Chapter 22

Species diversity in South Africa. The Cape Floristic Region, on the southwestern tip of South Africa, has an unusually high richness of plant species.

Photo by Hoberman/UIG/age fotostock.

The Cape Floristic Region on the southwestern tip of Africa is home to more than 9,000 species of plants, which makes it one of the most species-rich places on Earth. For comparison, Florida is about the same size and is positioned at the same latitude but has only about 4,200 species of plants. Moreover, nearly 70 percent of plant species on the cape are found nowhere else on Earth.

Why is this region such a hotspot of biodiversity? Research on the phylogenies of the most species-rich plant genera and families revealed that their ancestors all originated in the cape region. Therefore, the evolution of so many species is somehow connected to factors existing in southwestern Africa. Some scientists hypothesized that the high diversity of the region was the outcome of historic changes in climate. Approximately 5 million years ago, as the positions of the continents of the world were shifting, the climate in the Cape Floristic Region became cooler and drier. Perhaps these climate changes favored a period of rapid speciation in plants. However, when researchers tested this hypothesis by examining when speciation events occurred in the plant phylogenies, they found that much of the speciation had predated the changes in climate. Thus, the climate hypothesis was refuted.

Other scientists proposed that the region’s great diversity is the result of having plants with highly specialized pollinators and narrow flowering periods. Both factors would allow populations of existing plant species to become reproductively isolated from each other and to diverge genetically over time. This would favor an increased rate of speciation. Researchers tested this idea by determining whether closely related species differ more in flowering time or in their pollinators than we would expect to occur at random. If these differences occurred more than would be randomly expected, it would suggest that they were the driving factor in the evolution of new species from a common ancestor. In 2011, researchers refuted this hypothesis, too, when they found that closely related plants often have similar flowering times and use similar pollinators.

A final hypothesis proposed that the mountainous topography of the region created conditions that favored the evolution of new plant species. This topography contains tremendous heterogeneity in soil types that would favor the evolution of new species adapted to each. In support of this hypothesis, researchers discovered that closely related species of plants prefer very different soil types. These preferences suggested that the heterogeneity of soils offered a diversity of habitat conditions that favored the evolution of new species. Two other factors also contributed to the diversity of plants in this region. Because many mountainous areas are isolated, evolving populations would also be reproductively isolated, further favoring evolutionary divergence from conspecifics living in other areas. In addition, for the past several million years the climate in the region has been stable; this has slowed the rate of species extinctions, which has contributed to the high number of species today.

The story of the Cape Floristic Region demonstrates the importance of exploring the historical causes of species richness. It illustrates how a diversity of habitats facilitates a diversity of species, and it highlights the fact that we sometimes need to think about ecology at large spatial scales to understand the distribution and composition of species on Earth. In this chapter, we will explore ecology at large spatial scales to examine the distribution of species both across landscapes and around the world.

Throughout this book we have considered the role of physical conditions and of species interactions at particular locations. We have seen how these factors can affect the ecology of individuals, populations, communities, and ecosystems. When we explored these topics, we often found it helpful to focus on relatively homogeneous areas of land or water. However, as we moved across landscapes and across continents, we observed that terrestrial and aquatic ecosystems vary from place to place. In this chapter, we will consider much larger areas, from landscapes that include a variety of habitats to entire continents that contain a range of climates. In taking this large-scale approach, our goal is to understand why we find different numbers of species and often very different types of species in different places around the globe and why we sometimes find very similar types of species on widely separated continents. Once we understand the patterns of biodiversity, we can understand the processes that affect diversity and develop plans to conserve it, which is the topic of the next chapter.