Temperature affects food-chain length and macroinvertebrate species richness in spring ecosystems

I assessed effects of the widest range of temperatures ever examined on maximal food-chain length (L) and macroinvertebrate species richness (S) by a worldwide comparison of spring ecosystems with mean water temperatures (T) ranging from 4.5 to 93°C. Eukaryotic L averaged ∼3.2 and varied independently of T between 4.5 and 31°C. However, over the relatively narrow T range of 35 to 50°C, L dropped abruptly to 0 and remained so up to 91°C. The negatively nonlinear relationships of L vs T and macroinvertebrate S vs T both deviated from predictions based upon metabolic theory, and the negative effect of T on S contrasted with positive relationships observed at larger regional scales. Thermal tolerance limits apparently play a major role in causing these relationships, but other factors also may be involved (e.g., availability of colonists adapted to different temperatures and temperature-dependent rates of resource use and species interactions that affect population establishment and persistence).