What is one effect of a loss of biodiversity in an ecosystem?
The ecosystem is less stable and resilient.
I'll start with the foundation for this: trophic levels. Every species fits somewhere in the trophic pyramid, whether they're a producer, decomposer, or primary, secondary, or tertiary consumer. No matter what, they are important to maintaining the ecosystem's balance. If the producers die off, the ecosystem has no food; If the top predators die off, population explosions can drive the ecosystem down; bottom line, every trophic level is crucial.
Higher biodiversity means there are more species in each trophic level, which means that on every level of the food chain, there is a larger menu of food. This means that if one species dies off, other species can sub in to make sure the trophic level stays intact, and that ensures that the ecosystem can survive.
Low biodiversity means there are less species per trophic level, which makes that ecosystem very vulnerable to disturbance. If one species goes in this type of an ecosystem, the entire pyramid could collapse because the trophic levels are increasingly reliant on individual species instead of a collective of species in each trophic level.
Desert ecosystems aren't very diverse because of the extreme environment. They have a few species of producer, a couple primary/secondary consumers, and a couple top predators. Let's say that a virus swept through a desert and killed off most of the coyotes, a top predator, in an area. This would allow the populations of jackrabbits, kangaroo rats, mice, etc. to explode in population.
The higher a population, the more food it needs. So all of these consumers will now be competing for all the vegetation in the area, and, if the situation is bad enough, could actually deplete the vegetation completely. If nobody has any food to eat, the ecosystem will die off.
If there were more predators that could have stepped in to fill the role of the coyote, the balance of the ecosystem could have been maintained, but because coyotes were one of the only ones, the ecosystem was vulnerable.