Final answer:
A disease outbreak among grey wolves would negatively affect Yellowstone's grey wolf carrying capacity by directly decreasing their population and thereby affecting ecosystem stability and health.
Step-by-step explanation:
The event among the listed options that would negatively impact Yellowstone's grey wolf carrying capacity is a disease outbreak among grey wolves. Carrying capacity is defined as the maximum number of individuals of a particular species that an environment can support sustainably. An increase in prey population would generally increase carrying capacity, as there would be more food available for the wolves. The introduction of a new predator could potentially impact carrying capacity, but not necessarily negatively; it could lead to more balanced ecosystem dynamics. A decline in vegetation could have indirect negative effects by reducing prey populations, but it's not as directly impactful as a disease outbreak. A disease outbreak among the wolves themselves would directly decrease their population by increasing mortality, thus immediately affecting carrying capacity negatively.
The reintroduction of wolves to Yellowstone, a keystone species, dramatically shaped the ecosystem. By changing grazing behaviors of elk and allowing vegetation like willow and cottonwood to grow, wolves helped decrease erosion and improve habitats for other species, such as beavers and fish. Such ecosystem changes demonstrate the importance of predators in maintaining healthy ecosystems and support the notion that a disease outbreak among wolves, by potentially reducing their numbers, would negatively impact not just their carrying capacity but also the wider ecosystem health.