A number of factors may help account for the difference in biodiversity between low and high latitudes.
When we look at the way in which biodiversity (biological diversity) is distributed over the land surface of the planet, we find that it is far from even. The tropics contain many more species overall than an equivalent area at the higher latitudes. This seems to be true for many different groups of animals and plants.
Why is it that higher latitudes have lower diversities than the tropics? Perhaps it is simply a matter of land area. The tropics contain a larger surface area of land than higher latitudes-a fact that is not always evident when we examine commonly used projections of Earth's curved surface, since this tends to exaggerate the areas of land in the higher latitudes-and some biogeographers regard the differences in diversity as a reflection of this effect. But an analysis of the data by biologist Klaus Rohde does not support this explanation. Although area may contribute to biodiversity, it is certainly not the whole story; otherwise, large landmasses would always be richer in species.
Productivity seems to be involved instead, though perhaps its influence is indirect. Where conditions are most suitable for plant growth-that is, where temperatures are relatively high and uniform and where there is an ample supply of water-one usually finds large masses of vegetation. This leads to a complex structure in the layers of plant material. In a tropical rain forest, for example, a very large quantity of plant material builds up above the surface of the ground. There is also a large mass of material, developed below ground as root tissues, but this is less apparent. Careful analysis of the aboveground material reveals that it is arranged in a series of layers, the precise number of layers varying with age and the nature of the forest. The arrangement of the biological mass ("biomass") of the vegetation into layered forms is termed its "structure" (as opposed to its "composition," which refers to the species of organisms forming the community). Structure is essentially the architecture of vegetation, and as in the case of some tropical forests, it can be extremely complicated. In a mature floodplain tropical forest in the Amazon River basin, the canopy (the uppermost layers of a forest, formed by the crowns of trees) takes on a stratified structure. There are three clear peaks in leaf cover at heights of approximately 3, 6, and 30 meters above the ground; and the very highest layer, at 50 meters, corresponds to the very tall trees that stand free of the main canopy and form an open layer of their own. So, such a forest contains essentially four layers of canopy. Forests in temperate lands often have just two canopy layers, so they have much less complex architecture.
Structure has a strong influence on the animal life inhabiting a site. It forms the spatial environment within which an animal feeds, moves around, shelters, lives, and breeds. It even affects the climate on a very local level (the "microclimate") by influencing light intensity, humidity, and both the range and extremes of temperature. An area of grassland vegetation with very simple structure, for example, has a very different microclimate at the ground level from that experienced in the upper canopy. Wind speeds are lower, temperatures are lower during the day (but warmer at night), and the relative humidity is much greater near the ground. The complexity of the microclimate is closely related to the complexity of structure in vegetation, and generally speaking, the more complex the structure of vegetation, the more species of animal are able to make a living there. The high plant biomass of the tropics leads to a greater spatial complexity in the environment, and this leads to a higher potential for diversity in the living things that can occupy a region. The climates of the higher latitudes are generally less favorable for the accumulation of large quantities of biomass; hence, the structure of vegetation is simpler and the animal diversity is consequently lower.
Though land area选项对应二段，所以A选项正确。
Regions possessing 选项对应三段，所以该选项正确。
A structure of varying heights选项，文章并没有提及在热带雨林和温带森林都有不同高度的结构。所以该选项错误。
The difference in microclimate选项针对第四段。原文没有提及到地面树冠和上层树冠的微气候差别决定了每个树冠层中栖息的物种的数量。所以该选项错误。
The more complex the structure选项针对四段，原文有，the more complex the structure of vegetation, the more species of animal are able to make a living there. 所以该选项正确。
The temperature 选项针对第四段，但原文没有把气温的变化当作动物的数量的一个决定因素。另外，该选项叙述更符合高维度地区特征，也可认为与原文相反。