When the same habitat types (forests, oceans, grasslands, etc.) in regions of different latitudes are compared, it becomes apparent that the overall number of species increases from pole to equator.
Topic
The increase in species diversity from poles to equator: why there are more species in the tropics than at higher latitudes.
Framework
Answer / Explain
Main Point
The passage explores several hypotheses to explain why there are more species near the equator than near the poles but favors the idea that higher speciation (and not higher extinction) in the tropics best explains this pattern. (Most valuable sentence: Last sentence of P5: )
P1: The Pattern—Latitudinal Diversity Gradient
Scientists have noticed that as you move from the poles toward the equator, the number of species in similar habitats increases, with the effect likely being even bigger than currently known.
P2: The Time Theory Hypothesis
One idea ("time theory") says tropical regions have more species simply because they've had more time without disruptive ice ages for new species to evolve, but this doesn't fully account for differences among various regions.
P3: The Species-Energy Hypothesis
Another hypothesis suggests that more sunlight in the tropics means higher growth and reproduction rates, which could lead to more biomass and thus more species, but the supposed chain of cause-and-effect has several weak links and is unproven.