Urban Agroecology

Following are some basics that apply generally to all ecosystems. This page presents these basics in a simplified general ecosystem model and walks through the key features. Once you have this down, you can go on to the specific ecosystems (often microecosystems) that are described in this website.

Three critical concepts

First, ecosystems have boundaries. Or, more clearly, when we study them, we impose boundaries on them. For example, if we look at the entire earth as an ecosystem, the boundary is the biosphere, made up of the earth’s atmosphere and the thin top layer of the earth’s surface that contains living matter. We can also make the boundaries very small, say a crock of fermenting sauerkraut. Sometimes the boundaries are obvious natural boundaries, like the shoreline that separates a redwood forest ecosystem from an ocean ecosystem. Sometimes the boundaries are arbitrary, like in a one-meter square of soil being studied by a scientist. But, in all cases, the point of the boundary is to make studying and understanding a set of living organisms and their environment tractable. So, don’t worry too much about boundaries. Just accept that some activities occur within the ecosystem and some occur across the boundaries.

Second, ecosystems are dynamic, that is, they change. They are rarely, if ever, static. Changes sometimes come from outside the ecosystem, such as sunlight shining into your backyard ecosystem or a meteor entering the earth ecosystem. Other changes come from inside the ecosystem. For example, as organisms evolve (itself a change, usually in response to something else), their behavior can change, altering their relationships with other organisms. Or, as a plant grows in response to sunlight entering it’s ecosystem, it changes the microclimate around it by creating shade and, perhaps, making the air more humid.

You’ll note a key point from some of these examples. As one thing changes, other things change in response to them. This is our third important concept: ecosystem components are related to each other. This factor is why studying individual factors of a living environment, such as what nutrients an organism needs to grow, tells a very limited story. This is why the science of ecology was developed, to study the bigger picture of how ecosystem components interact with each other. This is also a key difference between agroecology and industrial agriculture. Most factors related to food production in industrial agriculture are looked at individually instead of systemically. For example, fertilizers are usually applied to increase plant growth. But, there is often excess fertilizer that isn’t used by the target plants and this excess can run off into natural waterways, changing the natural ecology of streams. But, the effect on streams is ignored, while attention is focused on the plants targeted by the fertilizer. In agroecology, attention is given to the entire ecosystem and alternative means of enhancing plant growth are used, such as composting and manuring, that release nutrients slowly over time as they can be absorbed by the plants that need them.

The basic model

Let’s look at the basic, generic model of an ecosystem and discuss its key elements.

At the heart of all ecosystems are the living organisms that make up the system. In all ecosystems, there are some very small, microorganisms. For example, cup your hand and scoop up some dirt or soil. You won’t see much action, maybe a sow bug or a worm. But, there are billions of micro-organisms there in your hand and all of them, including the sow bugs, are after one thing: energy. These organisms all interact, sometimes competing for energy, sometimes cooperating to obtain it.

All life requires energy. In fact, one way to think about living organisms is that they are simply complex batches of chemicals organized to capture and use energy. That applies to the smallest micro-organism to the largest fish, birds and mammals. Sometimes, as shown in the model above, energy comes from outside the ecosystem. This might be sunlight coming into your backyard, which is converted to energy by plants. Or, in the smaller soil ecosystem in which the plant grows, the plant is pushing out roots into the soil, bringing some of that stored energy into the soil in those roots. Most other organisms, including humans, get their energy from other organic matter, either living or dead. The important point is that they all need energy, so tracking energy flows into, through and out of an ecosystem is critical to understanding how the system works. (There are many ways energy leaves ecosystems. The dotted line above represents that loss of energy from the system.)

All organisms require water to survive. Most organisms need air, too. Plants need air to get carbon dioxide (CO₂), which is combined with sunlight during photosynthesis to enable plants to store energy in carbohydrate, fat and protein molecules. In the process, oxygen (O₂) is produced. Other organisms (including you, me, and the micro-organisms in a compost pile) use this stored energy. If air is available, then carbohydrates, fatty acids (components of fats) and amino acids (components of proteins) are used efficiently, using O₂ from the air and releasing CO₂ back into the air. This is an aerobic process, that is, one that uses O₂. If air isn’t available, then the molecules are processed for energy using different, anaerobic processes (without O₂). Good compost piles allow air to flow so that aerobic processes occur. Good fermentation restricts air flow, so that anaerobic processes can occur.

Individual organisms require a number of critical macro-nutrients, that is substances that make up a major part of their structure. These include carbon (a key structural element for plants and critical for storing energy), nitrogen (a key component of proteins) and phosphorus (important for storing the genetic code of all organisms in RNA and DNA, transporting energy around cells in adenosine triphosphate, and more). Many micro-nutrients, substances needed in lower quantities but still playing vital roles in living organisms, are also important. These nutrients come from various sources and organisms of different species often cooperate to help each other obtain nutrients. Ecologists sometimes focus on specific key nutrients, such as nitrogen and carbon, to identify how they flow and cycle through ecosystems.

Environmental conditions play a critical role in ecosystems. Every organism has a temperature range within which it functions best. The average levels of temperature, the ranges of temperatures, and periodic fluctuations of temperature on a daily or seasonal basis are all important in some way to different organisms. Finally, depending upon the ecosystem, other environmental factors such as acidity, salinity and alcohol levels can be important.

Population dynamics

Because ecosystems are composed of interacting populations of different species, and because ecosystems change, populations of those species change over time as conditions change. Let’s assume some species has a certain population level, but something changes in the environment that causes that level to increase. A hypothetical curve (called a logistic curve) describes how the population might change over time (see picture of Model-1).

In this picture, a change in environment allows population to increase, but as it does so, other changes take place in the environment that eventually cause population growth to slow and a new population level to be reached. Of course, lots of changes can occur in an environment and some of these might cause a population to decline (see picture of Model-2).

Because of interactions taking place in ecosystems, including reactions to the changing population of a species, more complex changes can occur (see picture of Model-3).

These are, of course, simplified diagrams. In real ecosystems, population dynamics are usually much more complex and are often difficult to predict. What’s important for our purposes is that these populations do change. As you will see in examples of specific ecosystems, these changes can often be manipulated to accomplish some purpose, like speeding up decomposition in a compost pile or baking bread. It is your job as an agroecologist to learn to manage conditions to best accomplish your purposes.

There are many other important topics in ecology, but we now have enough background to get started looking at specific kinds of ecosystems important to agroecologists.