Natural Phenomena

Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use. 

Interspecific competition where native species are competing with nonnative species for resources (e.g., nutrients, space, water, etc.)

The sequence of biochemical changes undergone by nitrogen, wherein it is used by a living organism, liberated upon the death and decomposition of the organism and converted to its original state of oxidation.

Source (via NALT): Schwarz, Charles F.; Thor, Edward C.; Elsner, Gary H. 1976. Wildland planning glossary. Gen. Tech. Rep. PSW-13. Berkeley, Calif.: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station