Summary:
Food webs have long been known to exhibit intervality: species can be ordered along a single axis in such a way that the prey of any given predator tend to lie on unbroken intervals. Although the meaning of this axis, or niche dimension, has remained a mystery, it is widely assumed to be the reason behind the highly non-trivial structure of food webs. For decades, therefore, most food-web modelling has been based on assigning species a niche value by hand. However, going on empirical evidence and a simple self-assembling network model, we show here that realistic intervality — and many other relevant features — can come about as a consequence of biologically plausible mechanisms that do not require an a priori ordering. This calls for a fundamental change in perspective as to how ecosystems arise and persist.