Among the most remarkable features of life is its astonishing diversity. Biological diversity, or biodiversity, is the product of evolution, but it is shaped and sustained by ecological interactions among organisms and between organisms and the physical environment.

To begin, an estimated 500,000 species of photosynthetic organisms fuel the carbon cycle in all but a few deep-sea and subterranean environments. In terms of carbon and energy metabolism, all these species do pretty much the same thing. Why, then, is there such a diversity of photosynthetic organisms? Why don’t just a few species dominate the world’s photosynthesis?

The example of the pond and forest helps to make the basis of photosynthetic diversity clear. In the forest community, the leaves of several different tree species form a photosynthetic canopy above the forest floor. Below, shrubs grow, making use of light not absorbed by the leaves above them. And below the shrubs are grasses, herbs, ferns, and mosses that can grow in the reduced light levels of the forest floor. In the nearby pond, a few species of aquatic plants line the water’s edge, but beyond that algae and photosynthetic bacteria dominate photosynthesis, with some species anchored to the pond bottom and others floating in the water column.

Locally, then, the different photosynthetic species that transfer carbon atoms from CO₂ to organic molecules subdivide the forest on the basis of light, water, and nutrient availability—a pattern complicated by grazing and environmental disturbances such as fire and landslides (Chapter 47). On a larger scale, climate and topography vary tremendously from one region to another, and these features, as well, help to explain how plants can build and maintain diversity by performing comparable metabolic functions in different habitats (Chapter 48). In the forests of New England, the tree species that thrive in wet regions differ from those found on well-drained hillsides. And seasonally dry woodlands in southern California support yet another set of plant species. In general, plants of varying size, shape, and physiology inhabit physically and biologically distinct environments, and the same is true of photosynthetic organisms in lakes and oceans.

Thus, the immense diversity of photosynthetic organisms found today does not reflect evolutionary variations in the biochemistry of photosynthesis (although some of that occurs; see Chapter 29) so much as it does structural and physiological adaptations. These adaptations allow the effective gathering of light, nutrients, and—critical to life on land—water, in widely varying local environments. Natural selection, acting on local populations, links the diversity of photosynthetic organisms to the carbon cycle.

If half a million species function as primary producers, an estimated 10 million species help to cycle carbon through respiration. These include the plants, algae, and bacteria that generate carbohydrates by photosynthesis, as well as animals, fungi, and microorganisms that obtain both carbon and energy from organic compounds in photosynthetic organisms or the consumers that eat them. These organisms are essential to the completion of the short-term carbon cycle, returning carbon atoms to the environment as CO₂.

Heterotrophic bacteria, amoebas, and humans may use essentially the same biochemical pathway to respire organic molecules, but they differ markedly in how they feed and, therefore, in what they can eat. Bacteria (and also fungi) absorb molecules from their environment, but amoebas and many other eukaryotic microorganisms can capture and ingest cells—they are capable of predation. Animals capture prey, as well, but commonly feed on organisms far too large for an amoeba to eat. As photosynthetic organisms have adapted structurally and physiologically to local environments across the globe, consumers have adapted by means of locomotion, mouth and limb specialization, perception, and behavior to obtain their food. So, as you read about bacteria, amoebas, and insects in the chapters that follow, think about how each operates within the carbon cycle and so contributes to the operation of Earth’s diverse ecosystems.