describe the positive interaction between populations of different species in a community?

Positive interactions are the interactions in which both the interacting species are benefited but neither is harmed. The different types of positive interactions are mutualism or symbiosis and commensalism.  

• Mutualism or symbiosis − It is the positive interaction , in which both the partners are benefited. For example, lichens, interaction of algae and fungi, where both are benefited. The fungi give support to the algae while the algae prepare the food for the fungi.
• Commensalism − In this interaction, one of the partners is benefited while the other is neither benefited nor harmed. For example, an orchid growing as an epiphyte on the mango tree. The orchid gets support while the mango tree is unaffected.