Because the amount of precipitation is equal to the amount of evapotranspiration and runoff.
Answer is B. Pioneer communities contain few species, while climax communities have many species.
Pioneer species are the first species that start life in an area. Their growth is fast and soon replaced by next community. Whereas, climax communities are made of many species. They are stable is not replaced by any other community. They are supporting new life.
If I'm right I'd say They control and direct the quantity of food passing through the digestive organs and prevent backward movement of partially digested food. Hope it's helpful.
