As we move from the equator towards the poles,species diversity $.........$.

In the tropical forests of different continents,the slope of the $Z$ line for frugivorous birds and mammals is found to be $...........$.

Approximately $25\%$ of all insects are known to be phytophagous (feeding on plant sap and other parts of plants).

Why do the tropics have greater biological diversity?

Given below are two statements.
Statement-$I$: Availability of intense sunlight,warmer temperature and higher annual rainfall in the tropics are responsible for species richness of fruit-yielding trees.
Statement-$II$: The variety of frugivorous organisms is more in the tropics as compared to the temperate regions.
In light of the above statements,choose the most appropriate answer from the options given below.

According to the hypothesis proposed by environmental biologists,a relatively constant environment in the tropics promotes: