How does the once-ler affect the animals?

The Once-ler's actions significantly harm the animals in the ecosystem by exploiting the resources of the Truffula Trees for profit. As he cuts down the trees to produce Thneeds, the environment becomes degraded, leading to the displacement and endangerment of various creatures, such as the Bar-ba-loots, Swomee-Swans, and Humming-Fish. The pollution and destruction caused by his factory further deteriorate their habitat, ultimately forcing them to leave in search of a healthier environment. This highlights the broader theme of environmental degradation and the consequences of prioritizing profit over ecological balance.