Niche construction is the process whereby organisms modify their own and/or each other’s niches through their metabolism, activities, and/or choices. This can result in changes in one or more natural selection pressures in the external environment of populations. Niche-constructing species may either alter the natural selection pressures of their own population, of other populations, or of both. In ecology, foundation species are species that have a strong role in structuring a community. Cancer cells act as a foundation species and also act as ecosystem engineers to construct new system niches. This may lead to increased genetic instability through evolutionary adaptation. Most constructed ecosystems eventually reach a point of homeostasis (equilibrium) that creates an environment that allows the foundation species to thrive. There is no evidence that cancer reaches this ecological endpoint. It is worth exploring if this can be exploited as a therapeutic target.