The primitive food chain is mutually exclusive, so that different species can continue to survive in nature, piranhas have a nemesis even if they are fierce.

The sea otter is a very cute looking animal that is known to be a professional in being cute, and there are various expressions of it circulating on the internet for internet users to love. Sea otters are not only cute in appearance, but also in their activities such as eating, swimming and sleeping, which all reflect their cute side. In 2013, it was ranked fifth on the list of the world's cutest species.

The existence of reproductive isolation, so that these situations can not occur, even if there are some individuals of different species sometimes complete the mating, but they generally can not produce offspring, or produce offspring can not continue to reproduce