As a rule, biodiversity decreases as you travel from the equator towards the poles. Biodiversity refers to species richness and the general health of an ecosystem or biome. Typically, equatorial regions have warmer climates and direct insolation. This allows for more producers which provide food for the many food chains operating in the equatorial biomes.Therefore, there are greater numbers of species found in equatorial regions as compared to the poles. Any organisms in the poles must be able to withstand a cold, harsh climate. There are much less diverse food chains and webs that exist in these locations. Climate is a big factor in determining which biomes on the planet have greater biodiversity.
To answer this question, you need to look at the two environments. Plants need sunlight and precipitation to grow. Animals need plants to eat, or they eat other animals that eat plants. For an ecosystem to have rich biodiversity, you need abundant moisture and sunlight.
This ecosystem with abundant moisture and sunlight exists around the equator. The equatorial areas play host to most of the world's tropical rainforests. Unlike the northern and southern hemispheres, the equatorial areas do not experience major seasonal changes in sunlight. Precipitation does change, and there is a wet season and a dry season. However, the sun shines all year round. Conditions are excellent for abundant plant and animal growth. Forests grow quickly and have many layers. There are also many different niches in these forests, providing different habitats for different animal species.
At the poles, life is much more difficult for plants and animals. Most of the year it is very cold. There is a narrow summer window when plants can grow and animals can mate. At other times, conditions are harsh. Only those animals and plants that are adapted to cool, dark weather can survive.
In betweent the poles and the equator are environments where there are varying amounts of sunlight and rainfall. These environments are less diverse than the equator and tend to be more diverse than the poles.