One big reason for this is because individuals of the same species often have to fight over shared resources (this means shared food and shared environment).
Because they fight over a limited amount of space and food, competition between individuals within one species can be present and very tough.
On the other hand, having members of different species also possibly means different territories and different food sources. A lion has no quarrel with dolphins, while it can have a lot of fights with lions from different prides (large group of lions).
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The lichens show mutualism and they are found in every biome of the Earth. The lichens have algae and fungus associated with them and they live in a mutualistic relationship with each other. Lichens can grow on soil where no other species can grow like rocks, barren soils and they enrich the soil on dying where other plants and trees can grow.
Lichens being pioneer species change the environment by building up the soil and liberating nutrients. Also the lichens contribute to the chemical weathering of the soil releasing phosphate along with erosion and weather conditions.
When the simple plants die, the wildflowers and other plants start growing replacing lichens or mosses.
From the above explanation it can be concluded that lichens alter the abiotic environment and favor other species of plants to grow by breaking down rocks from soil and providing nutrients.