New York,located at $41^{\circ} N$,has $X$ species of birds,and Greenland,located at $71^{\circ} N$,has $Y$ species of birds.

Describe species-area relationships.

Choose the incorrect sentence$(s)$ regarding latitudinal gradients.
$(I)$ Biodiversity throughout the world shows uneven distribution.
$(II)$ According to latitudinal gradient,there is no major difference in biodiversity.
$(III)$ Species diversity increases as we move away from the equator towards the poles.
$(IV)$ Biodiversity increases as we move from temperate to tropical regions.

Identify the correct statement.

Ecologists have discovered that the value of the $Z$ line in a small area is $....P....$ and the value of the $Z$ line in a very large area is $.....Q....$.

Give three hypotheses for explaining why tropics show the greatest levels of species richness.