Biodiversity of Hong Kong 3 is a four-episodes series of natural history documentation of Hong Kong Wilde Life. The focus is the interaction between different species of organisms. The commensalism, the mutualism, the parasitism , the competition and the predation.
Parasitism means that one benefits the other and the other suffers from it. As being highly efficient is one of the most successful ways of surviving, many species have adapted to live the parasite lifestyle.
"Lianli tree" are not really in love with each other, hermit crabs catch each other's shell. Butterflies climb the woodpecker to court, and egrets build their nests. Life seems to be avoiding that competition.
Nature is not necessarily ruthless. Some species originally lived together, and two species would bring benefits when they came together. Sometimes it is difficult to survive and reproduce without each other. This relationship is called symbiosis.