Embedded in ecosystems are non-random stabilizing structures that allow ecosystems to persist in the face of environmental variability. Food web structure is a vital part of this architecture because it determines the flow of energy and nutrients through ecosystems. Food web structure is flexible because it reliably changes with environmental conditions in time and space, thus promoting ecosystems’ capacity to adapt. Flexible food web structure arises when species exhibit rapid, predictable responses to environmental change through shifts in foraging behaviour based on their traits. Ecologists have examined the foraging responses of only single species, but understanding the flexibility of whole food webs requires examining the foraging responses of the many species that comprise ecosystems; however, studying whole food web flexibility requires detailed, large-scale food web data on short timescales. In this thesis, I study the Canadian boreal shield lakes to expand our understanding of flexibility in the whole food webs structure in three important ways. In Chapter 2, I show that key food web members display paired foraging and behavioural responses to increased temperature, generating flexible food web structure along multiple axes. In Chapter 3, I use behaviour as a proxy for feeding data to show that species within thermal guilds display aggregate behavioural responses that imply whole food webs flex with warming. In Chapter 4, I determine that DNA-based stomach content analysis increases prey detection and food web resolution relative to traditional morphological approaches, implying this technique could reveal subtle foraging shifts and flexes in food web structure on short timescales. Taken together, my thesis (a) establishes that numerous species consistently respond to environmental variability based on their traits and drive predictable flexes in whole food web structure that will determine the impacts of climate change on entire ecosystems, and (b) demonstrates that ecologists possess the complementary toolset necessary to study rapid flexes in food web structure. I conclude that species responses represent a potentially powerful, repeated mechanism to stabilize food webs and that flexibility of whole food webs supports the notion that ecosystems are indeed complex adaptive systems. Importantly, human activities erode this flexibility, but by embracing variability, we can seek ways to conserve the fundamental stabilizing structures ingrained throughout ecosystems.