Final answer:
Since Columbus' time, biodiversity has fluctuated, with periods of both increase and decrease in species numbers, largely influenced by speciation and extinction rates. Recently, there has been a significant decline in biodiversity due to human impact.
Step-by-step explanation:
The question addresses the changes in biodiversity since the time of Columbus, focusing on the rise and fall of the number of species in our world. Analyzing various indications from ecological data and historical accounts, it is evident that there has been a fluctuation in species numbers due to factors such as global warming, overfishing, and changes in ecosystem conditions. This is further confirmed by discussing the balance between speciation and extinction rates that drive the variation in biodiversity over time.
The fossil record indicates that the number of species has seen both rises and declines, driven by these two processes. The correct answer to the original question would be that the number of species has both risen and declined (C) since Columbus. However, in recent times, particularly since World War II, there has been a significant decline in biodiversity, largely due to human activities.