ecosystems thus depends on the scale and dimension (spatial or temporal) of productivity.

基于生态系统开发假说（Ecosystem Exploitation Hypothesis），生态系统生产力可预测营养级复杂度，但目前尚不明确生产力的空间与时间驱动因子是否会产生类似影响。针对荒漠这类动态变化的生态系统，开展长期研究是捕捉其营养结构时间动态影响的必要前提。本研究于澳大利亚中部辛普森沙漠开展了为期22年的采样工作，期间降雨量波动幅度达10倍之巨，研究团队在此期间采集了蚂蚁样本并测定了植物资源状况。为考量生产力的空间异质性，我们分别对沙丘洼地（养分含量更高）与沙丘脊线（养分含量更低）进行了采样，旨在探究生产力的时空变化如何影响蚂蚁营养功能群的多度。降雨可提升植被覆盖度，且该差异在沙丘脊线处更为显著；同时植物结实与开花情况亦随降雨增加而提升。广谱性蚂蚁类群的活动强度随时间推移而增强，且不受生产力水平影响。捕食者类群在生产力更高的洼地生境中活动更为频繁，即生产力的空间影响在最高营养级处最为显著。与之相对，植食性类群（包括种子取食者与糖分取食者）的活动强度随长期降雨增加而提升；种子取食者的活动同时也随植物结实量增加而增强。由此可见，生产力的时间影响在低营养级类群中最为显著。因此，生产力变化究竟会驱动形成自上而下还是自下而上构建的生态系统结构，取决于生产力变化的尺度与维度（空间维度或时间维度）。