Other types of species interactions include commensalism, which is an interaction that stimulates one organism but has no effect on the other. Another example is protocooperation, which is an interaction that stimulates both partners, but it is not obligatory because stable growth continues in the absence of the interaction. Common examples of mutualism are mycorrihizae, nitrogen fixation, and pollination. Mycorrhizae are fungal associations with the roots of higher plants. Types of mycorrizae are ectomycorrhizae and endomycorrhizae.
Nitrogen fixation is the conversion of atmospheric nitrogen gas into organic ammonium.
Next I want to discuss weeds. A weed by definition depends on the opinion of the one doing the defining. A weed is simply a plant found growing rapidly and abundantly in a place that it is not wanted. From ecological perspective weeds may be regarded as colonizers, plants that are adept at capitalizing on disturbed areas. Few weeds are actually native to the United States.