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.

依据生态系统开发假说（Ecosystem Exploitation Hypothesis），生态系统生产力可预测营养级复杂性（trophic complexity），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。针对荒漠等动态变化的生态系统，开展长期研究方能捕捉其营养结构所受的时间维度影响。本研究于澳大利亚中部辛普森沙漠开展，为期22年，期间降雨量波动幅度达10倍；研究期间我们开展了蚂蚁采样工作，并测定了植物资源指标。我们对沙丘洼地（养分含量更高）与沙丘脊部（养分含量更低）进行采样，以纳入生产力的空间变异因素。本研究旨在探究生产力的时空变异如何影响蚂蚁营养功能群（trophic guilds）的丰度。降雨量提升可增加植被覆盖度，且该效应在沙丘脊部更为显著；同时，植物结实与开花情况也随降雨量增加而改善。广食性蚂蚁类群的活动强度随时间推移而提升，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，亦即生产力的空间效应在最高营养级上表现最为显著。与之相对，植食性类群（包括种子采食者与糖食者）的丰度随长期降雨量增加而提升；种子采食者的丰度同时也随植物结实量增加而增长。由此可见，生产力的时间效应在低营养级类群上表现最为显著。因此，生产力变异究竟会造就顶下行（top-down）还是自下而上（bottom-up）的生态系统结构，取决于生产力变异的尺度与维度（空间或时间维度）。