The fossil record provides evidence that for the first 30 to 40 million years of their evolution, angiosperms were neither diverse nor ecologically dominant. In fact, fossil pollen indicates that, between about 140 and 100 million years ago, gnetophytes diversified just as much as angiosperms. Later in the Cretaceous Period, beginning about 100 million years ago, angiosperm diversity began to increase at a much higher rate. Trees belonging to the magnoliids, the branch of the angiosperm tree that today includes magnolias, black pepper, and avocado, emerged as ecologically important members of forest canopies. More important, however, was the divergence of the two groups that would come to dominate both angiosperm diversity and the ecology of many terrestrial environments: the monocots and the eudicots.

Before looking at the diversity of these two groups, let’s consider how characteristics of angiosperms may have promoted their diversification. While many factors likely have contributed, we’ll focus on two themes. The first is that angiosperms are more efficient at building their bodies and completing their life cycle than are other plants. Recall that the xylem of angiosperms has vessels supplying water required for photosynthesis and fibers providing mechanical support. Because water transport and mechanical support are separated, angiosperms are able to grow efficiently into a wide variety of forms, ranging from short-lived herbaceous plants to trees with large spreading crowns. This diversity of shapes and sizes reduces competition for light and space.

Angiosperms complete their life cycle as efficiently as they build their bodies. Not only are animal-pollinated plants able to reduce their production of pollen, but also double fertilization reduces the costs of reproduction by allowing angiosperms to delay provisioning their offspring until after fertilization. Angiosperms can thus reproduce quickly and with a minimum of resources. One possible consequence is that angiosperms can make use of habitats and resources that are only fleetingly available, such as ephemeral pools or open areas that appear following a fire.

A second theme focuses on the ways in which angiosperms interact with other types of organisms. The interaction of flowers and animal pollinators contributes to angiosperm diversity. Plants whose flowers attract different pollinators tend to more readily diverge to form new species than wind-pollinated species. In addition, the ability of carpels to block the growth of pollen from different species and from closely related individuals (including self) suggests that these genetic recognition systems contribute to the reproductive isolation required for speciation (Chapter 22).

It is important to recognize that diversity results from both high rates of species formation and low rates of species loss. The fossil record suggests that the persistence of species may play a particularly important role in angiosperm diversity. Wind-pollinated plants can reproduce only when their populations have a relatively high density. Since no wind-carried pollen is likely to fall on isolated individuals, species with low population density are more likely to go extinct. In contrast, animal pollinators actively searching for rare species are much more likely to find them, and so animal-pollinated angiosperm populations can persist at low population densities.

Another reason rare species may be more persistent is that they are less likely to encounter soil pathogens and seed predators. These threats to survival tend to gather near an adult of the same species. The seeds of rare species are more likely to land away from an individual of the same species and thus suffer less from this density-dependent mortality than do more common species. However, this ecological strategy works only if rare isolated individuals can remain in contact with a large enough population of potential mates, underlining again the key role of flowers and animal pollinators in persistence.

If animal pollination has so many advantages, why did approximately 20% of angiosperm species subsequently evolve a dependence on wind for pollination? Wind-pollinated species grow primarily in seasonal environments, where fewer animal pollinators are available than in tropical forests, and the pressures resulting from soil pathogens and seed predators may be much less.

That angiosperm diversity is the result of multiple factors is reinforced by the observation that some of these “angiosperm” features are also present in other groups of plants. For example, cycads rely on animal pollination, whereas gnetophytes produce xylem vessels, and yet neither group is particularly diverse. The initially slow but increasingly rapid buildup of angiosperm diversity may reflect the compounding effect of coevolutionary interactions with pollinators and dispersers, as well as adaptive radiations triggered by chemical arms races with pathogens and herbivores (Chapter 32). This is what is meant by the adage “Diversity begets diversity,” a phenomenon further explored in Chapter 47.