As ecologists would soon discover, the loss of even one species can disrupt an entire ecosystem—from the health of giant wading birds right down to the movement of matter and energy.

Energy is the foundation of every ecosystem; it is captured by photosynthetic organisms and then passed from organism to organism via the food chain—a simple, linear path that shows what eats what. Any given ecosystem might have dozens of individual food chains. Linked together, they create a food web, which shows all the many connections in the community. Both food chains and webs help ecologists track energy and matter through a given community. They can vary greatly in length and complexity between different types of ecosystems. But most share a few common features, and all are made up of the same basic building blocks—namely, producers and consumers.

Florida wood storks sit near the top of a food chain that begins with sawgrass, other plants like cypress and mangrove trees, and a mix of algae and bacteria known as periphyton. These photosynthetic organisms are all known as producers. Producers capture energy directly from the Sun and convert it to food (sugar) via photosynthesis (see Chapter 8). They are then eaten by a wide range of consumers—organisms that gain energy and nutrients by eating other organisms. Animals, fungi, and most bacteria and protozoa are consumers. INFOGRAPHIC 10.1

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These different feeding levels are known as trophic levels. Consumers are organized into trophic levels based on what they eat. Primary consumers eat producers; secondary consumers eat organisms on lower trophic levels, predominately feeding on primary consumers; tertiary consumers primarily (or exclusively) eat secondary consumers, and so on, ending with the apex predators in the last trophic level. Of course, many consumer species often feed at more than one trophic level: Wood storks eating crayfish are feeding at trophic level 3, but when they eat small fish like bluegill, they are feeding at trophic level 4.

In an ecosystem as diverse and complex as the Everglades, there are dozens of different organisms at each trophic level, making for a wide variety of food chains. For example, periphyton might be eaten by grass shrimp (a primary consumer), which in turn might be eaten by bluegill (a secondary consumer), who fall prey to raccoons (tertiary consumers), who might be eaten by alligators (quaternary consumers). Alligators eat a wide variety of animals—turtles, fish, birds, even mammals like the raccoon—making them the apex predator in many Everglades food chains.

When any of these organisms die, they are eaten by an army of consumers known as detritivores—animals like worms, insects, and crabs that feed on dead plants and animals—and decomposers—organisms like bacteria and fungi that break decomposing organic matter all the way down into its constituent atoms and molecules.

As one moves up the food chain, energy and biomass (all the organisms at that level) decrease, creating a trophic pyramid. The reason is simple: Nearly every organism uses up the majority of its energy and matter in the complicated act of living. So when an organism is killed and consumed by a predator, it only passes on a small percentage of all the energy and matter it consumed during its lifetime. INFOGRAPHIC 10.2

A pyramid’s ultimate size is determined by its first trophic level—the one made up of photosynthesizing producers, namely plants. More plant growth means more food for primary consumers, which in turn means more food for those organisms above them, and so on up the food chain. The end result is a larger pyramid with more and larger trophic levels.

The amount of energy trapped by producers and converted into organic molecules like sugar is called productivity and is limited by sunlight and nutrient availability. Gross primary productivity is a measure of total photosynthesis. But plants use only a portion (actually less than 50%) of this energy to fuel their daily needs; the rest goes to growth. Scientists use the term net primary productivity (NPP) to describe that left-over energy—the energy that fuels the addition of new plant biomass (growth). The “net” is a measure of energy available to higher trophic levels. Terrestrial NPP is highest in tropical forests due to high year-round photosynthesis. Equaling or surpassing these tropical forests’ NPP is that seen in wetlands and estuaries (aquatic habitats where rivers meet oceans), especially tropical ones. Favorable temperatures, high levels of nutrients, and abundant water supplies make these the most productive ecosystems on Earth.

The Everglades are blessed with long summer days that favor plant and algal photosynthesis. But in many other ecosystems, winter months cast the downside of sunlight dependence into stark relief. In most temperate and boreal forests, for example, less sunlight and cooler temperatures limit photosynthesis during these months, causing plant productivity to slow or shut down completely, resulting in less new growth and thus less food for other organisms. This is why bears hibernate and birds fly south for the winter: The lack of productivity drives them to these extremes. NPP can be a window into the health of an ecosystem. If it rises or falls unexpectedly, ecologists can look for the cause of the change, which might be an invasion by a non-native plant or a sudden drop in a producer population. Anything that alters NPP can potentially affect organisms at every other level of the trophic pyramid.

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As researchers discovered in the 1980s, it was a kink in the food chain that hurt the wood storks. While they feed on many things, they prefer fish—and not just any fish, but those between 2 and 15 centimeters (1 and 6 inches) long. Most fish need more than a single season to grow this big; in fact, they need wetlands that are flooded for longer than a year and only very rarely go completely dry. It turns out that as humans altered water cycles in South Florida, there were fewer and fewer such areas, and thus fewer fish for the storks to feed their young.