Question

Net Primary Production (NPP) in ecosystems can be controlled either by limiting resources for photosynthetic (autotroph) organisms, such as nutrients, light, and temperature, or by the abundance of herbivores (primary consumers), which in turn may be controlled by predators (secondary consumers). The first scenario is called bottom-up control of NPP and the second top-down control.

1. Consider the saltwater ecosystem. Define the three important trophic levels. What are the most important photosynthetic organisms, herbivores, and carnivores in the saltwater ecosystem?
2. Outline an experiment to determine if NPP in the saltwater ecosystem is controlled bottom-up or top-down. What is the main variable to be measured in your experiment? What are your experimental treatments? Include controls and replication.

Answer 1

1) The freshwater ecosystem: in this ecosystem, the phytoplankton's are primary producers, zooplankton's, and small fishes are primary consumers while large fishes who feed upon smaller fishes or zooplankton are carnivores or secondary consumers.

2 In the above hypothetical ecosystem considered understudy could be tested whether it is controlled by bottom-up factors or by top-down factors as follows. To know top-down factor's effects, the large fish need to be removed from the experimental pond step by step. That means first take half of them from the pond then remove the remaining half of them and observe the number or abundancy of the other participants, how they are affected whether they increased or decreased etc. If any significant change observed in the overall ecosystem then consider it as top-down controlled.

To see whether it is controlled by bottom-up factors like light, gas exchange, etc. We can cover the pond partially later fully for a course of time and observe the effect of sunlight or gaseous exchange on the primary producers and how the whole thing reflects on the other participants in the ecosystem. If a significant change in the number or abundancy of the other hierarchies as well then we can consider that our hypothetical ecosystem is bottom-up controlled.