Ecosystems are comprised of all living organisms and the physical surroundings in which they live. Within an ecosystem we see energy flow and nutrient cycling. Of course, in a basic sense, the entire earth forms one ecosystem in that energy flow occurs between nearly all parts of the globe. The Artic Tern will migrate 24,000 miles during its annual migration, nearly circumnavigating the Earth. In this view, the Tern will collect energy in Alaska and then transfer that energy to the far reaches of South America, making a connection between these two distant lands. In practical terms, we can recognize that the amount of energy flowing between these two systems is relatively minor. It is reasonable to say that the grasslands are a distinct ecosystem from the coniferous forests of the Rocky Mountains.

Consider the example of the Greater Yellowstone Ecosystem (GYE).  What is the difference between Yellowstone National Park (YNP) and the GYE? The Park is contained within official boundaries set by human beings. In contrast, the GYE is an attempt to identify the land area that supports all organisms that live (at least part of the year) in the Park (eg bison, eagles, elk, wolves, etc). For example, a common means of entering the park is through Paradise Valley. As you travel down the valley, you will reach the Gardnier entrance, which marks the northern boundary of the Park. While this boundary is known to humans, the animals of the park
freely ignore the park boundary as they migrate up the valley in search of food during the winter months. As most biologist correctly point out, if the National Park System wants to effectively manage the Park, they need to find a way to influence the areas used by animals that don’t lie within the Park boundary. This is an
illustration of an ecosystem, a system in which the biotic and abiotic factors interact with energy flow and nutrient cycling. What land area falls within the Yellowstone ecosystem? In large part, think of it like this: plants contained stored energy and nutrients - if eaten by an herbivore, where will the energy and nutrients go? If a moose eats grasses, stores some of the energy from the grass as muscle, and then moves 100 miles south before being consumed by a wolf, then all these organisms fall within an ecosystem covering at least 100 miles (most ecosystems are much larger!). If there is a drought or disease outbreak in one part of an ecosystem, the effects will be felt by all organisms in that system. 

In a ecological view of Earth, we must begin with the sun. It provides (almost) all the energy that fuels life on Earth. (The only notable exception are the ecosystems that exist at the ocean depths or throats of geysers, where the sun provides no energy but organisms thrive by feeding off the energy and minerals provided by thermal vents.) Sunlight energy reaches Earth and a portion of the energy is accepted by autotrophs

What makes something like the grasslands a distinct ecosystem? Within each ecosystem, we can recognize unique species playing the roles of each of the following:

Primary Producer – The autotrophs (start with inorganic sources and store energy in organic molecules such as lipids, carbohydrates and proteins) harvest all the energy that will be used by other living organisms in the ecosystem. All energy used by other life forms in the ecosystem will be initially trapped/fixed by primary producers!! These organisms include plants but also include algae and autotrophic bacteria. There are two major groups of autotrophs: phototrophs (collect energy from sunlight) and the chemotrophs (collect energy from chemicals in their environment, such as deep sea vents or the geysers of Yellowstone).

Consumers – Any organism that feeds off other living tissue (or once-living tissue) is a consumer. Thus, lions on the African savanna are consumers but so are mushrooms that work to digest dead plants material (the decomposers and detrivores). We have herbivores (feed off plants), carnivores (feed off of other consumers), and omnivores (feed off both consumers and producers). Also, decomposers (digest food oustide of body) and detrivores (internally ingest dead and decaying matter) play important roles as consumers.

Trophic Levels - When all the producers and consumers are arranged by feeding levels, an energy pyramid can be described. Each organism is assigned a trophic (feeding) level, ranging from Primary Producer, to Primary Consumer, Secondary Consumer, Tertiary Consumer and, finally Quaternary Consumer. Why is it rare to find a consumer above the 4th or 5th trophic level?  We must look at how much energy is fixed (stored) at the base of the energy pyramid:

The base of any ecosystem is formed by the primary producers. To calculate how much energy is available, we calculate Primary Productivity. This is the total amount of energy/fixed carbon available to the trophic levels above the producers. We can measure Gross Primary Productivity, which is the total amount of energy fixed in the process of photosynthesis for an ecosystem. Net Primary Productivity is the total amount of energy fixed by photosynthesis minus the amount of energy used by autotrophs in respiration (remember: plants are both autotrophs and heterotrophs).

Above the producers, three to four levels of consumers are present in most ecosystems. Why only three or four? We have a general rule that only 10% of available energy is transferred between trophic levels. By the time we see tertiary or quaternary consumers, the available energy base is getting so small that higher trophic levels can’t be sustained.

When we look at the level of primary consumer, we recognize two types of food webs:

1. Grazing Web: A traditional field with cows grazing grasses and forbs is an example of a grazing food web. Think about how much of the grass is used by grazers: often these are endotherms and expend a great deal of energy keeping warm. Also, they don’t tend to use all of the plant material and some remains undigested in waste products. Thus, grazing systems are not the most efficient systems.

2. Detrital Webs: In this system, energy flows from autotrophs to decomposers (usually fungi and bacteria that digest and degrade dead material outside of their bodies) and detrivores (organisms such as worms and insects that ingest dead/dying material).  These webs are much more effecient than 10% because relatively little energy is expended for body heat and movement.

Biological Magnification: In food webs, we usually focus on energy flow and nutrient cycling. However, at times we must also include other abiotic factors, such as the insecticide DDT. DDT was used extensively in the United States prior to the 1970s as it is a very effective pesticide. At the same time, scientists were noticing a drop in the populations of many birds high on the trophic scale (tertiary or quaternary consumers) such as Bald Eagles, Osprey and Brown Pelicans. A closer looked revealed that the r-value for these species was falling largely because of low birth numbers, not elevated deaths. What was the cause of the decline in successful births (hatches) in these birds? It was a case of biological magnification.

Certain chemicals are not soluble in water and therefore are not easily removed by rain or living processes. Many of these chemicals are fat-soluble, meaning they dissolve and are stored in the fat of living organisms (remember chemistry? this molecules must be polar or nonpolar?). Because they are not water-soluble, they are not excreted in the urine, but can be stored for years. Over time, the concentration of such chemicals can greatly increase as you proceed up the trophic feeding levels.

This was the case for eagles and other birds with DDT. DDT would be blown into rivers and fields as it was sprayed around neighborhoods. When it got on any autotrophy, the primary consumer eating the autotrophy would ingest a certain amount of DDT and it would be stored in their fat. The book provides actual numbers, but just realize that the concentration of DDT was low in the primary consumers. Next, consider a fish feeding off the primary consumers. If this secondary consumer eats thousands of minnows or other herbivores, what will happen to all the DDT the secondary consumer eats? It will accumulate in the fat and the concentration will become magnified over time. By the time you get to eagles and osprey, the concentrations can get very high. In the US, the concentration was high and when birds lay eggs, it is very taxing on the body and they often end up burning energy stored as fat. The metabolism of the fat would release DDT and it turned out this was causing brittle eggshells and when the mother bird attempted to incubate the eggs, they would often crack. We banned DDT use in the US (but not other countries so it is still present on vegetables and fruit imported from other countries) and the populations of all raptors have benefited.

I often speak of fish tanks when discussing ecology because food is the only thing we commonly add to an established tank. Fish wastes, uneaten food and other products will build up to toxic levels if we don’t carefully manage a tank.  How can some tanks run for years with only an occasional water change?  Why don't the fish wastes make the water toxic for the fish?  The cycles of nitrogen, carbon and phosphorus are key to understanding ecosytems (and how to properly manage a fish aquarium).

Within ecosystems, we say that energy flows because it is collected by autotrophs and then lost as organisms feed off each other. In contrast, nutrients cycle, or are reused within the ecosystem and can therefore pass from heterotrophs to autotrophs.

Know these three cycles well:

The water cycle: study figure 49.15 from your book.

The carbon cycle: study figure 49.17 from your book.

The nitrogen cycle: In most ecosystems, nitrogen is a limiting factor.  Examine figure 49.22 from your text