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 above ground 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 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.

当我们在研究生物多样性在地球表面的分布方式时，我们发现这种分布是很不均匀的。总的来说，热带地区比高纬度地区同等面积要包含更多的物种。这对许多不同的动物和植物群体来说似乎如此。 为什么高纬度地区要比热带物种数量少？这也许仅仅是与土地面积有关。热带地区比高纬度地区土地面积更大——当我们在审视通常所用的地球曲面投影时，这一事实并不总是显而易见的，因为这往往夸大了高纬度的土地面积——一些生物地理学家认为物种多样性的差异反映了这一点。但生物学家克劳斯罗德的数据分析并不支持这样的解释。虽然面积可能影响生物多样性，这当然不是全部的影响因素；否则，面积大的陆块总会有更丰富的物种。 生产力似乎也有作用，但也许它的作用是间接的。那些条件最适合植物生长的地方，即温度相对比较高，温差小，有充足水源的地方，通常会有大量的植被。这就导致了一个复杂的植物体层级结构。例如，在一个热带雨林中，有大量的植物体生长在地面之上，也有大量的不显眼的根系组织位于地下。仔细分析地面以上的部分，发现它分成很了多层，层的具体数目随着森林的年龄和性质而变化。植被的生物量的分层形式被称为“结构”（区别于“组成”，指的是形成群落的各种物种）。结构基本上是植被的架构。对于热带森林来说，结构是非常复杂的。在一个成熟的亚马逊冲积平原的热带森林中，树冠（森林最上面的层，由树木的冠组成）就是一个分层结构。枯枝落叶层有三个明显的峰值，分别在距离地面3米、6米和30米的高度；在最高的那一层，约在距离地面50米高的地方，非常高大的树木并不属于主冠，而是形成自己的一层。所以，这样的森林本质上包含了四层树冠。温带的森林通常只有2个冠层，所以它们的结构没那么复杂。 结构对栖息在一个地方的动物的生活有很大的影响。它构成了动物觅食、在周围活动、生活和繁殖的空间环境。它甚至会通过影响光照强度、湿度以及最高温度和最低温度来影响当地气候（“小气候”）。例如，结构简单的草地，它在地面的微气候就和上层的树冠层不同。接近地面的地方，风速较低，白天气温也低（但夜间比较高），相对湿度更大。小气候的复杂性与植被结构的复杂程度密切相关，一般而言，植被的结构越复杂，在此生活的动物种类就越多。热带地区庞大的生物量导致了环境的更大的空间复杂度，也就使得住在这个区域的生物的多样性可能更高。高纬度地区的气候条件一般不利于大量生物的集聚，因此，植被结构更简单些，动物的多样性也相应较低。