托福口语 Task 3

In biology, the concept of 'commensalism' refers to a symbiotic relationship between two species in which one species benefits while the other is neither helped nor harmed. The benefiting species often gains food, shelter, or transportation, while the host species remains unaffected. Commensalism is distinguished from mutualism, where both species benefit, and parasitism, where one species is harmed. This relationship is common in nature and can be observed in various ecological contexts.

Professor: Let’s take a look at a classic example of commensalism—the relationship between cattle egrets and grazing cattle. Cattle egrets are birds that follow herds of cattle as they move through grasslands. As the cattle walk, they disturb insects and small creatures in the grass, making them easier for the egrets to catch. The birds eat these insects, so they clearly benefit—they get an easy meal without expending much energy. On the other hand, the cattle are completely unaffected by the presence of the egrets; they neither gain nor lose anything. The cattle are not helped by the birds, nor are they harmed. So, this relationship fits the definition of commensalism perfectly: the egrets benefit, the cattle are indifferent.