WHAT IS AN ECOSYSTEM?

Copyright Patricia S. Muir, 2000

This course deals with ecosystems: what is an ecosystem?

The word comes from the words "ecological" and "system." We will consider an ecosystem to be all of the organisms and the abiotic (nonliving) environment in a particular place, and all the interactions and flows between the organisms and the organisms and their environment. That is, an ecosystem is a system of interacting parts, in which some of the parts are living and some are nonliving.

You'll remember from introductory biology that we speak of the various levels of organization in life: molecules, organelles, tissues/organs, organisms, populations, and communities? An ecosystem continues this hierarchy, and represents all of the living entities below it in this hierarchy plus their nonliving environment and all of the interactions among the various constituents.

We often model ecosystems as a series of compartments, linked by flows or transfers between them, with inputs from and losses to the "outside." For example, a simple model of energy flow in an ecosystem might include compartments representing plants, animals, and soil. Each compartment would be connected to the other via a flow of energy (e.g., from the plants to the animals, and from both to the soil). (Recall the box and arrow model we built in lecture?)

Each compartment has associated with it a measurable state or condition; in this case, because we are modeling energy flow, kcal/m2

Each transfer between compartments also has a measurable coefficient, often a rate, such as kcal/m2/day

There are not only transfers within the system, but also outputs from the system (e.g., respiratory losses from each compartment) and inputs into the system (e.g., solar input into the plant compartment).

The compartments and transfers that we include are dictated by the aspects of the system that we want to look at. Our model would be different if we wanted to focus on nutrients, for example.

This then is a simple ecosystem model, and we will work with these throughout the term. We will show how human activities are transmitted through the various compartments of ecosystems, and affect the interactions between compartments and the compartments themselves.

You can see that there is some arbitrariness about defining boundaries of an ecosystem, and we will see that for many of the problems we discuss its only reasonable to be inclusive in our definition of the system of concern – that is, to consider it to be the globe.

Why use systems models like this?

What we just constructed is a model of reality. We use such models -- simplifications of reality -- in environmental science for several reasons:

1. We gain understanding of the system by attempting to model it this way -- what are its pieces and how do they interconnect?

2. We can use the model to predict effects of treatments on the system without risk of direct experimentation.

3. The systems we work with are often too large, complex or slow-behaving to experiment directly with, (for example, global systems) so we model the m. That is, models are useful in cases where it is impractical (or impossible!) to do direct experiments.

4. Models serve to remind us of the interconnectedness of various components of the living world. They help us to visualize these interconnections, and thus to be reminded about the place of humans, and human impacts, in these systems

5. Modeling a system can help to reveal our biases. If we have unknown, and inaccurate biases in our serception of the world, this may lead to faulty models. When the models don't behave like reality, (in cases where it is possible to check this!) we can then go back to the model to figure out where we are wrong.

We will focus this term on the impacts of human processes and activities on ecosystems, rather than on humans in particular. For us, humans will be treated as a part of the system, rather than something separate.

Effects on ecosystems do have effects on humans

As humans, while we may not like to admit it, we depend on ecosystems to maintain habitability and integrity of the planet. That is, we depend on ecosystems for their provision of "goods" such as fish in the oceans, wood from forests, and so on. We also depend on ecosystem services or ecosystem functions.

What are these ecosystem services? The Ecological Society of America (ESA), the scientific society to which many professional ecologists in the US (including me) belong, published a few years ago a special "Issues in Ecology" paper on ecosystem services, which provides a nice overview of the concept. The paper can be viewed (strictly optional!!) by visiting the ESA's home page and following links to this issue, or by clicking on Issues (then use "BACK" in your browser's toolbar to return to this page). See also several articles on the subject of ecosystem services and their valuation on the supplementary reading list for this section of the course.

A related question: If we are going to talk about human impacts on ecosystems, what are the ecosystem properties and functions that we will examine impacts on?

As humans we rely on these functions. We couldn't live without them, yet our actions affect them all -- and to an increasing extent (see the article in your assigned readings by Vitousek et al. on "Human Domination of Earth's Ecosystems" for a sobering perspective). Despite their importance in our lives, we tend to take these services for granted and don't have sufficient concern about actions that threaten the ecosystems that provide them for us.

Our oblivion about the value of ecosystem services results at least in part from the fact that until recently no one tried to put a dollar value on them. We do, of course, put economic values on ecosystem goods (timber, fish, etc), but until recently we haven't put such values on ecosystem services. Because we are used to thinking of the value of a thing in monetary terms, this lack of economic valuing of these services has made it difficult for them to be considered seriously in policy discussions. However, recently several groups have attempted to redress this, by estimating the value of various ecosystem services (see the volume edited by Daily (1997) and other articles listed on your supplementary reading list for some attempts). These assessments invariably conclude that the global value of these services is in the trillions of dollars.

In a few cases, estimates of the economic value associated with ecosystem services have been used to assist decision-making. This typically is based on the calculation of replacement values. For example, New York city was recently faced with drinking water that was below EPA's standards for such water. Planners recognized that they could approach the dilemma in one of two ways:

The cost of the necessary restoration of the watershed was estimated at $1 - 1.5 Billion, while the cost for the new treatment facility was estimated at $6 - 8 Billion. Guess which New York chose to do? Of course; to work on restoring the watershed!

This example (see Conservation Biology 12(3): 497-498 [1998] for more details) points out that it is critically important that ecologists and economists work hard to develop methods of assessing and valuing ecosystem services!

Another recent study estimated the pollination sevices provided free of charge to pollinator dependent crops in California's agriculture by wild bees is "worth" about $2.4 billion per year! (Frontiers in Ecology and Environment Aug. 2011). In addition, this study found that these wild bees depend on suitable habitat surrounding the farms and orchards that they provide pollination for -- in particular, they need rangeland habitat (grasslands, meadows, savanas, and shrublands), hence an argument that fostering these hotspots of natural habitat will benefit agriculture can clearly be made!

Recently, some formal markets that sell, essentially, ecosystem services have emerged. One involves wetlands. As you probably know, wetlands in the US are protected under the Clean Water Act, because they provide many valuable ecosystem services -- in particular, they retain and purify water and provide habitat for many plant and animal species. If an individual (or another entity) wishes to drain or fill or otherwise influence wetlands, as for building on them, they must comply with "no net loss" provisions under the Clean Water Act. One way this is attempted is by mitigating for the lost wetland acreage by restoring, enhancing, or creating a wetland in another area. This can be done by paying someone who is managing a certified "wetland mitigation bank," which is an area in which restoration, enhancement, or creation of wetland area is occurring. Some former grass seed farmers in the Willamette Valley, for example, are working to convert former grass seed fields back to their previous wetland status, and, by selling mitigation credits on these lands they actually make a good deal of money!

Another emerging market is associated with carbon credits. We'll talk about this when we talk about global climate change policies, but, in a nutshell, a landowner can sell carbon credits to another individual or entity in exchange for managing the land in a way that fosters storage of carbon in the soil or vegetation.

In Dec. 2008, the US Department of Agriculture announed the creation of an Office of Ecosystem Services and Markets. This entity is charged with developing guidelines and science-based methods to assess values associated with ecosystem services and help create markets for them. A sign that ecosystem services and their valuation is being taken seriously!

Of course, most valuation attempts are almost completely human-centered, neglecting the value of ecosystem services for other inhabitants of this planet. Is a human-centered valuation ethical? necessary?

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Page maintained by Patricia Muir at Oregon State University; last updated Oct. 28, 2011.

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