Natural ecosystems have far greater biodiversity. They are natural systems in which the plants and animals present are there because they are adapted to the particular conditions of that area. When humans plant gardens/farms, they spend a lot of energy in adapting the conditions to the plants they want to grow. They change the content and pH of the soil, eliminate native plants and insects, and usually grow what are essentially mono-cultures--one plant type in a set area. This is not the natural condition, and it makes it much easier for an insect, a virus, bacteria, or herbivore to come in and destroy the whole crop. In a natural system, plants are much more diverse,making such an occurrence unlikely. Diverse ecosystems are natural, and healthier; human-made ecosystems require a lot of additions that are not generally found in that area.