Which of the following statements explains why net primary productivity is
higher near the tropics?
More predators in the tropics kill herbivores, increasing plant growth in
the tropics
Increased species diversity results in increased plant growth
Availability of newly weathered inorganic nutrients
Water and warm temperatures in the tropics promote
photosynthesis
Net primary productivity (NPP), the biomass plants produce via photosynthesis after respiration, reaches its peak near the equator due to optimal environmental conditions. The correct statement is: Water and warm temperatures in the tropics promote photosynthesis. These factors enable year-round plant growth, abundant sunlight, and minimal seasonal disruptions, driving higher NPP in rainforests compared to temperate or polar regions.
Option Breakdown
More Predators Kill Herbivores, Boosting Plant Growth
This is incorrect because increased predation on herbivores does not directly enhance plant growth; it may regulate herbivore populations but often maintains ecosystem balance rather than boosting NPP. In fact, heavy predation could indirectly limit plant access to resources if it disrupts food webs.
Increased Species Diversity Boosts Plant Growth
While tropics host greater biodiversity, this is often a result of high NPP rather than its cause. Diversity supports complex ecosystems but does not primarily drive the elevated photosynthesis rates seen in tropical latitudes.
Availability of Newly Weathered Inorganic Nutrients
Nutrients from weathering contribute in some ecosystems, but they are not the main driver globally; leaching in wet tropics can deplete soils. Temperature and water availability overshadow nutrients as key NPP predictors.
Water and Warm Temperatures Promote Photosynthesis (Correct)
Tropical climates provide consistent warmth (above 20°C) and rainfall, maximizing enzymatic reactions and stomatal opening for CO2 uptake. This sustains high photosynthetic rates, doubling NPP near the equator versus higher latitudes.
