Towards a mechanistic understanding of variation in aquatic food chain length

Ecologists have long sought to understand variation in food chain length (FCL) 42 among natural ecosystems. Various drivers of FCL, including ecosystem size, 43 resource productivity and disturbance, have been hypothesized. However, when 44 results are aggregated across existing empirical studies from aquatic ecosystems, we 45 observe mixed FCL responses to these drivers. To understand this variability, we 46 develop a unified competition-colonization framework for complex food webs 47 incorporating all of these drivers. With competition-colonization tradeoffs among 48 basal species, our model predicts that increasing ecosystem size generally results in a 49 monotonic increase in FCL, while FCL displays non-linear, oscillatory responses to 50 resource productivity or disturbance in large ecosystems featuring little disturbance or 51 high productivity. Interestingly, such complex responses mirror patterns in empirical 52 data. Therefore, this study offers a novel mechanistic explanation for observed 53 variations in aquatic FCL driven by multiple environmental factors.