Biology is considered preparatory work for a degree in ecology because of its use in understanding processes in living matter. Beginning biology courses emphasize ecological processes and review patterns/processes that influence the evolution and distribution of living organisms. More advanced courses, such as Biometrics, require students to design biological experiments while providing analysis and data.
Chemistry is at the heart of any scientific degree, but is especially important to ecology because it helps to understand the environmental impact of chemical forces, such as pollution. Chemistry courses emphasis a firm grasp of periodic elements, kinetics and molecular structure. Chemistry also involves a fair amount of math, which is used in stoichiometry and other equations.
Courses in environmental science vary widely, but will cater specifically to the type of ecology degree you pursue. Examples involve oceanography, environmental toxicology, environmental risk assessment and climate change. Many courses will overview ecological processes in specific environments, such as streams, wetlands and forests.
While many degree programs in ecology do not require math courses as part of the curriculum, it is assumed and sometimes required that you have learned calculus. A solid grasp of advanced algebra and analytical geometry is useful, if not necessary, to complete upper-level courses in an ecology degree.