Mutualism is the way two organisms of different species exist in a relationship in which each individual benefits from the activity of the other. Similar interactions within a species are known as co-operation. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other. Symbiosis is a broad category that is defined to include relationships that are mutualistic, parasitic, or commensal. Mutualism is one type of symbiosis.