Finally, there are instances where increased biodiversity may be deleterious. For example, although diverse assemblages of natural enemies (predators, parasitoids and pathogens) are frequently more effective in reducing the density of herbivorous pests, diverse natural enemy communities sometimes inhibit biocontrol, often because enemies attack each other through intra-guild predation. Another example relates to human health, where more diverse pathogen populations are likely to create higher risks of infectious disease, and strains of bacteria and viruses that evolve drug resistance pose health and economic burdens to people. Such examples caution against making sweeping statements that biodiversity always brings benefits to society.Also:
For example, in one data synthesis, 39% of experiments in crop production systems reported that plant species diversity led to greater yield of the desired crop species, whereas 61% reported reduced yield.