Final answer:
Multiple species of primary producers contribute to a robust ecosystem by enhancing gross primary productivity, increasing resource efficiency, and improving nutrient recycling. They also ensure ecosystem stability by creating multiple energy flow pathways and facilitating mutual benefits among species.
Step-by-step explanation:
The question asks why a robust ecosystem needs multiple species of primary producers. A diverse range of primary producers is crucial because they contribute to higher levels of gross primary productivity, which is the rate at which these producers capture and convert energy to biomass. Primary producers within different niches contribute to ecosystem resilience and stability. This diversity allows for a more efficient nutrient recycling and better utilization of resources, thereby maintaining ecosystem services and mitigating disturbances. Field studies have shown that biodiversity enhances ecosystem productivity and reduces productivity fluctuations, especially in the face of perturbations such as droughts.
Moreover, varying primary producers ensure the compartmentalization of energy flow, creating multiple pathways for energy transfer and nutrient cycles, which contributes to a more efficient and stable ecosystem. This sampling-competition effect increases the probability that a particularly productive species will be present in any given environment, facilitating overall ecosystem functioning. The complexity introduced by a rich tapestry of producers also gives rise to what is known as the facilitation effect, where different species improve conditions for one another, boosting the ecosystem's overall function and stability.