Gause's Rule
Russian biologist Gause - are you that competition may occur between populations within an ecosystem for available resources like food space light or shelter. consequently one services and other gets eliminated. Basically only the fittest survive.
Reasons
When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche.
his experiment on paramecium aurelia and paramecium caudatum showed that separately they both survived when culture together paramecium caudatum was eliminated.
exceptions do exist but poorly understood. for example gause rule predicts less planktons species should survive on limited resources solar energy and minerals in sea. still multiple plankton species thrive in small areas.
In humans
hunter-gatherer groups surrounded by other hunter-gatherer groups in the same ecological niche will fight. while hunter-gatherers surrounded by groups with a different means of subsistence can coexist peacefully. Fog Agber 2017.