Zhao, Q.; Wang, Y. X. G.; Xu, C.; Sugihara, G.; De Laender, F.

Across species, body size and population abundance are often assumed to be tightly linked, with smaller species reaching higher abundance. Consequently, either size or abundance is commonly used as a proxy to assess responses to abiotic and biotic environmental changes and to inform conservation strategies. However, whether the tight relationship holds within species remains unclear. Here, we analyzed 41 natural ecosystems spanning 10 to 48 years and found that species abundance and size are not tightly linked over time. Only 14.0% of species abundance was causally linked to size, while 21.5% of size was causally linked to abundance. Only 7.7% of species had bidirectional causal links among size and abundance. Warming was associated with decoupling between abundance and size. Our findings suggest that species abundance and size tend to be decoupled over time under climate change. Thus, abundance and size time series can provide distinct insights into species responses to climate change and should be considered separately in ecosystem conservation and stability assessments.