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Beyond playing a critical role in energy flow and nutrient cycling, plants interact with and impact their environment in many other ways. Their presence provides not only food but also habitat for other organisms. Plants influence temperature and other aspects of climate. They also compete with one another for resources in the environment. These and many other phenomena demonstrate that plants are not just a passive backdrop on the landscape, but are a dynamic part of their environment.

A central principle of ecology is that organisms must have traits which help them fit and survive in their environment. For example, a cactus produces shallow roots that allow it to rapidly absorb any rainfall in the desert and specialized cells in its stem that swell to store that water. Instead of conducting photosynthesis, the leaves are modified into spines that protect the cactus from animals that may try to eat it. Photosynthesis occurs in the outer layers of its succulent, green stem.

The traits of the cactus described above are its phenotype, which is any structural, biochemical, or behavioral characteristic expressed by an organism. The genes in the DNA that code for the phenotype are the genotype. Genetically based phenotypic traits that promote survival and reproductive success of an organism in its environment are adaptations. For example, the shallow roots, photosynthetic stems, and spines are adaptations that promote cactus survival in the desert.