Ecosystem Ecology

For most ecosystems on Earth, the ultimate origin of energy comes from the Sun. A few exceptions use energy from inorganic molecules and Earth's thermal radiation, but that is beyond the scope of the course.

The energy is transferred from the Sun to primary producers, so named because they produce their own food using this energy along with some reactants from their environment. This energy is then passed to the primary consumers. From there, some of the energy may get passed to secondary consumers (if present).

However you will notice that all of the energy eventually makes its way either to detritus (dead matter to be broken down by detritivores, or decomposers) or out of the system via heat. This latter case is a result of the inefficiencies of energy transfer. Every time energy changes forms, some will be lost due to inefficiencies.

Energy transfer between organisms can be simply represented with a food chain or a food web. A food chain is simply a linear transfer of energy from primary producers to primary consumer to secondary consumer, etc.

A food web, although more complex, it simply a collection of food chains from the same ecosystem. Often an organism eats more than one type of food, or multiple predators consume the same prey. So a food web is a more accurate representation of the energy relationships in an ecosystem.

It is crucial to note that the arrows of a food chain or web depict the energy flow, so should point from source to destination. In other words, arrows must from prey to predator.

Often absent from food chains and webs are the decomposers - do not forget that energy of course is transferred to them from the biomass of dead organisms at ALL trophic levels. They are simply left off of these diagrams to make the image less dense and arguably more useful.

Each of these energy transfers have their own inefficiencies with energy being lost as heat and the remaining energy being used by each organism along the way to grow, reproduce, move, etc.

Energy is traced across multiple trophic levels, the name given to primary producers, primary consumers, etc.

As energy travels from one level to the next, most energy is used up or lost. Thus, as your travel up the food chain, there is less and less energy available at each level.

Thus, the most common representation for the energy across trophic levels is a pyramid. The base of the pyramid contains the most energy with it decreasing each level up.

In fact, an average of only 10% of the energy from one trophic level is made available to the subsequent trophic level. This is referred to as the 10% rule, and is, of course, an oversimplification. Real data is rarely so clean, but will work as an approximation.

It is tempting to say that the losses to biodiversity are simply a problem for biologists and wildlife-lovers. Surely it won't actually affect humans significantly, right? Wrong.

Biodiversity is actually crucial to our livelihood, as not only do many of our industries (including our crucial and fragile food industry) rely on living organisms, but there is also a huge amount of potential uses from organisms that we have not yet discovered or made possible.

For example, horshoe crab blood's utility for the medical field was not described until 1956. Yet horseshoe crabs evolved around 450 million years ago. If they had gone extinct during the Permian extinction 250 million years ago, for example, we would not have known of their benefits to human society and health. To learn more about the use of horseshoe crab blood, please read here.

Keystone species, much like their architectural namesake, represent a group that is so essential to an ecosystem that it will collapse if removed.

As opposed to a foundation species, keystone species are not necessarily present in large numbers, so can be more susceptible to disturbances within an ecosystem.

Thus, when protecting an ecosystem, it is important to identify and protect keystone species. Often keystone species are top predators due to the top-down model of trophic control.

The most commonly discussed historic example of the removal of a sea otter population (for its pelts) holds true. With the removal of their only predator, the sea urchin population boomed and consumed the kelp population to extinction, eventually wiping out the entire community.

Invasive species are species that are introduced (intentionally or unintentionally) to an ecosystem and cause harm or destruction to this new ecosystem.

Often invasive species are so successful in their new environment due to a lack of natural predators or a plethora of resources available.

Often when thinking of invasive species, people think of humans intentionally bringing the species to the area (i.e. the European starling now found all over the United States).

However, often human intervention causes invasion indirectly or accidentally. A classic example of this is the now-prevalent zebra mussel (shown here).

Zebra mussels have made their way across the eastern United States primarily via water transfer from one body of water to another in boats. Not only do these mussels cause problems for wildlife, but they have also been known to clog pipes and destroy boat engines (among other issues)!