In natural ecosystems, the presence of predators lessens the effects of among the prey..

The answer is competition. 

Competition is an interaction between organisms. If resources, such as food, water, sunlight, territory, are limited and several species depend on these resources, then, they must compete with other species to gain the resources. So, if predators are present, this means that they will maintain the size of prey population on some optimal level. With fewer individuals in the population, there will be less competition for resources.
Therefore, In natural ecosystems, the presence of predators lessens the effects of competition among the prey.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.