Final answer:
The tsetse fly is called a 'biological barrier' as it transmits the parasite Trypanosoma brucei, causing African sleeping sickness and limiting human and livestock presence in affected areas.
Step-by-step explanation:
The tsetse fly is referred to as the biological barrier because it keeps humans and cattle out of wilderness areas. The tsetse fly is a carrier of Trypanosoma brucei, the parasite responsible for African sleeping sickness in humans and nagana in cattle and horses. Its presence in certain areas makes them inhospitable for human and livestock habitation due to the threat of disease transmission. These flies contribute significantly to the shaping of the geographical and socioeconomic landscape in affected areas, influencing where people can live, farm, and raise livestock safely without the risk of disease.