15.1–15.2: Ecosystems have living and non-living components.

Flamingoes feeding at Lake Nakuru National Park, Kenya.

Picture a lush nature scene: some greenery, a bit of rotting wood, and abundant wildlife. Grazing animals abound, while predators feed on other animals and their eggs. Parasites are poised, looking for hosts, and, just below the surface, scavengers find meals among the organic detritus. It would seem to be the quintessential ecosystem, a community of biological organisms plus the non-living components with which the organisms interact. But now imagine that the entire scene gets up and walks away! The “camera” in your mind pulls back to reveal that the scene is playing out on the back of a beetle no more than 2 inches (5 cm) long (FIGURE 15-1).

Figure 15.1: “Be it ever so humble …” A small-scale ecosystem can exist on the large weevil of New Guinea.

The host of this mini-ecosystem is a beetle from New Guinea called the large weevil. The weevil is camouflaged from its predators by lichens—which consist of fungi and photosynthetic algae living together—while the lichens are given a safe surface on which to live. And the garden of lichens supports a wide range of other organisms, from tiny mites to a variety of other microscopic invertebrates, some free-living and others parasitic.

Not all ecosystems are the obvious assemblages of plants and animals that we usually picture—ponds, deserts, or tropical forests. A similar scene can just as easily be found in your large intestine, where several hundred bacterial species flourish. These ecosystems are contained within ecosystems that are contained within ecosystems. The scale can vary tremendously. The closer you look, the more you find.

What is important is that the two essential elements of an ecosystem are present: the biotic environment and the physical (abiotic) environment (FIGURE 15-2).

• 1. The biotic environment consists of all the living organisms within an area and is often referred to as a community.
• 2. The physical, or abiotic, environment, often referred to as the organisms’ habitat, consists of:
  
  • the chemical resources of the soil, water, and air, such as carbon, nitrogen, and phosphorus, and
  • the physical conditions, such as the temperature, salinity (salt level), moisture, humidity, and energy sources.

Figure 15.2: What makes up an ecosystem?

Biologists view communities of organisms and their habitats as “systems” in much the same way engineers might, hence the term ecosystem. Biologists monitor the inputs and outputs of the system, tracing the flow of energy and various molecules as they are captured, transformed, and utilized by organisms and later exit the system or are recycled. They also study how the activities of one species affect the other species in the community—whether the species have a conflicting relationship, such as predator and prey, or a complementary relationship, such as flowering plants and their pollinators. On a small scale, such as the back of a beetle, making some of these measurements can be easy. But there are also some well-studied giant ecosystems, such as the Hubbard Brook Experimental Forest in New Hampshire, which covers 7,600 acres. The same principles apply to the study of ecosystems regardless of size: observe and analyze organisms and their environments, while monitoring everything that goes into and out of the system.

Why should researchers bother with such a methodical—and tedious—analysis of ecosystems? Using the principles of scientific thinking and carrying out experimental tests of hypotheses have led to numerous valuable discoveries, from understanding how the clear-cutting of forests dramatically reduces soil quality to understanding the link between the use of fossil fuels and the creation of destructive acid rain. As we see in this chapter and the next, humans, perhaps more than any other species in history, are significantly affecting most of the ecosystems on earth. And, in addition to improving our understanding of environmental issues, ecosystem studies can also tell us about the microbial ecosystems living inside humans and thus lead to advances in public health.