Ecosystem Ecology

ECOSYSTEMS

An ecosystem consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows

Autotrophs vs. Heterotrophs

Organisms get their food in one of two ways.

Autotrophs (or producers) make their own food using light or chemical energy. Examples of autotrophs include plants, algae, and some bacteria.

Heterotrophs (or consumers) get organic molecules by eating other organisms or their by-products. Animals, fungi, and many bacteria are heterotrophs. Specialized heterotrophs, called decomposers break down dead organic material and wastes.

Energy Flow Through Ecosystems

Defining an Ecosystem

The ecosystem is the last level we will be discussing in the context of ecology. It is one step larger than a community. An ecosystem consists of all biotic and abiotic elements of a given area - essentially it consists of a community plus all the abiotic factors the community relies on.

For the purposes of AP Biology, the focus will be on energy within an ecosystem, but you should also be aware of the basics of nutrient cycling as seen below.

Energy's Origin & Pathways

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 form some will be lost due to inefficiencies.

While you can see that chemical nutrients will be recycled within the ecosystem, energy travels in one direction. So if it loses its energy source, the Sun, the ecosystem will collapse.

So what happens to all of this energy? Why is it even necessary for life? Shown here is an example of the energy consumed by a caterpillar as it consumes parts of a plant.

About half of the available energy will actually not be accessible to the caterpillar and will be passed on via feces.

The other half will be used for partially for growth and some eventually forms ATP via cellular respiration.

Please note that these proportions are not definitive, but general. Some organisms are able to get more energy out of a given food source than other, for example.

Gross primary productivity, or GPP, is the rate at which solar energy is captured in sugar molecules during photosynthesis (energy captured per unit area per unit time). Producers such as plants use some of this energy for metabolism/cellular respiration and some for growth (building tissues).
Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it's the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem.

Trophic Structure & The 10% Rule

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.

Trophic Levels

A trophic level refers to a level or a position in a food chain, a food web, or an ecological pyramid. It is occupied by a group of organisms that have a similar feeding mode. In an ecological pyramid, the various trophic levels are primary producers (at the base), consumers (primary, secondary, tertiary, etc.), and predators (apex).

Nutrient Cycling

Nutrients are crucial resources within an ecosystem without which individuals would die.

Your textbook provides the following simplified diagram of nutrient cycling within an ecosystem.

While simplified, it accurately depicts nutrients moving between different reservoirs through a number of biotic and abiotic processes.

This simplification will suffice for AP Biology as long as you know that these ideas apply in some form to crucial compounds and elements such as nitrogen, carbon, phosphorus, and even water!

Importance of Biodiversity

Ecosystem Services

It is tempting to say that the losses of 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, horseshoe 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.

Below are some examples of ways by which ecosystems perform ecosystem services, ways by which humans benefit from these ecosystems.

Ecosystem Resistance & Resilience

In order to provide such services, an ecosystem must remain relatively healthy. Many things can cause disruptions to ecosystems (examples below), but some ecosystems can resist these disturbances better than others.

An ecosystem's resistance is its ability to withstand disturbances with minimal loss. Its resilience is its ability to recover to normalcy after incurring significant damage from these disturbances.

Research has shown time and time again that biodiversity is crucial to an ecosystem's resistance and resilience, and, thus, its ability to perform ecosystem services from which humans benefit.

Disruptions to Ecosystems

Keystone Species

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

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)!

Negative Human Activity

Of course, it is well-established at this point that the most destructive force on ecosystems are not necessarily natural - it is us. Humanity has caused irreparable damage to countless species, many causes of which have been outlined below.

Positive Human Activity

But believe it or not, it is not all bad - many humans have worked very hard to mitigate our negative impacts both individually and as a species. Some of the positive impacts we have had on ecosystems have been outlined below

Hope for the Future?

Ultimately, it is not likely sustainable for humanity to focus solely on the needs of itself or solely on the needs of the natural world. True sustainability, if it is even possible, will come from balancing reasonable human needs and lifestyles while working to minimize and reverse much of the damage we have done and will do.

Page updated

Report abuse